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An atom is cooled by a standing-wave light field between two high-quality mirrors. Cavity cooling avoids the usual light scatter into the surroundings. Instead, the light leaking out of the mirrors is blue-shifted to a higher frequency (image credit: Pepijn Pinkse Max Planck Institute of Quantum Optics)
Artist's impression of an atom cooling device
NECF Meeting Abstracts

75th New England Complex Fluids Meeting
MIT | Friday, June 15, 2018
Registration deadline: Wednesday, June 13, 2018
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Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Invited; Julia Ortony

"Controlling state-of-matter on the nanoscale"

2. Rogers; W. Benjamin Rogers

Brandeis University
"Using microfluidics to quantify the dynamics of colloidal self-assembly"

3. Weitz; TBD


4. Zhou; Shuang Zhou

Harvard University
"Living Liquid Crystals"

Sound Bites

1. Arango; Maria A. Torres Arango1; Konstantinos Sierros2

1Brookhaven National Laboratory, 2West Virginia University
"Multiphase Emulsion Systems as Design Space for Complex Metal-Oxide Mesostructures"
Metal-oxides represent an important group of materials due to their vast application across different fields of science and technology. Moreover, these material systems and their processing provide unique opportunities to study materials’ fundamentals and accordingly tune their properties. Particularly, metal-oxide based multiphase emulsion inks may be viewed as a versatile approach for designing hierarchical mesostructured materials, by exploiting the properties and tunable nature of their precursor constituents and their interactions. Illustrating this approach, the design and tailoring of 3D printable mesoporous ceramic foams from TiO2-based emulsion inks are discussed, highlighting its environmental benignity and great potential for application. This approach may be transferred to other metal-oxide systems beyond TiO2, further opening to novel and exciting knowledge frontiers.
Keywords: Emulsions, 3D printing, Ceramic foams, Tuning surface properties

2. Asmi; Rhita Asmi, Renal Backov, Gareth H. McKinley, Thibaut Divoux

"Rheological properties of aqueous suspensions of carbon black particles"
Carbon black refers to colloidal soot particles that result from the incomplete combustion of fossil fuels. These fractal particles, of typical size 200-500 nm, tend to form reversible and weakly linked agglomerates when dispersed in a liquid hydrocarbon. Indeed CB particles interact through a short-range attractive potential, whose depth is estimated at about 30 kT. Small volume fractions are sufficient to turn the dispersion into an interconnected network, hence into a colloidal gel. Such gels are involved in a wide variety of industrial applications including paints, coatings, rubbers and tires. However, for some applications such as flow batteries, there is a growing need for aqueous dispersion of carbon black gels. Soot particles being hydrophobic, one can use a third component, such as a polymer to make an aqueous dispersion of soot particles. Here we report rheological measurements on cellulose-based carbon black aqueous dispersions. We show that the critical onset of a solidlike network occurs at a volume fraction that is much larger than the one measured in non-aqueous solvent. We determine the scaling of the viscoelastic spectrum as a function of the particle concentration and compare systematically the rheological properties of aqueous and non-aqueous suspensions of carbon black particles.
Keywords: Carbon Black, aqueous gels, yield stress fluid, viscoelastic spectrum

3. Atis; Bryan T. Weinstein, Andrew W. Murray and David R. Nelson

Harvard University
"How do growing microbial colonies generate their own propelling flow in liquids?"
Range expansions coupled with fluid flows are of great importance in understanding the organization, transport and competition of microorganism populations in liquid environments. We have recently created an extremely viscous medium that allows us to grow cells on a controlled liquid interface over macroscopic scales. Experiments revealed that the growth of populations of the budding yeast, Saccharomyces cerevisiae, on unstirred, viscous media generates fluid flow that makes the population dynamics and structure on liquid media radically different from those seen on solid, agar medium. We show with laboratory experiments, combined with numerical modelling, that this flow originates from a baroclinic instability due to buoyancy differences induced by microbial metabolism in the liquid media. Changing the viscosity of the medium has profound effects on population dynamics and structure, and can lead to a rich variety of morphologies, including a Rayleigh-Plateau like instability at the colony frontier.
Keywords: microbial growth, fluid dynamics, fingering instability

4. Bell; Jay Tang

Brown University
"Rotational speed oscillations in body-tethered C. crescentus: evidence for a whirling flagellum"
Swarmer cells of Caulobacter crescentus are often found to tether to glass at a point on the cell body. The rolling of the freely-rotating flagellum near the glass surface causes the cell body to rotate. We describe the discovery of oscillations in the rotational speed of these cell bodies, which depend on accumulated angle rather than time. We discuss possible explanations for these oscillations, including variations in motor torque, and suggest that a mechanical instability in the flagellar motion provides the most credible explanation.
Keywords: bacterial motility, flagellum

5. Berman; Justin Berman and Katharine Jensen

Williams College
"Solid Adhesive Detachment Dynamics"
Keywords: TBD

6. Brouchon; Julie Brouchon, Xu Zhang, Yuan Yuan, John Heyman, Jaime M. Calvo-Calle, David A. Weitz

Harvard University
"Microfluidics for high-throughput immune-cell interaction analysis"
We developed a microfluidics method to perform high-throughput functional assays in micron-sized droplet. We are using this technique to isolate immune cells based on activation after specific interaction with a target cell.
Keywords: Microfluidics, Immune cell, Functionnal assay

7. Busupalli; Joanna B Dahl

University of Massachusetts Boston
"Membrane dynamics in artificial vesicles"
Closed membranous structures such as vesicles are widely used as model protocells to study membrane dynamics including membrane bending and stretching and scission. We here describe how a little tweak in the preparation process and the membrane stretching moduli measurements threw a hint on one such vesicular system viz. polymer vesicles which eventually led us to realize membrane scission or fission in the membranes of the same leading to two daughter polymer vesicles. In a similar way, our current studies involve thorough analyses of membrane properties of lipid vesicles or liposomes prepared via electroformation and other elegant methods. We employ spectroscopic techniques such as vibrational spectroscopy and, also high-resolution microscopy to investigate the membrane dynamics in such liposomes to gain insights into the same and to thoroughly understand the membrane properties of these liposomes. Our collective expertise on polymersomes and liposomes could provide us with the knowledge to understand the liposomal membrane dynamics in more detail that could add to the existing knowledge on the membrane properties of the liposomes.
Keywords: Vesicles, liposomes, polymersomes, membrane dynamics, electroformation, vibrational spectroscopy.

8. Campanaro; Prof. Irmgard Bischofberger

ETH Zürich
"Fingers and Fractures – Morphologies of Failure Modes in Shear-Jamming Suspensions"
Stabilized dense suspension are a class of complex fluids that exhibit both shear-thickening and shear-jamming behavior as a response to an applied stress. These dynamic liquid-to-solid transitions have important consequences for the displacement of a dense suspension by another fluid: upon the injection of air, intricate patterns arise in the suspension, leading to flow or fracture of the material. The pattern formation is studied in a quasi-2D geometry, where a cornstarch suspension, confined between two plates, is displaced by a pressure controlled injection of air. Depending on the concentration of cornstarch and the applied stress, it is possible to access very diverse patterns: smooth fingering in the fluid regime and various modes of fractures, ranging from slow branched cracks to single fast fractures.
Keywords: dense suspensions, shear thickening, shear jamming

9. Chang; Tyler K. Lytle, Charles E. Sing, and Sarah L.Perry

UMass Amherst
"Sequence Control of Complex Coacervation"
Complex coacervation is a liquid-liquid phase separation phenomenon driven by the electrostatic and entropically-driven complexation of oppositely charged polyelectrolytes. The resulting coacervate phase retains significant amounts of water and ions and displays an extremely low surface tension that has enabled the use of these materials for many applications, such as underwater adhesives, drug delivery, food and personal care products. There also has been increasing interest in coacervate-like droplets occurring in biological systems. The majority of these so-called membraneless organelles involve a combination of intrinsically-disordered proteins and RNA, and phase separate due to a combination of long-range charge effects and short-range hydrophobic effects. While evolution has optimized the self-assembly of these types of biological polymers over millions of years, our ability to design such materials remains limited, in part because the relevant interactions that occur over a wide range of different length scales. The goal of this research is to establish molecular-level design rules as to how chemical sequence can modulate the formation and properties of complex coacervates. While studies to date have focused on the effect of parameters such as the charge stoichiometry, temperature, pH, salt concentration, stereochemistry, polymer architecture, and the density of charges present, the ability to pattern the sequence of charges and other chemistries has been rarely studied. The scarcity of such studies is due largely to the difficulty of synthesizing polyelectrolytes with equal chain length and charge density, but different distributions of charge or other functionalities. However, polypeptides represent a model platform for the synthesis and study of polyelectrolytes with precisely controlled polymer architecture and sequence patterning at the molecular level, while retaining relevance to a variety of biological, medical, and industrial applications. Experimental measures such as turbidimetry and optical microscopy, coupled with isothermal titration calorimetry were coupled with theoretical and computational approaches to study how variations in the patterning and overall fraction of charged groups along the polymer affect the resulting coacervate phase behavior. Increasing the number of charged residues increased the salt resistance and the size of the two-phase region. More interestingly, a comparison between polypeptides with the same overall charge fraction, but different periodic repeating patterns of charged monomers (e.g., alternating, every two residues, every four, etc.) showed an increase in coacervate stability with increasing charge block size. Thermodynamic data, coupled with insights from simulation showed that the increase in stability was entropic in nature, resulting from differences in the one-dimensional confinement of counterions along the patterned polymer. Expanding these efforts to also address the potential impacts of increasingly hydrophobic residues within these patterns, we are looking to understand the physics driving sequence-specific complex coacervation as a composition-independent strategy for modulating the phase behavior and physical properties of designer materials for a range of applications, as well as providing insight into phase separation in biological systems.
Keywords: Complex Coacervation, Polyelectrolyte Complex

10. Cohn; Andriy Sherehiy, Jeremy M. Rathfon, Hiroya Abe, Robert W. Cohn

University of Louisville
"Shape Transformation Photolithography: Self assembled arrays of suspended MEMS structures from patterned polymer membranes"
Shape transformation photolithography (STP) is a method for forming suspended arrays of delicate micromechanical elements by capillary force directed self-assembly. It is the first micro/nano fabrication method for patterning fiber bridges at arbitrary locations and arbitrary directions in parallel, and which therefore, supports high manufacturing rates. A film of polymer (around 80 nm thick) is suspended above the surface of a substrate (e.g. on a micropillar array). Modest resolution holes are patterned in the film. Above the glass transition the holes expand and transform into capillary bridges that further transform and thin into well-defined fiber bridges and interconnected fibers anchored between the pillars. STP could be used to add a MEMS layer to the process flow for mass produced integrated circuits. The method would be valuable for the fabrication of high aspect ratio, highly delicate structures that could otherwise be damaged by capillary forces (e.g. “snap down”) during a conventional release step in which the underlying spacer is removed by dissolution. In current 193 nm production optical steppers, the exposure dose can be as high as 1.1 mJ/cm2 per laser pulse. Depending on film thickness, molecular weight and dye concentration we have thermally ablated holes in polystyrene with doses as low as 10.5 mJ/cm2 for single 533 nm laser pulses and 3.3 mJ/cm2 for 0.5 s exposures of 3,000 pulses. If a polymer can be developed with an exposure sensitivity that is below the dose rate of current wafer steppers, STP could be implemented without modification of current steppers, and could maintain economically sustainable production rates of greater than 50 wafers per hour.
Keywords: Fabrication technology, lithography, self assembly, capillary force, polymer flow, fiber spinning

11. Costalonga; Michiel Hack, Tim Segers, Stefan Karpitschka, Hermann Wijshoff and Jacco Snoeijer

Massachussetts Institute of Technology
"Attraction and repulsion of droplets on a viscous film"
Every morning at their breakfast, cereal eaters can see that floating objects on a liquid bath attracts to form clusters : this is the so-called Cheerios effect. It has been shown recently that droplets on elastic substrates also interact, either attracting or repelling each other depending on the local slope of the substrate where they lie. Here we present an experiment extending these results to the interaction of droplets deposited on a thin viscous film. By measuring independently the velocity of the droplets and the surface topography of the film, we identify non-monotonic interactions that are due to waves appearing on the film. The drag force exerted onto the droplets is also investigated. We show that the thickness of the film below the drop is intrinsically selected by the velocity of the drop, by a mechanism similar to Bretherton’s bubble rising in a confining tube.
Keywords: viscous film, droplet, interaction, cheerios effect

12. Cui; Huidan Zhang; Yamei Cai, David Weitz

Harvard University SEAS
"Ultra High-Throughput Targeted sequencing in single cells using droplet barcoding microfluidics"
Single cell targeted sequencing is a powerful tool for genetic mutation detection, which play an important role in studying genetic heterogeneity and clonal evolution in many complex illness and diseases. Studies show that clonal evolution can contribute to treatment failure, drug resistance and metastasis in cancer. For example, Intratumoural EGFR heterogeneity in Non-Small Cell Lung Cancer (NSCLC) revealed a close relationship with tumor shrinkage under chemotherapy treatment. To provide a good representation of the whole tumor, large number of cells need to be processed due to large number of tumor cells present. Current technology could only study 96 cells at a time using plate based assays or commercially available platform, which is very difficult and expansive to scale up. We have developed a droplet-based microfluidics technology that is capable of first encapsulating single cells in droplet, amplifying multiple targeted genes in each drop and then molecularly barcode them using hydrogel barcoded beads with very high efficiency. The ability to barcode tens of thousands of cells in each experiment with such low cost makes this a great tool for many clinical applications. 
Keywords: single cell; sequencing; drop-based microfluidics

13. Deveney; Werner, Joerg; Payamyar, Payam; Weitz, David

"Experimental System for Studying Nucleation Kinetics of Hard Sphere Colloids"
Although the energetics of solid crystal nuclei formation are well described by homogeneous nucleation theory, the kinetics of nucleation as measured in experiment and as computed in simulations yield nucleation rates that differ by over 20 orders of magnitude. In an attempt to improve the understanding of nucleation processes and associated kinetics we are developing a model system using colloidal hard spheres where the volume fraction and nucleation behavior of the colloids can be controlled through variable-volume chambers. The technique is compatible with confocal microscopy for direct observation of the nucleation process.
Keywords: Nucleation

14. Farokhirad; Abhay Ranganathan, Jacob Myerson, Vladimir R. Muzykantov, Portonovo S. Ayyaswamy, David M. Eckmann, and Ravi Radhakrishnan

University of Pennsylvania
"Quantitative basis for design and vascular targeting of flexible polymeric nanoparticles"
Targeted nanometer-sized particles (50-200 nm) filled with therapeutics or imaging agents that are directed to precise locations in the body promise to improve the treatment and detection of many diseases. Targeting of nanoparticles (NPs) functionalized with antibodies to vascular endothelial surface molecules depends on several physiological factors such as antibody density, receptor expression, cellular mechanical factors, hydrodynamic conditions such as hematocrit density, blood flow rates, and vessel diameters. In addition, several design factors such as size, shape, flexibility, NP architecture influence targeting efficacy (i.e., tissue selectivity as well as avidity). However, the individual contributions of these multitudes of factors are often not discernable leading to largely an empirical and exhaustive search for optimal design. In particular, there is a need for precise control of the specificity and selectivity of binding of the NPs to the target (inflamed or diseased) tissue of interest under varying pathophysiological and hydrodynamic conditions in the vasculature. Using a theoretical model and coarse-grained Brownian dynamics simulations to compute the structural and dynamic properties of a deformable core-shell polymer-based NP, we explore the effects of wall-confinement due to endothelium, physiological factors including the glycocalyx layer and margination due to blood particulates such as RBCs, and NP synthesis factors such as the degree of cross-linking and targeting antibody density on the NP microstructure and transport properties. Through quantitative modeling and experimentation, we uncover rational design principles for engineering polymeric NPs through mechanistic studies of hydrodynamic interactions and multivalent binding for achieving efficient margination and enhanced binding to the endothelium. The reported combined computational and experimental approach and results are expected to enable fine-tuning of design and optimization of flexible NP in targeted drug delivery applications which are quite distinct from rigid and regular-shaped NPs.
Keywords: Brownian Dynamics, Drug Delivery, Cell Adhesion, Vascular targeting

15. Flagg; Ian Hunter, Seth Fraden

Brandeis University, Hampton University
"Using the Belousov-Zhabotinsky Reaction to Understand Small Networks of Neurons"
Studying small systems of Belousov-Zhabotinski reactions will reveal information that can be used to understand how large populations of neurons form patterns. The BZ reaction is a stable limit-cycle that imitates natural oscillations like the nervous system, heartbeats, or the movement of an eel. Using microfluidic cells, we create large networks of BZ reactions and focus on the inhibitory coupling that occurs when bromine diffuses through PDMS from one cell to another.

16. Gault; Zsolt Terdik, Joerg Werner, Frans Spaepen, David Weitz

Harvard University
"Visualization and mechanical study of a transparent filled rubber"
Filled rubbers are composite materials containing two interpenetrating phases: crosslinked elastomers, and a ‘filler’ consisting of colloidal particle aggregates. Above a critical volume fraction, the colloidal aggregates form a system-spanning subnetwork that reinforces the elastomer network and introduces a new energy loss mechanism at low strains of only 1-5%. This low-strain energy loss mechanism, known as the Payne Effect, is one of the mechanical hallmarks of filled rubbers and is a major contributor to rolling friction in tires. The goal of this project is to probe these microstructural dynamics in a model filled rubber, in combination with bulk rheological tests, to gain new insight into physics of the Payne effect.

17. Gerber; Will Steinhardt, Shmuel Rubinstein, David Weitz

Harvard SEAS
"Particle transport and structuration in fractured gels. Application to proppants."
During oil and gas well production or underground disposal of waste, proppants are added to frac fluids to help maintain conductive fractures. While a large number of particles with various sizes, shapes or composition have been empirically tested, the understanding of their propagation and structuration during fracking is still very poor. Here, we generate controlled penny-shaped fractures in brittle transparent hydrogels. The fluid invading the fracture is a suspension (5 to 20vol%) of index-matched beads. We follow the transport of the beads and their interaction with the surface of the fracture by high-speed imaging, and deduce the parameters relevant to propping efficiency.
Keywords: proppants, fracture, gel, transport

18. Giso; Timothy Atherton

Tufts University
"Packing Particles on a Sphere Under Gravity"
Packings on curved surfaces may resolve geometric frustration by introducing defects or elastic deformations. Here, I investigate the scenario where the surface is only partially covered. Soft, spherical particles are constrained to a sphere under the force of gravity and a Hertzian interaction and low-energy structures are found by simulated annealing. The defect structures obtained depend crucially on the coverage fraction and the elasticity of the particles.
Keywords: Packing,defect structures

19. Govinna; Nelaka Govinna, Papatya Kaner, Davette Ceasar, Anita Dhungana, Cody Moers, Katherine Son, Ayse Asatekin, Peggy Cebe

Tufts University
"Fouling resistant electrospun fiber membranes for oil-water separation"
Non-woven super-hydrophobic fiber membranes have potential applications in oil-water separation. The membranes are blends comprising poly(vinylidene fluoride), PVDF, and random zwitterionic copolymers of poly(methyl methacrylate), PMMA and sulfobetaine methacrylate, SBMA, and sulfobetaine-2-vinylpyridine, SB2VP. PVDF imparts mechanical strength to the membrane, while the copolymers enhance membrane roughness and introduces fouling resistance. Blend composition was varied by controlling the PVDF content of the blends. Non-woven fiber membranes were obtained by electrospinning solutions of PVDF and the copolymers, at 20% w/v in a mixed solvent, N,N-dimethylacetamide/acetone, 7/3 v/v. The crystal phases and crystallinities of the blends were studied using wide angle X-ray diffraction and differential scanning calorimetry. The degradation profiles were determined using thermogravimetry and the degradation temperatures varied systematically with the composition of the blends. The hydrophobicity of the fibrous membranes was determined using contact angle testing. Addition of both copolymers showed significant decrease of fouling and at 10% of PMMA-r-SB2VP, fouling resistance rose to excellence with a five-fold decrease of foulants retained.
Keywords: PVDF, SBMA, SB2VP, Zwitterion, filtration membrane, electrospinning

20. Han; Yu Long Han, Pierre Ronceray, Guoqiang Xu, Andrea Malandrino, Roger D. Kamm, Martin Lenz, Chase P. Broedersz, and Ming Guo

"Cell contraction induces long-ranged stress stiffening in the extracellular matrix"
Animal cells in tissues are supported by biopolymer matrices, which typically exhibit highly nonlinear mechanical properties. While the linear elasticity of the matrix can significantly impact cell mechanics and functionality, it remains largely unknown how cells, in turn, affect the nonlinear mechanics of their surrounding matrix. Here, we show that living contractile cells are able to generate a massive stiffness gradient in three distinct 3D extracellular matrix model systems: collagen, fibrin, and Matrigel. We decipher this remarkable behavior by introducing nonlinear stress inference microscopy (NSIM), a technique to infer stress fields in a 3D matrix from nonlinear microrheology measurements with optical tweezers. Using NSIM and simulations, we reveal large long-ranged cell-generated stresses capable of buckling filaments in the matrix. These stresses give rise to the large spatial extent of the observed cell-induced matrix stiffness gradient, which can provide a mechanism for mechanical communication between cells.
Keywords: cell–matrix interactions, cell mechanics

21. Haney; Joerg Werner, David A. Weitz, Subramanian Ramakrishnan, and Kayla Oden

Harvard University, Florida A&M University
"Stable Pickering Emulsions Using Janus Particles via Microfluidics"
Pickering emulsions are important in systems where controlled confinement of an oil or water phase is needed. For example, micro-capsules incased by particles can serve as rigid vehicles for drugs or precious food ingredients. These coated emulsions can harbor oils in water or even water in oils. Nonetheless, the stability of the Pickering emulsions depend on the wetting properties of the particle. Amphiphilic, “Janus”, particles with two distinct surface chemistries are ideal for stabilizing emulsions due to the ability to tune wetting properties by changing the chemistry. Microfluidic techniques are used to create Janus particles to form stable water in oil as well as oil in water Pickering emulsions. These particles are composed of polyethylene glycol hydrogel as the hydrophilic side and polypropylene glycol as the hydrophobic side. The hydrogel and hydrophobic polymer’s abilities to respectively absorb the water and oil in an emulsion system enable the particles stronger attachment to the oil-water interface. The flowrates in a glass capillary device are varied to demonstrate complete control over particle sizes and hydrophilic to hydrophobic domain ratios. Using UV light, these droplets are cross-linked via photo-polymerization to form monodispersed particles. By manipulating the inner-phase flowrate, larger sized particles are fabricated which give larger stable emulsions. By adjusting the ratio of inner-phase hydrophilic fluid to hydrophobic fluid, the sizes of the two individual sections of the Janus particle are controlled. Emulsion stability is tested via centrifugation. Use of these larger particles to stabilize the emulsions allows direct visual observation, which lead to better understanding of how factors such as the particle orientations at the L-L interface affect Pickering emulsion stability.
Keywords: Pickering Emulsion, Hydrogel, Microfluidics, Amphiphilic

22. Harris; Isabelle Bauman, Annika MacEwen

Brown University
"Impact of a superhydrophobic sphere onto a bath"
Small particles impacting a water surface can either stick to, pass through, or rebound completely from the interface (Lee & Kim, Langmuir 2008). In the present work, we focus on the bouncing dynamics of millimetric superhydrophobic spheres impacting the surface of a quiescent water bath. Particular attention is given to the dependence of the normal coefficient of restitution and contact time on the impact velocity.
Keywords: Free surface, surface tension, superhydrophobic

23. Hu; Xiaoyi Hu

Stony Brook University
"Viscous Wave Breaking and Ligament Formation in Microfluidic Systems"
Rapid layering of viscous materials in microsystems encompasses a range of hydrodynamic instabilities that facilitate mixing and emulsification processes of fluids having large differences in viscosity. We experimentally study the stability of high-viscosity stratifications made of miscible and immiscible fluid pairs in square microchannels and characterize the propagation dynamics of interfacial waves, including breaking and viscous ligament entrainment from wave crests at moderate Reynolds numbers. For large viscosity contrasts, parallel fluid streams adopt widely different velocities and provide a simple model system to probe the role of inflectional instabilities in relation with classic inviscid-stability theory. We reveal novel viscous wave regimes and discuss flow transitions using several dimensionless groups including capillary and Reynolds numbers based on characteristic flow parameters. The natural wave emission frequency appears independent of interfacial properties and is found in direct proportion with the interfacial velocity of stratifications. Relationships between wave celerity, wavelength, and amplitude suggest the existence of optimal operation conditions for passively disturbing flows and continuously dispersing low-and high-viscosity fluids at the small scale.
Keywords: Hydrodynamic instability, microfluidics, viscosity-stratification

24. Hunter; Mike Norton, Youssef Fahmy, Lanijah Flagg, Seth Fraden

Brandeis University
"Function following form: Dynamical consequences of symmetries in nonlinear networks"
Understanding how neural tissue creates spatially, and temporally varying patterns will help medicine, and guide development of technologies which mimic their functionality. An experimental system of confined micronscale oscillatory chemicals volumes coupled by selectively permeable membranes mimic the complexity of neural systems. The light sensitive, oscillatory chemical reaction in the microfluidic volumes can be controlled individually through a retrofitted projector, and collectively through temperature control. Recent theoretical work has also begun to reveal why observed system steady states are tightly related to the point symmetry group of the nodes of the coupled chemical oscillators under observation. These theoretical developments, and experimental motivation will be presented.
Keywords: Complex networks, nonlinear dynamics, BZ reaction, out of equilibrium dynamics

25. Katsikis; James Cybulski, Alexandre Breant, Anatoly Rinberg and Manu Prakash

Massachusetts Institute of Technology
"Droplets in sync: harnessing magnetic fields for synchronous microfluidics"
Droplets in microfluidic chambers are a growing platform for both physics and biology experiments; droplets can be produced at high throughput, perform chemical reactions as miniature beakers and carry biological entities. Here, we present a microfluidic platform for droplet control based on logic operations that automatically compute where droplets are stored or directed, thereby enabling parallel and synchronous control. We demonstrate that our platform enables error-free physical computation via synchronous universal logic with microdroplets. Our platform uses a rotating magnetic field that enables parallel manipulation of arbitrary numbers of droplets on permalloy tracks. The manipulation of the droplets is magnetophoretic; the rotating magnetic field activates the permalloy tracks to create a dynamic energy landscape that sets the droplets in motion. We show that our platform can control both ferrofluid droplets which are paramagnetic, and water droplets which are practically non-magnetic. Through a reduced-order model and scaling laws for understanding the underlying physics, we showcase droplet-based AND, OR, XOR, NOT and NAND logic gates, fanouts, a full adder, a flip-flop, a finite-state machine, and a droplet generator. Our platform enables large-scale integration of droplet logic, analogous to the scaling seen in digital electronics, and opens new avenues in mesoscale material processing.
Keywords: Droplets, microfludics, computation, logic gates, ferrofluids

26. Kosmrlj; So Nagashima, Hyun Dong Ha, Do Hyun Kim, Howard A. Stone, and Myoung-Woon Moon

Princeton University
"Capillarity-induced folding of wrinkled skin films"
We explore what happens, when a liquid droplet is placed on a wrinkled surface constructed by compressing a soft PDMS substrate with stiff skin. It was found that for sufficiently compressed wrinkled skin, the liquid from droplet starts entering wrinkled channels and forms a periodic array of liquid filaments. Capillary forces of water filaments squeeze such wrinkled channels into tight folds, while they pull and flatten the neighboring dry wrinkles. By placing a liquid droplet containing DNA molecules on a wrinkled substrate, we exploited the effect described above for the formation of periodically spaced DNA nanowires.
Keywords: wrinkling, liquid drop, folding, DNA nanowires

27. Liu; David Weitz, Martin Zhang (MGH)

Harvard University SEAS
"Largely upregulating Aβ levels by engineered polypeptide for Alzheimer’s disease pathology study"
Cerebral amyloid β-peptide (Aβ) accumulation and the formation of amyloid fiber plaque is regarded as one of the most important factors to induce Alzheimer’s disease (AD). But what factors causing the over-production and aggregation of Aβ are rather complicated. Here we find that elastin-like polypeptide which is originated from connective tissues is an important agent to speed up Aβ production. The added amount of elastin-like polypeptide for AD modeled nerve cell culture is positively relative to the Aβ levels. Remarkable, one magnitude upregulating Aβ level was founded in the cell lines. Further investigation indicated that the level of amyloid precursor protein secretase was increased by the elastin-like polypeptide additive. This might be a reason for the largely upregulating Aβ levels in the experiments. These results also suggest that native elastin proteins embedded in extracellular matrix may have unexpected correlation with the formation of AD.
Keywords: Alzheimer’s disease; amyloid β-peptide; elastin-like polypeptide;

28. Liu; Mike Norton, Seth Fraden

Brandeis University
"Tracking Topological Defects in Confined Active 2D Nematics with Deep Learning"
The active nematic we are researching is made up of microtubule bundles driven by ATP-powered kinesin clusters. The active nematic continuously flows in a turbulent-like manner and produces motile topological defects. By confining the suspension within a circular disk, a total topological charge on the system is enforced requiring, at minimum, two +1/2 defects. Under certain confinement conditions, these two defects co-rotate around each other in the well. In experiment, this circulating state is constantly disturbed by the nucleation of new defects near the boundary followed by annihilation of old defects. In order to understand these dynamics more fully, defects must be identified reliably. Our current tracking method requires us to first identify the nematic director field and then search for disclinations – a process which works well only under ideal experimental conditions. A deep learning approach is proposed to identify defects directly from images of the fluorescently labeled microtubule system. Currently, a library of sample defects is being built by manually identifying defects in experimental images. These instances will be used to train a Deep Convolutional Neural Network(CNN) to identify defects from experimental images. In the second phase of the project, a Deep Recurrent Neural Network(RNN) will be constructed to predict defect nucleation.
Keywords: liquid crystals, image processing, deep learning

29. Lowensohn; Janna Lowensohn, Benjamin Rogers

Brandeis Physics
"Linker-mediated binding of DNA-grafted colloids: New phase diagrams and how to predict them"
A central goal of nanotechnology research is to prescribe interactions between nano- and microscopic components, such that they self assemble into prescribed structures. DNA is an implementation of this idea. We examine the assembly of DNA coated colloids that can only interact via oligonucleotides dispersed in solution which act as linkers to mediate binding between the grafted strands of DNA. We report the melting temperatures and phase behavior of this system under changing linker concentrations, grafting densities, and linker binding affinities, as well as develop a model to predict the melting temperatures of future experiments. Our results show that increasing grafting density and binding affinity will produce an increase in the melting temperature. We also report on a previously unreported region of phase space in which particles fail to aggregate at high linker concentrations. We find that the linker binding affinity has no effect on this re-entrant concentration. Finally, we explore more complex systems in an effort to develop competitive linker systems able of achieving aperiodic and/or photonic structures.

30. Martin; Johannes Zwanikken

University of Massachusetts Lowell
"The importance of directionality in the clustering behavior of self-propelled squares."
Self-Propelled particles have the ability to self-assemble into large-scale structures not unlike Brownian particles. However, there are radical differences with respect to the fluctuations inside these structures, the time scales of assembly, the range of accessible structures, and the number of tuning parameters. We demonstrate, using extensive molecular dynamics simulations, that there are radical transitions in the structural and dynamical properties of ensembles of self-propelled squares brought by a change in the direction of self-propulsion while keeping the other material properties constant. Squares that self-propel in the direction perpendicular to a side rapidly reach a steady state with a characteristic cluster distribution and structure. After tilting the direction of self-propulsion towards a corner, the particles form large and dense clusters that show a transient collective motion, and display remarkable fluctuations over long time scales. Understanding this behavior could offer new design rules for programmable materials, and grant further insights in the dynamic processes that nature employs for self-assembly.
Keywords: Active Matter, Self Propelled Particles, Active Squares

31. Michel; T. Divoux, I. Bischofberger

"Hydrogel reinforcement: unexpected particle size dependence"
Polymer hydrogels are composed of a network filled with a solvent that confers upon the gel viscoelastic properties. At low polymer concentrations of typically a few percents in mass, hydrogels mechanical properties remain weak and brittle. A practical solution to reinforce the gel elastic properties consists in adding stiff nanoparticles. Here we investigate the impact of polystyrene nanoparticles on the linear viscoelastic properties of agarose gels. We show that reinforcement only occurs when the particle size is at least as large as the mesh size of the gel. Surprisingly, we observe that the latter reinforcement effect is independent of the particle size, showing that the reinforcement is not a mere function of ratio particle size/mesh size. Finally, we observe that particle reinforcement occurs beyond a critical particle volume fraction.
Keywords: gels, elastic modulus, nanoparticles, reinforcement

32. Millay; Zsolt Terdik, Dave Weitz, Frans Spaepen

Williams College and Harvard University
"Relaxation Phenomena in Hard-sphere Colloidal Glass"
We will be analyzing the effects of osmotic pressure on a colloidal glass during formation and relaxation. Using a confocal microscope to track particle positions, we will observe the time dependent fluctuations of short-range, purely repulsive monodisperse particles. The gravitational force due to density difference between the particles and the solution creates the osmotic pressure that varies as a function of height. In order to probe the effects of osmotic pressure, we will collect data about the bulk modulus, a sensitive measure of local structure. We will extend this and analyze the relaxation structure after centrifugation.
Keywords: Colloidal glass, osmotic pressure, relaxation, bulk modulus

33. Moustaka; Maria Eleni Moustaka, Baptiste Blanc, Mike Norton, Viktor Horvath, S. Ali Aghavami, Irving Epstein, Seth Fraden

Brandeis University
"Partition, reaction and diffusion coefficients of Bromine in PDMS for the study of non-linear chemical dynamics in BZ networks"
The goal of this study is to measure the partition coefficient, diffusivity and reaction rate of bromine in the ubiquitous elastomeric material, poly(dimethylsiloxane) (PDMS). The dynamics of chemical oscillator networks comprised of PDMS well lattices filled with the oscillatory and bromine-producing Belousov-Zhabotinsky chemical reaction depend critically on the coupling strength between wells. The PDMS membrane between wells is responsible for dynamically coupling neighboring wells by selectively transmitting bromine; understanding its physiochemical properties are therefore essential for understanding network dynamics. All data are collected with the use of absorption spectroscopy techniques and analyzed by fitting to a numerically solved reaction-diffusion model. The reaction we identify is heretofore unreported but justified due to the presence of free vinyl groups in Dow-Corning Sylgard 184 PDMS.
Keywords: BZ networks, poly(dimethylsiloxane) (PDMS), bromine, Partition coefficient, reaction coefficient, diffusion coefficient

34. Murray; David Murray, Zsolt Terdik, Dave Weitz, Frans Spaepen

Georgia State University, Harvard University SEAS
"Phase Behavior of Colloidal Crystals with Tunable Interaction Strengths"
Colloidal particles can be assembled into a variety of crystalline structures, including FCC and BCC lattices. Many physical properties of the colloidal crystal -such as packing structure, packing fraction, and elastic constants- depend on the colloid-colloid interaction potential. We synthesize colloid particles with tunable interaction potentials, from long range repulsive to hard sphere, using a recently developed synthesis. The tunable interactions principally arise from salt concentration in the suspending solvent and the polyelectrolyte stabilizing brushes. Using these colloidal particles, we will experimentally map out the equilibrium phase behavior and search for solid-solid phase transformations (presumably FCC <-> BCC) driven by mechanical compression.
Keywords: synthesis, colloidal crystals, tunable interactions, solid-solid phase transformations

35. Norton; Achini Opathalage, Seth Fraden, Zvonimir Dogic

Brandeis University
"Confined active nematic suspension exhibits multi-timescale dynamics"
Confining an active nematic suspension composed of microtubules and kinesin motor clusters to a disk below a critical diameter tames the otherwise turbulent flows observed in bulk into a single, system-sized vortex. This circulating flow is driven by two, topologically required +1/2 defects. In contrast to hydrodynamic theory, which predicts stable orbits, this state is periodically disrupted by boundary nucleation of a +/- 1/2 defect pair on a time scale longer than the defect procession period. The nature of these slow periodic dynamics that are superimposed on the faster, procession dynamics will be presented.
Keywords: active suspensions, liquid crystals

36. Ovryn

Colorado State University
"Single molecule tracking of tagged glycans in live cells and modeling of beads-on-a-string structures along membrane nanotubes"
The surface of eukaryotic cells is covered with a dense layer of glycans; using a combination of bioorthogonal click reactions, super-resolution imaging and single molecule tracking, we are able to observe the dynamics of cell-surface glycans on live cells with high spatial and temporal resolution. This approach has enabled dynamic visualization of tunneling nanotubes connections between cells and the tracking of glycosylated receptors along the surface of these membranes. Our custom microscope also implements a form of confocal interference microscopy, based upon laser feedback interferometry, to simultaneously measure the topography of the membrane. Although many membrane nanotubes appear as stable liquid cylinder connections between cells, we have recently observed from single molecule tracking of glycoproteins in live cells, that membrane nanotubes can manifest stable, “beads-on-a-string structures”. Our analysis of the free energy landscape including membrane bending energy and a heterogenous density of transmembrane proteins is in agreement with several of our observations.
Keywords: single molecule imaging, interference imaging, glycans, nanotubes

37. Parada; Xuanhe Zhao

"Ideal Reversible Polymer Networks: Viscoelastic Hydrogels with single relaxation times"
Ideal Reversible Polymer Networks have well-controlled network structures similar to ideal covalent networks but exhibit viscoelastic behaviors due to the presence of reversible crosslinks. We developed a theory to describe the mechanical properties of these ideal reversible polymer networks, and predicted that the networks behave as a single Maxwell element of a spring and a dashpot in series, with the instantaneous shear modulus and relaxation time determined by the concentration of elastically-active chains and the dynamics of reversible crosslinks, respectively. Due to the use of short polymer chains, we expected no contributions from polymer chain entanglements or chain relaxation, as the Rouse relaxation time is much shorter than the reversible crosslinks’ characteristic time. The theory also provided general methods to (i) independently control the instantaneous shear modulus and relaxation time of the networks, and to (ii) quantitatively measure kinetic parameters of the reversible crosslinks, including reaction rates and activation energies, from macroscopic viscoelastic measurements. To validate the proposed theory and methods, we then synthesized and characterized the mechanical properties of a hydrogel composed of 4-arm polyethylene glycol (PEG) polymers end-functionalized with reversible crosslinks. All the experiments conducted by varying pH, temperature and polymer concentration were consistent with the predictions of our proposed theory and methods for ideal reversible polymer networks.
Keywords: hydrogels, transient network, rheology

38. Patino; Zsolt Terdik, Dave Weitz, Frans Spaepen

Williams College and Harvard University
"Solid-solid Phase Transitions in Colloidal Crystals"
We are investigating the microscopic mechanisms that occur during a solid-solid phase transition in a model colloidal crystal. The colloidal crystal is an ordered packing of charged colloidal particles; the phase behavior is controlled by the packing fraction and exhibits two distinct crystalline phases (BCC and FCC) at high packing fraction. The colloidal crystal can be imaged in 3D using a confocal microscope. Moreover, the packing fraction, and hence phase behavior, can be externally controlled using an experimental technique known as an electric bottle . The three dimensional visualization combined with control over the packing fraction provides an opportunity to directly interrogate the microscopic mechanisms that occur during the solid-solid phase transition.
Keywords: Solid-solid phase transition, colloidal crystals, confocal microscopy

39. Polly; Gatien Polly, Philippe Bourrianne, Thibaut Divoux, Gareth H. McKinley

Massachusetts Institute of Technology, HML
"Shear-thickening in polymeric fluid"
Shear-thickening fluids are known as excellent energy absorbers due to their remarkable rheology. Indeed, these liquids exhibit a dramatic increase of viscosity under shear, an observation reported on a broad range of fluids but not yet fully understood. Different mechanisms have been proposed to account for this phenomenon. On the one hand, numerical studies suggests that frictional contact forces between particles might be responsible for the increase in viscosity. On the other hand, recent experiments show that inter-particle hydrogen bonding could enhance contact friction and thus that particle surface chemistry would play a key role in the shear-thickening phenomena. Here, we report experimental results on the shear-thickening response of fumed silica suspensions in polypropylene glycol. We show that varying the surface chemistry of the silica particles provides an efficient way to tune the magnitude of the shear-thickening transition.
Keywords: Rheology, shear-thickening, colloids, dense suspensions

40. Prasanta Pal; Remko V. Lutterveld, Daniel Theisen, Michael Datko, Andrea Ruf, Judson A. Brewer

University of Massachusetts Medical School, Yale University
"Material flow signature of mental flow state"
By definition "FLOW" also known colloquially as being in "THE ZONE", is the mental state of operation in which a person performing an activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. In essence, flow is characterized by complete absorption in what one does, and a resulting loss in one's sense of space and time. Is there a material truth to this long standing definition of mental state? We investigate flow signature similar to those of the material world which may correspond to the classical psychological flow description. We collected EEG data from a group of subjects performing baseline and multi-state meditation tasks trained by Harvard psychologist Dan Brown as well as subjects without any exposure to the training. We calculated the mean square displace (MSD) of the electrical potential collected from each sensor from human subjects undergoing various cognitively distinct tasks. The slope and length of the MSD plot at different time scales serve as the measure of material signature of mental FLOW. We found characteristic signatures of these states that may serve as preliminary basis for a material proof of psychological FLOW state. Such a stable measure of mental state would help accelerate the diagnostic evaluation and treatment of mental health issues.
Keywords: Brain state, EEG, Medical imaging, Flow

41. Pucci; Ian Ho, Daniel M. Harris

Brown University, School of Engineering, 184 Hope St., Providence (RI), 02912
"Drag on super-hydrophobic sliders"
In our daily experience, an object tends to sink into water if its density is larger than the water density. However, when the surface of the object is hydrophobic it can be made to float on the water surface due to the effect of surface tension. We prepared floating disks of different geometries and coated them with a commercially-available super-hydrophobic spray coating. We then made them slide across the water surface, guided by an externally-actuated magnetic field. Our custom setup allowed us to characterize the friction of these super-hydrophobic sliders as a function of their shape and mass. We find that the observed friction it is predominantly due to the viscous stress in the boundary layer beneath the sliders and suggest designs of sliders that substantially reduce the friction.
Keywords: fluid interface, skin friction, super-hydrophobicity

42. Rendos; Nourin Alsharif, Brian L. Kim, and Keith A. Brown

Boston University
"Elasticity and Failure of Liquid Marbles: Influence of Particle Coating and Marble Volume"
When coated with microscale hydrophobic particles, macroscopic liquid droplets can become non-wetting liquid marbles that exhibit many fascinating solid-like properties. Specifically, the force required to uniaxially compress liquid marbles depends on their volume, but it is unclear if the particle coating plays a role. In contrast, failure of marbles upon compression does depend on the particle coating, but the conditions for failure do not appear to change with marble volume. Here, we experimentally study the elastic deformation and failure of liquid marbles and, by applying a doubly truncated oblate spheroid model to quantify their surface area, explore the role of marble volume and particle composition. First, we find that the work required to compress liquid marbles agrees with the product of the fluid surface tension and the change in surface area, validating that the elastic mechanics of marbles is independent of the particle coating. Next, we study marble failure by measuring their ductility as quantified by the maximum fractional increase in marble surface area prior to rupture. Not only does marble ductility depend on the particle coating, it also depends on the volume with smaller marbles being more ductile. This size effect is attributed to an interaction between marble curvature and particle rafts held together by interparticle forces. Building on these results, we seek to better understand the role of interparticle interactions on marble ductility by conducting microscopic and macroscopic experiments using particles with tunable shape and interactions. In particular, macroscopic experiments were performed on particle-laden interfaces in which the interaction forces of the particles can be tuned from weakly to strongly interactive. These experiments elucidate the analogy between ductility and the macroscopic behavior of particle-laden interfaces. Tailoring the strength of these interactions is essential for realizing controlled rupture of liquid marbles for applications ranging from smart fluid handling to pollution mitigation.
Keywords: liquid marbles, failure, fracture, hierarchical materials

43. Ruiz; Zsolt Terdik, Frans Spaepen, David Weitz

Harvard University
"Visualizing the Structure and Dynamics of Dense Sediment Flows"
Turbidity currents can be described as sediment flows driven by gravity and the turbulent energy of the fluid. Typically, the sediment load is of a heterogeneous composition, with grain sizes spanning several orders of magnitude-- from microns to meters. Due to the complex, multi-scale interactions of the suspended grains with both the fluid and erodible bed, there are many unanswered questions regarding the basic physical mechanisms that give rise to erosion, sedimentation, and auto-suspension of the grains. Our aim is to develop experimental tools to visualize the three-dimensional micro-structure of dense sediment flows and directly measure the microscopic forces that arise from complex particle and fluid interactions during sediment transport.
Keywords: fluid mechanics, sedimentation, rheology

44. Saintyves; L. Mahadevan, I. Bischofberger

"Drying patterns in a highly deformable confined hydrogel"
In many constraint systems, from paintings to muddy soils, the evaporation of a solvent leads to the formation of complex drying patterns. Commonly these patterns are characterized by brittle cracks. Here, we report very different structures that emerge during the drying of thin films of hyperelastic hydrogels confined between two glass plates. The evaporation front advances by intermittent bursts, leading either to disordered or worm-like structures.
Keywords: Patterns, Drying, Hydrogel

45. Shayegan; Yinan Shen, David Weitz

Harvard University
"Active multi-point microrheology of biopolymer networks "
Rheology is the field that can describe viscoelastic behavior of a material in response to applied force or deformation. Active microrheology is a technique in which particles can be manipulated by an external force, in contrast to the passive one, in which thermal fluctuations of particles are recorded. One experimental approach to active microrheology uses optical tweezers, which trap a μm-sized particle located within the material and excite it with an oscillating force. In this study, we use optical tweezers to oscillate a particle inside reconstituted biopolymer networks and by measuring the response of multiple neighboring particles in response to the excitation of a reference particle, we are able to determine the oscillatory force propagation inside network, which is a direct measure of the viscoelastic response of the material.
Keywords: Microrheology, Biopolymer, Optical tweezers

46. Simonetti; Seth Fraden, Mike Norton, Ian Hunter, Bolun Chen

Brandeis University
"Understanding the dynamical effects of symmetries on nonlinear oscillatory networks"
Many complex systems such as neural tissue can be modeled mathematically as discrete oscillator networks, whose connectivity is described by a graph. Nodes of these graphs correspond to the components of the network, such as neurons, while the edges correspond to the interactions among these components. These interactions are nonlinear in nature, so although the dynamics are deterministic, the existence of multiple, stable dynamical steady states make predicting the long term behavior of these systems non-trivial. Our goal is to predict general features of dynamics by understanding how symmetries of the network (permutations of the network that maintain the connections among nodes) constrain dynamics. We also explore how steady states change when symmetry is purposefully broken.
Keywords: Oscillatory Networks, Nonlinear Dynamics

47. Slomka; Piotr Suwara, Jörn Dunkel

"The nature of triad interactions in active turbulence"
Generalised Navier–Stokes (GNS) equations describing three-dimensional active fluids with flow-dependent narrow spectral forcing have been shown to possess numerical solutions that can sustain significant energy transfer to larger scales by realizing chiral Beltrami-type chaotic flows. To rationalize these findings, we study here the triad truncations of polynomial and Gaussian GNS models focusing on modes lying in the energy injection range. Identifying a previously unknown cubic invariant for the triads, we show that their asymptotic dynamics reduces to that of a forced rigid body coupled to a particle moving in a magnetic field. This analogy allows us to classify triadic interactions by their asymptotic stability: unstable triads correspond to rigid-body forcing along the largest and smallest principal axes, whereas stable triads arise from forcing along the middle axis. Analysis of the polynomial GNS model reveals that unstable triads induce exponential growth of energy and helicity, whereas stable triads develop a limit cycle of bounded energy and helicity. This suggests that the unstable triads dominate the initial relaxation stage of the full hydrodynamic equations, whereas the stable triads determine the statistically stationary state. To test whether this hypothesis extends beyond polynomial dispersion relations, we introduce and investigate an alternative Gaussian active turbulence model. Similar to the polynomial case, the steady-state chaotic flows in the Gaussian model spontaneously accumulate non-zero mean helicity while exhibiting Beltrami statistics and upward energy transport. Our results suggest that self-sustained Beltrami-type flows and an inverse energy cascade may arise generically in the presence of flow-dependent narrow spectral forcing.
Keywords: biological fluid dynamics, complex fluids, turbulence theory

48. Srinivasan; C. Nadir Kaplan, L. Mahadevan

Harvard University
"Shape, form and dynamics of bacterial swarms and biofilms"
Collective microbial swarming and biofilm colonization on hydrated surfaces are important in many clinical settings, such as in the contamination of implants, catheters and during chronic infections. What are the physical constraints that set the limits to the structure and dynamics in expanding microbial colonies? To address this question, we develop a single unified multiphase framework that couples geometry, fluid flow, mechanical stresses, nutrient and osmolyte transport with localized cell growth and biomass production. Our model describes steady-state swarm expansion as fluid-mediated, and governed by osmolyte production and fluid exudation from the substrate. In contrast, transient biofilm colonization is nutrient-transport mediated, and governed by localized zones of exopolysaccharide production that drives expansion. Our unified framework allows us to explain a range of recent experimental observations associated with the shape, form and dynamics of Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa swarms and biofilms in terms of underlying mechanical and physical forces. In this manner, we demonstrate how hydrodynamics and transport serve as key physical constraints in regulating biological organization and function in microbial communities.
Keywords: bacterial swarms, bacterial biofilms, thin-film dynamics, capillarity, osmotic flows, front propagation

49. Stolovicki; Elad Stolovicki, Lloyd Ung, Roy Ziblat and David A. Weitz

Harvard John A. Paulson School of Engineering and Applied Sciences
"Drop chemostats: What is it good for?"
The production of chemicals using cells or enzymes, is increasingly employed as it results in higher, overall greener chemical processes. Bio-production is also ideally suited in cases where the selectivity of enzymes for a specific molecule enantiomer (chirality) is critical. By using an emulsion of small drops of growth medium in oil as a micro-reactor, we can optimize the production yield of a desired bio-product, preforming toxicity tests or response-resistance assays. Another key feature of our system is that each, individual droplet fully emulates a larger-scale bioreactor that can be grown using batch, fed-batch or continuous culture methods. Thus, drop micro-reactors have the advantages of reducing R&D production time and cost by having thousands of parallel experiments, greatly reduced quantities of reagents, and compact space requirements.
Keywords: white biotechnology bio-production, drop micro-reactors, microfluidics, green chemistry

50. Su; Qing Zhang, Irmgard Bischofberger, Rama Bansil

Boston University, Harvard University, MIT
"Viscous Fingering in Porcine Gastric Mucin Solution"
The mammalian gastric mucosa is regularly exposed to highly acidic environment due to the hydrochloric acid (HCl) secreted by the gastric glands. There is a pH gradient from neutral at cell surface to acidic (4 or lower) at the mucosal surface. Purified mucin glycoprotein which is responsible for the viscoelasticity of the mucus layer undergoes a sol-gel transition at pH4, above which it is solution-like and below pH 4, the mucin gels. There are several schools of thoughts on the interplay of this pH-dependency and the mechanism that protects the gastric epithelium surface from being digested by hydrochloric acid. One of the major issues is on the acid transport across the gastric mucus layer, from the gastric glands to the gastric lumen, without acidifying the mucus as a whole. Previous studies have reported macroscopic viscous fingering of acid in purified mucin solutions as well as observed acid channels forming from crypt openings in rat stomachs in vitro. Here, we demonstrate the viscous fingering of HCl through porcine gastric mucin (PGM) using a microfluidic device and examine the microrheology of the mucin in the vicinity of the acid channels by measuring the Brownian motion of micro-particles. We found that HCl creates an acid finger when injected with proper pressure into neutral PGM, which leads to sol-gel transition immediately and a gelled PGM channel forms around the finger. While the PGM have gelled almost everywhere in the microfluidics device after a few minutes of acid flow, the mean square displacements of the particles suggest more gelation around the finger and less so farther away. The diffusion mechanism also became sub-diffusive around the finger, and maintained normal diffusion away from the acid.

51. Tagliaro; Irene Tagliaro

University of Milano-Bicocca, Harvard University
"Colloidal approach to prepare new elastomeric nanocomposites "
Melt mixing is the main used compounding technique to produce elastomeric nanocomposites because of its suitability for industrial application. Regarding the dispersion of hydrophilic particles, having low affinity for hydrophobic matrices, other compounding methods based on colloidal systems have been proposed as alternative and promising routes (i.e. latex compounding and in situ polymerization). In this context, this research project aims at studying the structural-mechanical relationship of elastomeric nanocomposites compounded through innovative aqueous-based and environmental friendly methods with applications in the field of tyres.
Keywords: elastomeric nanocomposite, tyre

52. Terdik; David Weitz, Frans Spaepen

Harvard University
"Traction Force Rheology of Colloidal Solids"
We present a new technique, traction force rheology, to directly measure the mechanical response of colloidal solids (crystals and glasses) in response to imposed shear strain, while observing the dynamics of individual colloidal particles. The technique consists of forming a composite bilayer consisting of a colloidal solid on top of a soft, polymer gel with embedded tracer particles. A precisely controlled shear strain is applied to the bilayer leading to controlled deformation of the colloidal crystal/glass. In addition to directly observing rearrangements and defects that occur within the colloidal crystal/glass during plastic deformation, we also measure the deformation of the tracer particles embedded in the polymer gel. Given the observed deformation of the polymer gel and the measured mechanical modulus, the traction forces exerted on the polymer gel by the colloidal solid can be inferred using traction force microscopy. Experimental details, challenges, and current results will be discussed.
Keywords: Viscoelasticity, Colloids, Stress-Strain relations, glass, polycrystalline

53. van Rees; L. Mahadevan (Harvard)

"Simulation and design of thin shape-shifting structures"
Shape-shifting structures consist of materials that undergo local expansion or compression when subjected to an environmental stimulus, such as heat, humidity, light, or a magnetic field. Such materials are typically patterned into non-uniform thin planar sheets using additive manufacturing methods. After application of the stimulus, metric frustration inside the sheet leads to out-of-plane buckling. These soft structures can therefore undergo complex shape changes, with potential applications ranging from drug delivery to actuation and robotics. To exploit the rapidly evolving experimental capabilities in this area, it is important to be able to predict and analyze the shape arising from a given initial design. Related to this is the challenge posed by the inverse problem: can we predict the growth factors and directions that will transform a given structure from its initial shape into a desired, non-trivial target shape? We give a high-level overview of our work on these topics. Using numerical simulations of the non-linear elasticity equations, we show that we can predict the shape change achieved by experimental samples. Further, by deriving a theoretical equivalence between monolayers and bilayers we can trivially solve the inverse-design problem for the subclass of anisotropically growing bilayers. Lastly, we demonstrate experimentally how lattice-based structures can achieve shape changes with not only changes in mean curvature, but also Gaussian curvature.
Keywords: 4d printing, elasticity, simulation, inverse-design

54. Wang; Rui Wang, Jian Wu and David A. Weitz

Zhejiang University Harvard University
"Counting DNA molecules with visual sigments based readout in minutes "
An ultrafast and extremely simple approach was proposed to count the number of DNA molecules via nano-segments based readouts in minutes, without any pre-modification or physical separation process. This method was generally based on ultrafast nucleic acid amplification, extremely weak convection and diffusion effect of amplicon molecules in capillary. In this paper, a home-made device was built to shuttle the flexible plastic tube between two water baths for DNA amplification within 5 min. After amplification, every DNA molecule was combined with the pre-added fluorescent dye and generated a fluorescent amplicon cluster with the size of around 20 nL, forming visible discrete nano-segment in the thin reaction tube. By directly counting the number of amplicon cluster nano-segments, the absolute amount of DNA molecules could be known easily. Furthermore, the diffusion process of amplicon cluster in thin plastic tube was tracked. A diffusion and amplification model was proposed to explain the size of amplicon nano-segments. Therefore, without any microfluidic-based operation or micro-fabricated chips, DNA counting at the level of single DNA molecule could be successfully realized.
Keywords: visual sigments, rapid PCR, DNA molecules counting

55. Werner; Hyomin Lee, Douglas V. Amato, Brendan T. Deveney, David Weitz

SEAS, Harvard University
"Thiol-ene polymerization in double emulsion drops"
Microcapsules are widely employed in the fields of agriculture, detergents, drug delivery, and cosmetics. The size, shell thickness, and composition of microcapsules is precisely controlled using double emulsion drop templating in microfluidic drop makers. Traditionally, the liquid shells of double emulsion drops are converted to polymeric shells by solidification of a dissolved polymer during solvent evaporation or by photopolymerization of a monomeric shell phase. Photopolymerization of monomer shells enables fast conversion of complex emulsion drops to polymeric microcapsules. (Meth)acrylates have been widely employed as monomers for microcapsules, but they suffer from inhibition of the polymerization by oxygen, polymerization induced stress development, incomplete reaction during curing, and most importantly, the formation of highly heterogeneous polymer networks. These drawbacks limit the applicability of complex emulsions as microcapsule templates, and the tunability as well as the functionality of the microcapsule shell. A promising alternative polymerization to achieve capsules with homogeneous and highly tunable properties is the use of thiol-ene chemistry, a robust, efficient, and simple radical-based reaction that offers flexibility from a large number of commercially available monomers. We employ thiol-ene photopolymerization in microfluidic emulsion drop templating to fabricate various functional microcapsules with tunable properties such as low permeability, degradability, and stimuli-responsiveness. Furthermore, we highlight the rapid cure kinetics afforded by thiol-ene chemistry in a continuous photopatterning device for hemispherical microparticle production.
Keywords: Microcapsules, Microfluidics, Complex emulsion drops

56. Xie; Chen Hao, Timothy J. Atherton, Patrick T. Spicer

Tufts University
"Restructuring of ellipsoidal viscoelastic droplets"
When two ellipsoidal droplets contact, the liquid meniscus instigate movement to minimize the surface energy, causing the droplets to undergo rolling and restructuring, due to the asymmetry. We investigate the behavior of this restructuring from both experiment and simulation. In the experiment, the ellipsoidal droplets are generated by millifluidic device with wax forming microstructure inside to preserve non-spherical shape. The final configuration of doublet is shown to be controlled by the balance between surface tension and internal elasticity. In simulation, we map the surface energy as a function of their relative orientation through Surface Evolver. By comparing experimental paths of restructuring with the simulation map, we understand how these forces dominate this phenomenon.
Keywords: ellipsoid, coalescence, restructuring

57. Xu; Qingrong Xiong, Roger Bonnecaze Matthew Balhoff

MIT; UT-Austin; University of Manchester
"Is Ostwald Ripening Important in CO2 Geological Sequestration? Pore-Scale Experiments, Theories, and Modellings of Bubble Repining Dynamics"
Long-term storage is necessary in order for CO2 geological sequestration in to be feasible. Leakage of CO2 by buoyant forces may occur if the trapping of CO2 in porous media is not stable with formation of large CO2 clusters. Ostwald ripening is a well-known phenomenon in two-phase mixtures that may affect bubbles’ stability. During an Ostwald ripening process in an open system, the gas in small bubbles dissolves in the surrounding fluid and diffuses to larger bubbles to grow them. Thus, there is concern about whether a coarsening of CO2 bubbles can occur and lead to such leakage after injection into porous media. Here we show that Ostwald ripening will not lead to considerable coarsening of CO2 bubbles trapped in porous media. The size evolution of bubbles in a micron-scale porous medium has been shown very different from that in an open system. Unlike the coarsening typically observed in open systems, an initially polydisperse population of bubbles will ultimately become monodisperse and there is egalitarianism in bubble size for sufficient confinement in a homogenous porous medium, with gas from the larger bubbles diffusing to smaller bubbles. Experiments conducted on a 2.5-D micromodel validate the ripening dynamics models, with the bubble population evolution dynamics well quantified. Our results show that this anti-coarsening effect is driven by the capillary pressure difference, and directed by the micron-scale geometric confinement. A physical model for the evolution dynamics on bubble population is derived from first principles with no empirical parameters, and this model matches our experimental data very well. Based on our experiments and models, we conclude that, Ostwald ripening can be a positive effect, rather than a negative effect, to improve bubble stability. If bubbles are initially dispersed into small size (similar to or smaller than pore size) during injection, no significant coarsening will happen that lead to gas leakage. In addition, understanding this anti-coarsening effect of bubbles/droplets in porous media is also of great significance for better description and operations in many other applications which involve time scales similar to or longer than several hours, such as the formation and concentration of oil and gas in reservoirs (millions of years), transport of NAPLs in soil (days or even years), foam-based enhanced oil recovery (months to years).
Keywords: Ostwald Ripening; Porous Media; CO2 Geological Sequestration

58. Zhang; Weixia Zhang, Liangliang Qu, David Weitz

John A. Paulson School of Engineering and Applied Sciences, Harvard University
"Thin shell microcapsules for encapsulation and release of biologics"
We report a single-step microfluidic approach to fabricate thin shell microcapsules for encapsulating various biologics. Using a glass capillary microfluidic device, monodisperse water-in-oil-in-water double emulsion droplets are fabricated with biologics in the core and a thin middle oil layer containing polymeric monomer that can be polymerized upon UV exposure to get a thin solid shell. The thin shell is tested to be impermeable to large biological molecules. By tuning the mechanical properties of the thin shell during the fabrication process, we demonstrate the encapsulated biologics within these thin shell microcapsules can be easily released with a small mechanical stress.
Keywords: Thin shell microcapsule, Encapsulation, Release, Biologics

59. Zhang; Angelo S. Mao, David J. Mooney Huanan Wang, David A. Weitz

Harvard University
"Microfluidic Templated Multicompartment Microgels for 3D Encapsulation and Pairing of Single Cells"
Controlled encapsulation and pairing of single cells within a confined 3D matrix can enable the replication of the highly ordered cellular structure of human tissues. Microgels with independently controlled compartments that can encapsulate cells within separately confined hydrogel matrices would provide precise control over the route of pairing single cells. Here, we present a one‐step microfluidic method to generate monodisperse multicompartment microgels that can be used as a 3D matrix to pair single cells in a highly biocompatible manner. This method allows microgels formation, microgel extraction from oil phase into one-step. We further demonstrated that by entrapping stem cells with niche cells within separate but adjacent compartments of the microgels, which can create complex stem cell niche microenvironments in a controlled manner and serve as a useful tool for the study of cell-cell interactions.
Keywords: Microfluidics, Stem cell, Alginate, Cell-Cell interaction

60. Zhang; Shuang Zhou, Irmgard Bischofberger

MIT, Harvard
"Pressure-induced restructuring of chromonic liquid crystal"
The application of pressure to a well-aligned liquid crystal in a quasi two-dimensional geometry leads to restructuring of the material. We investigate this phenomenon by injecting air at constant pressure into Hele-Shaw Cell which is filled with a circularly planar aligned liquid crystal. Remarkably, the disturbed region is very long-ranged; the liquid crystal loses its alignment far from the pressurized interface. We discuss how this long-range disturbance is related to a force balance between the applied pressure and the elastic energy of the liquid crystal due to the alignment.
Keywords: liquid crystal; long-range disturbance; Hele-Shaw Cell

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