Abstracts for Invited Talks and Soundbites

83rd New England Complex Fluids Meeting
UMass Amherst Online | Friday, June 5, 2020
Registration deadline: Wednesday, June 3, 2020

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Invited Talks

1. Kandula, Manasa

UMass Amherst

Understanding Glass Transition through Colloid Experiments
Understanding glass transition—transformation of a flowing liquid into a rigid solid on cooling without a perceptible change in structure, continues to pose a challenge. Decades of research saw huge debates whether glass transition is dynamic or thermodynamic in nature. In this talk, I will discuss the contribution from colloidal experiments towards understanding this puzzle. I will show how combination of direct visualization and ability to freeze particle configurations in real time in colloidal glass forming liquids can help test various theories of glass transition. Our experiments reveal growing amorphous order in the form of point-to-set correlation length with increasing supercooling. Interestingly, we find a non-monotonic density dependence of dynamic correlations. Further, we demonstrate that the non-monotonicity is accompanied by a change in morphology and internal structure of cooperatively rearranging regions—localized zones which govern relaxations in glass forming liquids. These experimental simulations are enabled by the selected system, in which each element of the supercooled liquid and eventual laboratory glass is imaged directly in real space.
Keywords: 

2. Kumar, Sanat

Columbia University

Using Crystallization to Control Filler Dispersion in Polymer Nanocomposites
To achieve the full promise of nanoparticle filled polymers, control over both filler organization and polymer morphology is required. Recent work demonstrated that there is a critical crystallization rate below which, nanoparticles will align in the interlamellar regions of semicrystalline polymers. This project explores the fundamental principles behind tailoring the organization of fillers by varying filler/matrix compatibility, filler diffusivity, and matrix molecular weight. Experimentally, we successfully align layers of nanoparticles in the interlamellar spaces in poly(ethylene oxide) (PEO) through isothermal crystallization. We have explored the impact of graft density, graft molecular weight, and crystallization rate on this response. To achieve this same alignment in a broadly used engineering polymer, we developed the first method for grafting olefins to nanoparticles with control over both graft density and molecular weight. We have found that the degree of filler-matrix compatibility leads to a competition between filler organization and filler agglomeration leading to interlamellar alignment at low molecular weight, and larger scale alignment in higher molecular weight systems. While the nanoparticles are ordered by the crystallization process, they are still templated by the spherutlitic nature of the semicrystalline morphology. We go beyond beyond this limitation, and created one-dimensionally ordered NP structures, by using zone annealing to create elongated spherulites along the flow direction.
Keywords: 

3. Yan, Jing

Yale University

Mechanical principles of biofilm formation revealed by single-cell resolution imaging
Biofilms are surface-associated bacterial communities embedded in an extracellular matrix. Biofilm cells are more resistant to antibiotics than their planktonic counterparts, which is a major problem in the context of chronic infections. Also, bacterial biofilms clog networks and filters in industrial settings. We still lack a fundamental biophysical understanding of how bacteria, in time and space, build these three-dimensional structures. In this talk, I will present a technique to image living, growing bacterial biofilms from single founder cells to ten thousand cells at single-cell resolution. Using the human pathogen Vibrio cholerae as a model biofilm former, we discovered that biofilms growing on flat substrates develop from a disordered, two-dimensional layer into a three-dimensional cluster with a nematically aligned core. Moving to more medically relevant environments, we began to investigate the mechanical feedback between biofilm growth and environmental deformation when they grow under mechanical confinement, at both the continuum level and at the single-cell level.
Keywords: 


Sound Bites

1. Aime, Stefano; M. Sabato, D. Weitz

Harvard University

Crack interaction in colloidal gels
We study the deformation field around the tip of a fracture that propagates in a colloidal gel by combining confocal backscattering and dynamic speckle holography, two novel optical techniques allowing the reconstruction of complex microscopic displacements in 3D. We find that the deformation field of an isolated crack is determined by the viscoelastic properties of the gels, and is distorted in presence of material defects, interfaces or other cracks: this distortion encodes the interaction between a crack and its heterogeneous environment, causing the crack to bend and accelerate in a way that can be rationalized by an effective interaction potential.
Keywords: fracture mechanics, rheology, light scattering

2. Alventosa, Luke

Brown University

Capillary Rebound: Droplets Bouncing on a Fluid Bath
The bouncing of drops on a gas-liquid interface is studied both experimentally and theoretically. Millimetric water drops are generated using a custom 3D-printed drop-on-demand (DOD) generator and are impacted on a deep bath of the same fluid. Drop trajectories, restitution coefficients, and contact times are reported. A weakly inviscid model based on an eigenfunction decomposition of the gas-liquid interface in a confined geometry is presented to compute the time dependent waves produced by the impact. The drop is modeled as an underdamped harmonic oscillator and its dynamics are directly coupled to the response of the interface. The model shows satisfactory agreement to the experimental results, and applications of this model to different capillary scale impact problems will be discussed.
Keywords: Impacts, Capillary Scale, Bouncing

3. Alventosa, Luke; Daniel Harris

Capillary Rebound: Droplets Bouncing on a Fluid Bath
The bouncing of drops on a gas-liquid interface is studied both experimentally and theoretically. Millimetric water drops are generated using a custom 3D-printed drop-on-demand (DOD) generator and are impacted on a deep bath of the same fluid. Drop trajectories, restitution coefficients, and contact times are reported. A weakly inviscid model based on an eigenfunction decomposition of the gas-liquid interface in a confined geometry is presented to compute the time dependent waves produced by the impact. The drop is modeled as an underdamped harmonic oscillator and its dynamics are directly coupled to the response of the interface. The model shows satisfactory agreement to the experimental results, and applications of this model to different capillary scale impact problems will be discussed.
Keywords: Impacts, Capillary Scale, Bouncing

4. Atzberger, Paul; Paul J. Atzberger

University of California Santa Barbara

Surface Fluctuating Hydrodynamics Methods for Fluid-Structure Interactions within Curved Fluid Interfaces
We introduce surface fluctuating hydrodynamics approaches for investigating fluid-structure interactions and transport within curved fluid interfaces. We formulate a mesoscale description of the surface mechanics taking into account the membrane geometry, discrete particle interactions, hydrodynamics, and thermal fluctuations. We also show how practical computational methods for simulations can be developed using spectral methods and meshfree approaches. We then show how the methods can be used to investigate the roles played by geometry in hydrodynamic transport and in the collective drift-diffusion dynamics of proteins within membranes, microscopic swimmers, and related systems.
Keywords: fluid-structure interactions, immersed boundary methods, fluctuating hydrodynamics

5. Bhaskar, Dhananjay; William Zhang, Ian Y. Wong

Brown University

Using Topological Data Analysis to Quantify Pattern Formation in Active Matter
I will outline a novel methodology to investigate phase transitions and collective behavior in active matter systems. The proposed method (arXiv: 2003.10008), based on persistence homology, can identify distinct patterns that emerge in simulations of soft tissues without any knowledge of the underlying biophysical model. It offers many advantages compared to a conventional approach requiring computation of order parameters, such as the availability of well-defined metrics for making comparisons, stable (w.r.t. noise) representation and robustness to changes in population size (number of agents).
Keywords: pattern formation, active matter, soft tissue simulation, topological data analysis

6. Clark, Andrew; Yajnaseni Biswas, Morgan E. Taylor, Ayse Asatekin, Matthew J. Panzer, Christoph Schick, Peggy Cebe

Department of Physics and Astronomy, Tufts Univeristy

Effects of free volume and backbone rigidity on thermal transitions of polyzwitterions
Polyzwitterions are polymers with side groups that contain a covalently linked anion and cation. Depending on the specific chemical structure, this can lead to high glass transition temperatures, occurring near the onset of degradation. In this study we are using fast scanning calorimetry to investigate how the free volume of the side-group and chain rigidity alter the glass formation of polyzwitterions. Poly(sulfobetaine methacrylate) (PSBMA), poly(sulfobetaine acrylate) (PSBA) and poly(ethyl sulfobetaine methacrylate) (PESBMA) were synthesized and cast into films. All polymers contain identical sulfonate anions allowing for a good comparison of the variation of side group volume and chain rigidity. Using fast scanning calorimetry, degradation was avoided by heating and cooling at 2000 K/s which allowed the first measurement of the glass transition process in these materials. We found large variation in Tg with PSBMA having the highest Tg and PESBMA having the lowest Tg. Analysis of the fragility reveals that the strength of glass formation goes from PESBMA as the strongest to PSBMA as the weakest.
Keywords: polymer, polyzwitterion, glass transition, dynamic fragility, fast scanning calorimetry

7. Cochard, Thomas; Y. Song, L. Xiao, D. A. Weitz

HARVARD UNIVERSITY

Hydraulic fracture dynamics
Hydraulic fracking of shale reservoirs is a process in which a highly pressurized fluid is injected into a rock formation, inducing fractures that create a connected path between the pores filled with hydrocarbons. Understanding the dynamics of those fractures is challenging as they grow quickly in opaque high strength materials and because they are difficult to predict. In the lab, the high strength material is thus made transparent by using stereo-lithography 3D printing (3GPa). The high-speed visualization is made possible by combining high speed imaging (75kHz) and piezoelectric transducers (>50 MHz). As a result, we made the first experimental observation of multiple dynamics during the fracture propagation due to different levels of energy dissipation.
Keywords: Hydraulic fracture, high speed imaging, 3D printing, Seismic wave propagation

8. Dimitriyev, Michael; Krishma Singal, Elisabetta Matsumoto

Georgia Institiute of Technology (now UMass Amherst)

From entangled elastica to emergent elasticity of knitted fabric
Knitted fabric is a soft elastic material formed from passing loops of yarn though each other in a two-dimensional arrangement, resulting in an extremely stretchy and flexible textile. Through slight modifications to the pattern of yarn loops, one can design a wide range of fabric elasticities and geometries. In this soundbite, I will briefly share the results of our computational model of knitted stitches and their comparisons to experiment, as well as how our results have informed the formulation of a set of nonlinear constitutive relations between stress and strain. Our results provide a stepping-off point for investigating the full 3D elasticity and geometry of knitted fabric.
Keywords: elasticity, knitting, fiber arts, geometry

9. Durey, Guillaume; Wan Luo, Guillaume Duclos, Bob Pelcovits, Tom Powers & Kenny Breuer

Brown University

Measuring the Stokes’ drag in a microtubule-kinesin active gel
The presence of activity invalidates many of the traditional assumptions of hydrodynamics. So far, many studies have been limited to characterizing and simulating activity-driven flows: few have measured forces and torques exerted by an active fluid on an external body. Yet, quantitative measurements of active stresses are required to achieve a complete description of active fluids and conceive practical applications of these new materials. This is what we tackled in our work, through the experimental measurement of the drag force on a sphere sedimenting through a microtubule-kinesin active gel. The sphere sediments under the combined effects of large-scale spontaneous coherent flows, mesoscopic turbulence and gravity. Its motion can be described by a nonlinear Stokes drag characterized by an effective viscosity and an effective diffusivity. Using a custom-built bright-field microscope, we demonstrate the ability to track active sedimentation with micron resolution, even for large bead diameters up to 100 µm, in three-dimensional microfluidic chambers, through continuous scanning of a moving experimental volume with a piezo-driven objective. This opens the door for developing the form of the Stokes drag for active media in experiments to be conducted in the near future.
Keywords: active matter; active stresses; Stokes' drag; 3D tracking;

10. Fan, Jinpeng; Cesar Castro, Jack F. Douglas, Francis W. Starr

Wesleyan University

Structure and Dynamics of Polymer/Star-Polymer Composites
Nanoparticles (NP) are versatile building blocks to create tunable particle superstructures and polymer nanocomposites with customizable properties. Star polymers with a large numbers of arms exhibit soft-colloidal behavior, making them interesting analogues of NP. While polymer composites with NP have been explored in detail, comparatively little is known about composites consisting of star polymers and chain polymers. We investigate how both the number of arms and molecular mass affect the structural and dynamical properties of polymer matrices and entire composites. In particular, we study how star polymers affect the glass transition of these composites and compare our findings with those of polymer-NP composites. In doing so, we also examine how the polymer/star-polymer interaction strength affects the structure and dynamics in the matrix, and how this potentially relates to changes in the mechanical and rheological properties of the material.
Keywords: polymer composite, star polymer, nanoparticle

11. Green, Yoav

Ben-Gurion University of the Negev

Approximate time-dependent current-voltage relations for currents exceeding the diffusion limit
For four decades it has been known that the steady-state ionic current transported into a permselective medium can exceed the theoretically predicted limiting currents. It has been shown that these above-limiting currents can be attributed to the formation of a non-equilibrium space-charge layer adjacent to the membrane. In this short talk, I will present the time-dependent behavior of one-dimensional ion transport into a permselective medium. I will show that the potential drop correlates to the time evolution of this space-charge layer.
Keywords: Ion transport, Electrokinetics, Nanofluidics

12. Hoffmann, Gwendolyn; Matthew M. Jacobsen, Matthew D. Layne, Michael L. Smith, Joyce Y. Wong

Boston University

Mechanically tunable extracellular matrix fibers
Most naturally occurring extracellular matrix fibers are difficult or impossible to extract for scientific studies or as natural materials. For development of materials based on natural fiber properties, our lab developed a powerful, yet simple, wet spinning technique to generate proteinaceous fibers. We found fibers can be generated from diverse proteins including fibronectin, laminin, collagen, silk fibroin, and albumin: moreover, the relationship between molecular weight and concentration required to form a continuous fiber follows a power law, which enables us to rapidly form fibers with specific compositions. To determine how variation of composition impacts fiber mechanical properties, we used wet spinning to generate collagen fibers with and without aortic carboxypeptidase-like protein (ACLP), a collagen-binding protein linked to Ehlers-Danlos Syndrome. The addition of ACLP to collagen fibers improves their mechanical properties (modulus, strength, toughness), demonstrating proteins such as ACLP can be used to modulate mechanical properties of wet-spun fibers. Therefore, our method of producing composite extracellular matrix fibers can be used to generate bioactive materials with tunable properties.
Keywords: Extracellular matrix, collagen, mechanics

13. Huang, Tina; Arash ManafiRad, Laura Arriaga, Anqi Chen, Anthony Dinsmore, David A.Weitz

Harvard University

Microfluidic Fabrication of Asymmetric Polymer Lipid Vesicle
Lipid vesicles are aqueous volumes surrounded by a bilayer of lipid molecules, which are amphiphilic molecules with their head groups facing water and tail groups facing oil. These vesicles are simple models that mimic cell membranes and can be used for drug delivery. Similarly, block copolymers are amphiphilic molecules that form vesicles by themselves or with lipids. Like lipid vesicles, polymer vesicles can also be used for drug delivery and cell membrane mimicry. One interesting type of lipid/polymer vesicle is the asymmetric vesicle, in which its bilayer is composed of two dissimilar lipid monolayers or a lipid monolayer and a polymer monolayer. Importantly, all eukaryotic cell membranes exhibit this type of asymmetry and asymmetry is also proposed to enhance mechanical properties of the membrane. Here, we use microfluidics to fabricate mono disperse and highly controllable asymmetric vesicles, which unlike the conventional methods that often end up with highly poly disperse samples. To achieve this, asymmetric vesicles are produced using water/oil1/oil2/water emulsions in a glass capillary device, with different lipids/polymers immersed in two different volatile oil phases. Using the asymmetric vesicles, we are trying to measure how mechanical properties are affected by this asymmetry and also how to improve the degree of asymmetry in our vesicles even more. In future, we envision asymmetric lipid/polymer vesicles could open a new door in the field of drug delivery.
Keywords: Microfluidic, Lipid, Polymer, Asymmetry, Vesicle

14. Hunter, Ian; Michael M. Norton, Bolun Chen, Chris Simonetti, Jonathan Touboul, and Seth Fraden

Brandeis University

Gait engineering: Studies of small synthetic neural networks
Living beings have evolved to use coupled neurons to perform autonomous functions, from breathing to walking. Local, often small, clusters of neurons known as central pattern generators (CPGs) perform these essential functions in the absence of constant guiding input from the brain. This modular method of control of whole organisms may present advantages over centralized, computational robotic control. An experimental platform, composed of diffusively-coupled PDMS microreactors each containing the oscillatory, light sensitive Belousov-Zhabotinsky (BZ) chemical reaction forming CPG-like patterns has been studied. Understanding experimental observations of quadruped gaits, and an explanation of a systematic method making synthetic CPGs with desired steady state gaits will be the focus of the presentation.
Keywords: synchrony, pattern formation, symmetry, nonlinear dynamics, network control

15. Katsikis, Georgios; Jesse Collins, John Sader, Scott Manalis

University of Melbourne, MIT

Inertial volume sensing without mass
Inertial sensors are widely known for measuring the mass of micro- and nanoscale particles but not their volume. Here, we discover a new modality of inertial sensors where fluid flow generated by the local rotation of the sensor interacts with a particle in a manner that depends on its volume but not mass. We develop a new theory which shows there is a signal proportional to the displaced fluid volume. This signal coexists with a signal proportional to the buoyant mass allowing for both volume and density to be calculated from a single measurement. We validate this theory with experiments across a wide range of Reynolds numbers expressing the ratio of inertial to viscous forces. We measure the radius and density of standard monodisperse polystyrene micro-nanoparticles with an accuracy better than 98$\%$ and then apply this measurement protocol to non-standard microparticles. Our new theory and experimental realization open new insights into the capacity of inertial sensors to characterize micro-nanoparticles.
Keywords: rotational inertia, particle volume, sensor, low Re

16. Lilin, Paul; Philippe Bourrianne, Irmgard Bischofberger

MIT

Blooming patterns in drying drops
The drying of drops of colloidal suspensions on a hydrophilic substrate leads to the formation of a close-packed solid particle deposit. The initial volume fraction of particles sets the shape and size of this deposit, from a ring at the edge of the drop to a solid film covering the initial wetted area. As the deposit forms, tensile drying stresses generate regularly spaced radial cracks. The radial cracks separate the deposit into a multitude of petals. Due to the negative water pressure inside the pores of the deposit, these petals bend upwards creating mesmerizing forms reminiscent of blooming flowers.
Keywords: colloidal suspension, drops, droplets, poroelasticity

17. Liu, Ari; Hamed Emamy, Francis W. Starr

Wesleyan University

Effect of Chain Length on the Structure and Dynamics of Polymer Composites
We use molecular dynamics simulations to study the effect of chain length on the properties of polymer-nanoparticle (NP) composites. We show that, independent of chain length, polymers slightly elongate near the NP interface and chains tend to align their longest axis with the NP interface. When we distinguish chains that bridge between NP, we find that they are significantly extended when the NP separation is large compared to chain dimensions. Dynamically, these bridging chains have a longer relaxation time than non-bridging chains, but they do not have a substantial effect on the overall relaxation of the composites. Accordingly, the variation of glass transition temperature Tg with chain length essentially mirrors that observed for pure polymer systems, where Tg increases with chain length and then saturates approaching the entanglement length.
Keywords: polymer nanocomposite, glass transition

18. Liu, Paige; Peter J. Beltramo

University of Massachusetts Amherst

Asymmetric large area model biomembranes
All biological cell membranes maintain an electric transmembrane potential of around 100 mV, due in part to an asymmetric distribution of charged phospholipids across the membrane. This asymmetry is crucial to cell health and physiological processes such as intracell signaling, receptor-mediated endocytosis, and membrane protein conformation and function as well as active processes involving flippase and floppase proteins. Experimental artificial membrane systems incorporate essential cell membrane structures in a controllable environment much simpler than living systems. It is of particular interest to study asymmetry in transverse lipid composition across the phospholipid bilayer on such a system to probe the effects of lipid composition and transverse asymmetry on the physicochemical properties of the membrane. By doing so, an understanding of how membrane asymmetry dictates membrane properties and in turn impacts cellular processes will be achieved. In this talk, I will introduce a platform was developed to fabricate and characterize compositionally controlled planar, free-standing, asymmetric membranes. This asymmetry was qualitatively demonstrated using a fluorescence quenching assay, and it has been quantified using charged lipids with a patch-clamp amplifier system. Initial measurements of a transmembrane potential on a partially asymmetric bilayer were found to be between 10 and 25 mV. Increasing membrane charge asymmetry increases the offset voltage, as expected, and also modifies the stiffness of the membrane. These initial successes demonstrate a viable pathway to fabricate and quantitatively characterize asymmetric bilayers that can be extended to accommodate more complex membrane processes in the future.
Keywords: biomembranes

19. Najma, Bibi

Brandeis University

Liquid to Gel Transitions in 3D Active Network
We are investigating the origin of hydrodynamic instabilities in 3D active networks composed of microtubules and kinesin molecular motors. The motors hydrolyze ATP and convert chemical energy into mechanical work as they step onto the microtubules. This input of energy drives the cytoskeletal network far from equilibrium. Aligned active nematic liquid crystals are unstable and when confined along the Z axis, the instability grows in the X-Y plan.

Lowering the ATP concentration induces the 3D active network to buckle in the out-of-plane direction (X-Z axis). We hypothesize that this is due to a change in the rheological properties of the network: at high ATP concentration, the active networks flow like a liquid, while at low ATP, the active network behaves as a viscoelastic gel which buckles along the shortest Z axis.
Keywords: 

20. Narayanan, Suresh; Qingteng Zhang, Eric Dufresne

Advanced Photon Source, Argonne National Laboratory

Probing dynamics in complex fluids
Advanced Photon Source at Argonne National lab is a national user facility providing intense x-rays to study a wide range of structural and dynamical properties in materials. I would present the capability with a few science cases to probe dynamics in complex fluids using x-ray photon correlation spectroscopy in the small angle scattering regime. We have in situ Rheology capability to study materials under shear.
Keywords: Dynamics, Rheology, X-ray photon correlation spectroscopy, small angle scattering

21. Niu, Wuqi; Maria Santore, Zhou Xu, Sylvia Rivera

University of Massachusetts Amherst

Reversible capturing of bacteria on a non-adhesive surface with depletion force
The interaction of bacteria with surfaces has important implications in a range of areas, including biofouling, biofilm formation, and the infection of plants and animals. Bacteria adhesion to surface by hydrodynamic and physicochemical (i.e. van der Waals and electrostatic forces) effects has been well studied. However, no study has discussed the role of depletion force in bacteria adhesion to surface, even though bacteria are often found in polymer rich environments where polymer driven depletion force is an effective attractive force. Here, we successfully captured non-motile Escherichia coli (E. coli) on polyethylene glycol (PEG) brush surfaces which are non-adhesive to E. coli on their own. The capture is reversible, and the cells could be released with the removal of depletion force. PEG with two different molecular weight were used as depletants. We also studied the influence of depletion driven aggregation on cell capturing. The depletion driven aggregation and depletion driven surface capture are two competing mechanism in this system.
Keywords: Cell-surface interaction, depletion force

22. OttinoLoffler, Bertrand; Mehran Kardar

MIT

Extinction Transitions in a Seascape Population Model
Population growth models with noise are an increasingly popular subject of study. However, it isn't always obvious how noise manifests macroscopically in such systems. Here, we present a logistic population growth model in a mean-field limit. We specifically focus on a type of ``seascape'' noise, wherein the population's fitness randomly varies is space and time. Despite the apparent simplicity the mean-field limit seems to afford, a number of unexpected transitions occur in extinction regime, revolving around the ratio of coupling to noise amplitude. We conclude by comparing the effects of fitness-parametric noise with the more conventional choice of demographic noise.
Keywords: 

23. Semwal, Shivani; Anand Yethiraj

Memorial University of Newfoundland

Dipolar depletion : study of a field tunable colloid-polymer system
In a colloid-polymer system, the addition of non-adsorbing polymer can control the magnitude as well as the range of an effective attraction between micron-sized colloidal particles.In such a system, various phases, including network- forming gels, can be observed. With switchable and tunable interactions, such systems could also be used to study phase transition kinetics. While the gas-liquid transition has been extensively studied, and transition to various gel and glass states have been observed, an external electric field would enable controlled crossing of phase boundaries that can help to understand the kinetics of the process.
Keywords: colloid, polymer, phase transition kinetics, external field

24. Sharma, Dipti; Shiv Kumar Pal2, Neeraj. Mehta2, Dipti Sharma1

BHCC/EC

Composition Dependent Calorimetric Study of SeTeSnGe Chalcogenide glasses (ChGs)
This collaborative research reports calorimetric study of SeTeSnGe Chalcogenide glasses (ChGs) varying Ge percent from 0% to 6%. Detailed results of glass transition peak of these samples are shown in this talk. Results show how percentage of Ge changes the appearance, shape, and peak position of the parent sample. Speciic heat capacity of grass transition changes significantly with the presence of Ge percent and can be explained in terms of composition’s structural change in the presence of Ge. Further research is under process to find more detailed reasons. Keywords: Glassy Alloys, Amorphous material, calorimetry, new generation material.
Keywords: Glassy Alloys, Calorimetry

25. Tjo, Hansen; Dr. Sarah L. Perry

Chemical Engineering, UMass Amherst, Amherst, MA

Unpacking the Role of Charge Density in Polyelectrolyte-Micelle Complexation
In solution, electrostatic complexation may drive oppositely-charged macro-ions to undergo an associative phase separation process termed complex coacervation. Two liquid phases form as a result: a complex coacervate phase – the dense, polyelectrolyte-rich liquid— and a dilute phase, the supernatant. The widespread applications of coacervates for biomolecular stabilization, controlled delivery, and personal care products have resulted in extensive analysis of the phase behavior and properties of polyelectrolyte-micelle systems. However, much of the work to date has focused primarily on characterizing the phase behavior of specific polyelectrolyte-micelle systems rather than using predictive design rules for a broader range of systems. The phase behavior of coacervate systems is governed by electrostatics: a 1:1 charge ratio between the macro-ions is expected for coacervation. However, polyelectrolyte-micelle coacervates typically involve mixed-micelles, consisting of one charged species and one neutral species, which introduces an additional variable Y: the micellar charge fraction. The critical micellar charge fraction Yc, where complexation is induced, is the typical means to characterize polyelectrolyte-micelle systems and has been shown in the literature to be a function of polymeric and micellar chemistries. Work by Dubin and co-workers have shown that the steric accessibility of micelle surface charges affects the observed composition for Yc, with a greater degree of steric hindrance shifting Yc to higher levels. We hypothesize that the charge densities of both the polycation and the micelle affect coacervate phase behavior, which we can test by varying both the charge density of the polymeric species and by using steric exclusion to decrease the apparent charge density of the micelles. Specifically, we utilize a combination of turbidimetry coupled with optical microscopy to characterize the phase behavior of a series of cationic random co-polymers of varying charge densities with a panel of anionic mixed-micelles with different hydrophilic head group sizes. Preliminary results support our hypothesis: for a given mixed-micelle, we saw a positive shift in Yc with decreasing polymer charge density. Similarly, for a given polymer we observed that increasing levels of steric exclusion correlated with increases in Yc; we believe these steric effects functionally decrease the micellar surface charge density. We are now applying electrophoretic light scattering to quantify the relationship between the zeta potentials ζ of each cationic co-polymer and the critical micellar charge fraction Yc of its corresponding polymer-micelle complex. Our goal is to establish design rules that take into consideration the head group chemistry of surfactant micelles and the effective charge density of each co-polymer to accelerate the design of new materials for complex coacervate-based applications
Keywords: Soft matter, charged polymers

26. Trevenen, Samuel; Samuel Trevenen, Peter J. Beltramo

University of Massachusetts Amherst

Gradient stretching for the production of variable aspect ratio colloidal ellipsoids
Developing reliable synthetic methods for producing shape-anisotropic polymer colloids is essential for their use in novel functional materials. In designing such materials from ellipsoidal particles, it is often necessary to screen a wide range of particle sizes and aspect ratios to appropriately understand how microscopic particle characteristics dictate macroscopic material response. Here we describe a technique whereby an array of monodisperse samples of colloidal ellipsoids with broad variation in aspect ratio are made in a single synthetic step from spherical polymer particles. The technique extends the traditional film-stretching approach to create ellipsoids by introducing a gradient in strain and film cooling, which results in varying degrees of particle stretching. We describe an empirical method to predict particle dimensions based solely on measurements of the film which enables consistent, selective harvesting of particles with narrow distributions in aspect ratio. The method is applied successfully to a wide range of seed particle diameters (500 nm – 10 micron) and enables the rapid synthesis of variable aspect ratio particles for systematic studies of anisotropic particles.
Keywords: colloids, ellipsoids, anisotropic particles, polymer microparticles, materials, complex fluids, self-assembly

27. Vaidya, Samiksha; Dr. Rajeswari Kasi

University of Connecticut

Studying the self-assembly and structure-property relationship of dye embedded liquid crystalline polymers within unique polymeric architectures
Stimuli-responsive dyes have received a great amount of attention due to their tunable optical properties upon application of mechanical force/pressure, heat, or UV light. Among these, mechanochromic dyes possess color-changing ability when mechanical force is applied and can be very useful for mechanical damage sensing, pressure sensors, crack detection, and for testing mechanical system failure. Allowing these dyes to interact with structural photonic polymers can create a new platform of tunable optical hybrid materials. Cholesteric liquid crystalline polymers (LCPs) are known to be thermally responsive as their optical properties can be tuned with temperature. For additional applications of cholesteric LCPs, we have synthesized mechanochromic dye incorporated cholesteric LCPs that can be useful for both mechanochromic and thermochromic applications. Molecular engineering of mechanochromic dye incorporated monomers is the key to couple mechanochromic and thermochromic properties. Tweaking the architectures of mechanochromic dye monomer within the liquid crystalline polymeric template promotes the unique synergistic and cooperative interactions between cholesteric liquid crystalline (LC) mesogen and mechanochromic dye molecules. Thus, a multi-stimuli-responsive elastomeric material showing both mechanochromic and thermochromic behavior could be a potential candidate for mirrorless lasing, actuators, thermal sensors, and photonic applications.
Keywords: mechanochromic, thermochromic, cholesteric liquid crystalline polymers

28. Wu, Jiwei; Jiwei Wu, T. Cochard, L. Xiao, D. A. Weitz

Harvard University

Experimental Investigations of Oil Transport in 2D Porous Media
The understanding of immiscible flow in porous media is a major interest for oil and gas recovery in conventional reservoir. After the secondary oil recovery (water flood), the oil network is disconnected in the pore structure. In order to understand how the single oil ganglia is being transported in such condition, we generate an artificial 2D porous media where we can trigger dominant parameters such as pore size, porosity and connectivity. Those artificial porous materials are then transferred into a PDMS chip. Thanks to our microfluidic design we can generate oil ganglia and look at its transport under certain condition: Type of fluids injected (polymer and surfactant mixture), surface properties of the PDMS device (change in wettability). Coupled with high speed imaging we can track the initial oil ganglia, look at its transport which can be achieved in several ways: continuous transport, break up into smaller ganglia but also study the coalescence of oil ganglia under certain conditions. New way of modelling the transport behavior are proposed and a framework of modelling the tertiary oil and gas recovery is suggested.
Keywords: artificial porous media; multiphasic flow; wettability; viscous flow; interfacial tension

29. Xie, Zhaoyu; Timothy J. Atherton

Tufts University

Explore a new jamming category on deformable surfaces: metric jamming
Aggregation of particles on the surfaces of liquid droplets can stabilize and produce shapes far from the equilibrium spherical state, which can serve for multiple purposes, such as encapsulation, delivery vehicles or reaction platforms. The interaction between particles prevent the deformation of surfaces due to surface tension minimizing the surface area, resulting in a jammed system. This jammed particle structure at the equilibrium state differs from the previous literature, in that it not only considers the particle interaction but also involves the surface energy of the deformable manifold the particles are embedded on. We propose a new jamming category-metric jamming to describe this structure. In this research we apply a simulation protocol to generate such metric jammed system and discover new properties by looking at the dynamical matrix.
Keywords: deformation, jamming

30. Xin, Weiyue; Hao Wu, Greg Grason, Maria Santore

UMass Amherst

Principles of Tension and Curvature-Controlled Solid Domain Interactions on Vesicle Membranes
Phase separation of phospholipids in biomolecular membranes is closely connected with signaling and trafficking in cells. This talk explores a fluid- solid coexistence system and pattern formation in phospholipid membranes, providing a new perspective on a classically studied system. The fluid- solid coexistence systems transcend biology to produce materials with potentially tunable interconnectivity and morphology. In our system, solid domains integrated into the fluid bilayer membrane on the curved contour of the vesicle, how curvature and tension in the fluid phase affect the arrangements of solid domains constitutes new physics that allows access to greater sophisticated interconnected patterns that form in response to touch.
Keywords: Domain Interaction, Membrane vesicles

31. Xu, Jiang; Alinaghi Salari, Michael C. Kolios, Scott S. H. Tsai

Harvard Medical School

Expansion-mediated breakup of bubbles and droplets in microfluidics
We report a breakup regime of bubbles and droplets that is caused by a sudden channel expansion in a microfluidic device. In this regime, bubbles or droplets generated at a flow-focusing geometry periodically breakup into smaller bubbles or droplets, respectively, upon entering an expansion. In addition to Capillary number Ca, which is previously shown to govern the dispersion breakup in such geometries, we find that, at a high-inertia regime, the Weber number We also plays a significant role in specifying the transition from non-breakup to breakup regimes. Furthermore, we identify different periodic breakup modes, for example, symmetric and asymmetric breakup, which are dictated by the Ohnesorge number. Our results demonstrate that, without modifying the geometry and by only tuning several dimensionless parameters related to the fluid flow, a microchannel expansion region can produce mono-, bi-, or tri-disperse bubble or droplet populations. These discoveries may find utility in the design of multi-disperse bubble or droplet populations using microfluidics.
Keywords: Bubbles, droplets, breakup

32. Zuraw, Sarah; Sarah Zuraw-Weston, Anthony Duprat Dinsmore, Mahsa Siavashpouri, Zvonimir Dogic, Thomas Gerling, Hendrik Dietz

University of Massachusetts Amherst

Experiments exploring controlled membrane remodeling by DNA origami nanorods
Membrane remodeling facilitated by the self-assembly of proteins on the membrane is essential for cellular function. Inspired by this system, we use DNA origami nanorods to illuminate the role of particle shape, adhesion strength, membrane tension and particle concentration on membrane reconfiguration. We combine giant unilamellar vesicles with oppositely charged nanorods and observe them with optical and electron microscopy. The binding affinity of the nanorods to the membrane is tunable via lipid composition, which reveals four primary membrane morphological behaviors. For weak particle binding vesicles adhere to one another either forming a stable gel or are deformed into elongated shapes exhibiting tubulation. At intermediate binding strengths the gel forms but is subsequently destroyed by avid binding of the nanorods. At higher binding strengths the vesicles rupture without forming a gel. Cryo transmission electron microscopy reveals in-plane ordering of rods on the membrane. These responses are robust and repeatable providing a physical understanding of the dependence on shape, binding affinity, membrane tension and particle concentration in membrane remodeling. The design principles derived from these experiments will lead to bio-inspired membrane materials that are stimuli-responsive and reconfigurable.
Keywords: Membrane, Vesicles, Materials, Nano particles, DNA Origami

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