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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

53rd New England Complex Fluids Meeting
Harvard University | Friday, November 30, 2012
Registration deadline: Wednesday, November 28, 2012
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Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Blair

Georgetown University
"Local probes of nonlinear rheology in biopolymer networks"
Soft and biological materials often exhibit disordered and heterogeneous micro-structure. In most cases, the transmission and distribution of stresses through these complex materials reflects their inherent heterogeneity. Using confocal-rheology we determine the micro-structural rearrangements that arise when biological filament networks are strained. In this talk, I will describe our recent results on the nonlinear rheology of in vitro collagen networks. We apply bulk shear strains to type I collagen gels that are adhered to a thin elastic polyacrylamide gel substrate with embedded fiduciary markers while performing confocal microscopy. By utilizing a modified version of traction force microscopy we can calculate the distribution of forces within the network as a function of the applied strain at the boundary. This provides precise information about the way stress propagates through networks near the percolation threshold. We find that the signatures of yielding in these materials follow a universal form, and that the distribution of stresses remain heterogeneous on scales much larger than the mesh size. This indicating that the transmission of stress is distributed over very few available paths.

2. Invited; SJ Claire Hur

Rowland Institute at Harvard University
"Differential Inertial Microfluidics: Deformability-based manipulation of disperse phase"
Alteration in single-cell deformability has been identified to be a useful indicator of changes in cellular phenotype of importance for biological research since mechanical properties of single cells are found to be strongly associated not only with their lineage but also with the progression of various diseases. A label-free deformability biomarker would likely have lower operating costs than current molecular-based biomarkers that require pre-processing steps, dyes, and/or costly antibodies. Furthermore, disease states of interest can be expanded to those without predetermined immunological markers as long as a correlation between deformability phenotype and clinical outcome is confirmed. Therefore, novel techniques, allowing high-throughput single-cell deformability measurement and target cell enrichment based on deformability, would expand the research use and clinical adoption of this biomarker.

Differential inertial microfluidic devices are great candidates for such tasks since they can continuously but differentially locate flowing rigid and soft particles to distinct equilibrium positions without need for additional external forces. Research findings have showed that dynamic equilibrium positions are strongly influenced by flowing particles’ physical properties (e.g., size, deformability, and shape), the flow speed as well as the channel geometry. Using differences in dynamic equilibrium positions, we adapted the system to conduct passive, label-free and continuous cell enrichment based on their physical properties. My lab focuses on developing high-throughput soft particle and droplet manipulation techniques, allowing (i) statistical analysis of large heterogeneous biological samples and (ii) detection and selective collection of clinically meaningful rare cells

3. Invited; Lisa Manning

Syracuse University
"The role of mechanics in embryonic tissues: Pattern formation and organogenesis"
Biological tissues often behave like elastic solids on short time scales and fluids on long time scales. Different tissue types exhibit different characteristic macroscopic mechanical properties such as surface tension and viscosity, and cell rearrangements in developing animal tissues are often governed by these "material" properties.  By exploiting analogies with foams and supercooled fluids, we develop two models for the emergent mechanical behavior in zebrafish tissues. The first "dynamic" model treats cells as individual units and introduces interactions between cells to capture intracellular degrees of freedom. We show that this minimal model, which is contains only three parameters and is carefully calibrated using experimental data, makes predictions for bulk structural and dynamical properties of tissues that we have quantitatively verified. It shows that dynamics of cells in the interior of a tissue are similar to the dynamics of molecules in supercooled fluids near a glass transition. A second "steady-state" model studies ensembles of mechanically stable "jammed" cell packings. This simple model makes verifiable predictions about cell shape changes during organogenesis and at tissue boundaries. Together, our results suggest that embryonic tissues are a strange viscoelastic "material": while the bulk properties are fairly generic, programmed cell shape changes can lead to interesting properties near surfaces and during organogenesis.

4. Invited; Fred MacKintosh

Vrijie University of Amsterdam
"Elasticity on the edge of stability: marginal networks inspired by the cell"
Much like the bones in our bodies, the cytoskeleton consisting of stiff protein biopolymers determines the mechanics of cells. Inspired by such networks, we describe recent theory and experiments on simplified fiber networks. We show that these exhibit a unique state of highly responsive matter near the isostatic point first studied by Maxwell. Here, such networks constitute a marginal state of matter with exceptional properties, including zero-temperature critical behavior and anomalous entropic elasticity.

5. Shalek

Broad Institute - Harvard University
"Single-cell transcriptomics: Unbiased single cell approaches for gaining insight into immune cell behaviors"
Recent molecular studies have revealed that, even when derived from a “homogenous” population, individual cells can exhibit significant differences in gene expression, protein levels, and phenotypic output, with important functional consequences. Existing studies of cellular heterogeneity, however, have typically measured a small number of pre-selected RNAs or proteins because genomic profiling methods could not be applied to single cells until very recently. Here, I will discuss the use of single-cell RNA-Sequencing (Seq) to investigate heterogeneity in the response of primary immune cells to pathogenic stimulation. In particular, I will detail some of our recent and unexpected findings on variability at the levels of cell state, molecular circuit usage, and transcript structure. Moreover, I will highlight how microfluidic approaches are enabling us to achieve the higher-throughput profiling necessary to fully reconstruct intracellular circuits, enumerate and redefine cell states and types, and fundamentally transform our understanding of cellular decision-making on the genomic scale.

Sound Bites

1. Adams; J. Bird, T. Franke, V. Manoharan, and D. A. Weitz

Harvard University
" Metamorphosis of “Caterpillar” Double Emulsions"
Familiar insects, such as caterpillars, have segmented abdomens with usually 10-12 segments per insect. How important is the number of segments? For caterpillars, we don’t know, but for ‘caterpillar’ double emulsions, which are cylindrically shaped drops containing inner drops, the number of ‘segments’ or inner drops plays a crucial role in their metamorphosis. Interestingly, not only is number of ‘segments’ a controlling factor in their transformation, but also the size and composition of these ‘segments’ as will be discussed.
Keywords: Caterpillars,Metamorphosis, Microfluidics, Double Emulsions

2. Arriaga; Esther Amstad and David A. Weitz

School of Engineering and Applied Sciences and Department of Physics, Harvard University, Cambridge, Massachusetts, USA
"Smart vesicles based on plasmonic nanoparticles"
Giant lipid vesicles have a high potential as biocompatible carriers with a large aqueous lumen cavity that can be loaded with large biomacromolecules. Vesicles are usually assembled by re-hydration techniques; this results in highly polydisperse samples. Advances in the design of delivery vehicles are hindered by their high polydispersity and the low encapsulation efficiency inherent to conventional preparative methods. It is still a major challenge to assemble smart vesicles with tunable and remotely controllable membrane properties. However, close control over these properties is crucial for their successful application as delivery vehicles. Using microfluidic techniques, we assemble monodisperse giant unilamellar lipid vesicles with plasmonic gold nanoparticles embedded in their membranes. Excitation of the surface plasmons of the nanoparticles produces localized heating inside the membrane. This heat increases the permeability of the liposome membrane resulting in an enhanced osmotic-driven flow of water across the membrane and causes an overall change in size of the vesicles. We aim to shed light on the underlying physical mechanisms involved in the changes in size and shape of these vesicles.
Keywords: giant lipid vesicles, nanoparticles, microfluidics

3. Balasubramanian; Bulbul Chakraborty, Aparna Baskaran

Brandeis University
"Prolate spheroidal swimmers in shear"
Active Matter are particles that draw energy from internal or external sources and dissipate this energy by moving through the medium they inhabit. Active Matter are known to self organize spontaneously whereas Passive colloids aggregate under forcing. We study a system of rod like swimmers under simple shear to try to understand the interplay between forcing and self propulsion in aggregation.
Keywords: active matter

4. Basu; Alex Shalek, Aviv Regev, David Weitz

SEAS, Harvard U.; Broad Institute
"Single-cell genomics using droplet-based micro-fluidics"
We study the genomics of single cells in isolation using a droplet-based micro-fluidic setup. We can follow a cell in response to its micro-environment by (a) uniquely bar-coding the genomic information from a given cell, and (b) encapsulating one cell per emulsion droplet. This enables study of cellular behavior and single-cell genomics in response to its micro-environment in the emulsion droplet.
Keywords: cell genomics, emulsion microfluidic

5. Chang; H. Zhang, A. Rotem, D. Weitz

Harvard University
"Directed Viral Evolution using High-Throughput Droplet-Based Microfluidics"
I will present the use of droplet-based microfluidics as an ultra-high throughput screening platform for the prediction of viral evolution. I will discuss the replication of viruses in drops and the use of this platform for the screening of rare viral recombinants. We hope this platform will aid in understanding viral epidemics and the emergence of future viral threats.
Keywords: Viruses, Directed Evolution, Microfluidics, "-omics"

6. Chen

Institute of Physics, Academia Sinica
"Dynamics of a Soft Particle in Shear and Poiseuille flow"
We investigate the cross-stream migration of soft particles in shear flow and Poiseuille flow in microfluidic channels. Soft particles deform and migrate in microflow due to the competition between shear forces, particle elasticity, particle diffusion, and particle inertia. At low particle Reynolds number ($Re < 1.0$), the soft particles migrate towards the channel center due to the coupling of particle elasticity and wall-induced hydrodynamic interactions. As particle Reynolds number increases to moderate values ($Re \ge 1.0$), the particle concentration profile has two maxima at off-center positions. The migration effect is also found to be enhanced for softer particles with longer elastic relaxation time. The variation of particle concentration profiles leads to non-linear variations of the mixture viscosity and the average flow rate.
Keywords: vesicles, migration

7. Collins; Vinothan N. Manoharan

Harvard SEAS
"Time-resolved DNA-colloid Cluster Assembly"
I coat micron-sized polystyrene spheres with DNA and control reaction conditions such that some species of beads bind only to others bearing complementary DNA strands. We watch 5-6 individual beads from 2-3 different species starting in high energy cluster states fold into energy minimizing configurations. Because these DNA-polystyrene composite building blocks are big enough to image, slow enough to track, few in number, isotropic and tunable in interaction strength, they are more amenable to experimental data collection and comparison with theory than the small, fast, and complex purely biomolecular systems with which they may share some interesting physics.
Keywords: DNA, colloid, self-assembly, clusters, protein folding

8. Dasgupta; Arshad Kudrolli

Clark University
"Speed of a swimming sheet in Newtonian and Viscoelastic fluid"
Motivated by a need for a fundamental understanding of microorganism locomotion through non-Newtonian fluids, we measure the swimming speed of a cylindrical version of Taylor’s swimming sheet in viscoelastic fluids, and find that depending on the rheology, the speed can either increase or decrease relative to the speed in a Newtonian viscous fluid. The swimming stroke of the sheet is a prescribed propagating wave that travels along the sheet in the azimuthal direction. The measurements are performed with the sheet immersed in a fluid inside a cylindrical tank under torque-free conditions. Later we make comparison of our results with that of Oldroyd-B model fluids.
Keywords: Taylor's swimming sheet, newtonian, viscoelastic

9. Datta; Harry Chiang, T. S. Ramakrishnan, David A. Weitz

Department of Physics, Harvard University
"Getting Out Of A Tight Spot: How A Fluid Navigates A 3D Porous Medium"
Filtering drinking water, squeezing a wet sponge, and watering a dry garden are all familiar examples of forcing a fluid through a porous medium. This process is also crucial to many technological applications in geology, chemical engineering, colloidal science, and biology. However, despite the ubiquity and importance of this process, the underlying pore-scale physics remains unclear. We have recently developed an experimental approach to directly visualize single- and multi-phase flow within a porous medium, in three dimensions, both at pore-scale resolution and over continuum length scales. I will describe how we use this approach to quantify the strong velocity variations that arise in the flow, and will show how these data can be understood within a simple mean-field picture.

10. Dimiduk; Jerome Fung, Rebecca Perry

Harvard University
"High Speed Tracking of Asymmetric Particles with Holography"

11. Dressler; L. Gottardo, M. Becker, U. Heck, R. Hufenus, M. Heuberger

UMass Amherst
"Design of a spinneret for bi-component fibers based on multi-phase flow experiments and simulation"
To design a spinneret for the production of bi-component fibers with a shear thickening core and a shear thickening sheath, we performed core-annular flow experiments of immiscible liquids using non-standard injection needles. Furthermore, we used the open source multi-physics package OpenFOAM for the computational fluid dynamics simulation of our model experiments to simulate the complex interface of a liquid jet in a co-flowing ambient fluid. As a first step, experiments and simulations have been carried out for immiscible Newtonian liquids at room temperature. The interFoam solver of OpenFOAM is able to predict various morphologies of the co-flow experiments, e.g. drops, slugs, and jets.
Keywords: fiber spinning, process design, computational fluid dynamics

12. Duncanson; Laura R. Arriaga, Tom E. Kodger and David A. Weitz

Harvard University
"Microfluidic Fabrication of Antibubbles: Packing Negative Foams"
A monodisperse foam is a dispersion of gas in liquid that consists of an array of bubbles; whereas, a negative foam is a dispersion of liquid in gas that consists of an array of antibubbles. These well-ordered structures enable the study of the packing arrangements of bubbles or antibubbles as well as the flow dynamics of gases in contact with liquids. However, negative foams have not yet been fabricated or studied because monodisperse antibubbles have not been produced. We use microfluidics to produce monodisperse antibubbles that pack to form negative foams.
Keywords: Antibubbles, Microfluidics, Negative Foams

13. Fluerasu

Brookhaven National Laboratory
"Dynamics in crowded systems with X-ray Photon Correlation Spectroscopy"
Over the last two decades, X-ray Photon Correlation Spectroscopy (XPCS) has become an increasingly important technique in the study of dynamical phenomena in materials. As a consequence, the Coherent Hard X-ray (CHX) beamline at the NSLS-II light source, one of the first six "project beamlines" to be build with the storage ring, will be dedicated to XPCS and the study of dynamics of materials using intense coherent beams. Here I will describe some recent results from XPCS studies of the colloidal glass transition including a comparision of the experimental data for structural relaxations with predictions from the Mode Coupling Theory and a "jamming transition interpretation".
Keywords: XPCS, colloids, glass transition, soft matter

14. Fung; Alexander Small, Vinothan N. Manoharan

California Polytechnic University, Pomona; Harvard University
"Scattering by a colloidal sphere at a liquid-liquid interface"
We discuss progress towards calculating the electromagnetic scattering of a spherical particle whose equator lies at a planar interface between two dielectric media with different refractive indices.
Keywords: scattering

15. Graham

"Sample title"

16. Guo; Linas Mazutis, Jeremy Agresti, Morten Sommer, Gautam Dantas, Peter Turnbaugh, Dave Weitz

"High-throughput single-cell PCR using microfluidic emulsions"
The human gut and other environmental samples contain large populations of diverse bacteria that are poorly characterized and unculturable, yet have many functions relevant to human health. Our goal is to identify exactly which species carry some gene of interest, such as a carbohydrate metabolism gene. Conventional metagenomic assays sequence DNA extracted in bulk from populations of mixed cell types, and are therefore unable to associate a gene of interest with a species-identifying 16S gene, to determine that the two genes originated from the same cell. We solve this problem by microfluidically encapsulating single bacteria cells in drops, using PCR to amplify the two genes inside any drop whose encapsulated cell contains both genes, and sequencing the DNA from those drops that contain both amplification products.
Keywords: PCR, single cells, microfluidics, metagenomics

17. Herranz; Laura Arriaga, Dongfei Liu, Sabiruddin Mirza,David A. Weitz,Hélder A. Santos

University of Helsinki
"Encapsulation of Mesoporous Silicon Microparticles via the Microfluidic Technique for Sustained Drug Delivery"
Taking advantage of the already known benefits of porous silicon (PSi) materials as drug carriers and the possibility of encapsulation offered by the microfluidic technique, we have developed a stable encapsulation system for hydrophobic PSi-based microparticles in order to improve their stability in suspension and their potential prolonged drug release effect. For instance, we have successfully adapted the bulk method elaboration of a solid-lipid composite comprising a solid-lipid matrix shell and a thermally hydrocarbonized-PSi (THCPSi) core to the microfluidic technology. In the microfluidics, THCPSi microparticles of sizes around 5 µm were encapsulated within a lipidic matrix. Compared to bulk method, the microfluidic technique allows a better control of the encapsulation efficiency of the THCPSi microparticles. In addition, using a similar approach, giant lipid-based vesicles or liposomes comprising an hydrophilic core surrounded by a lipophilic bilayer were also used to incorporate the THCPSi microparticles within the aqueous core. The main advantage of this system is that the THCPSi microparticles may be loaded with hydrophobic cargos allowing for the transport of these drugs within the aqueous core of the vesicle/liposome. Drug release studies employing the systems described above are currently being conducted in order to evaluate the drug release behavior/mechanism from the PSi-based microcomposites.
Keywords: Mesoporous Silicon Microparticles, encapsulation, drug delivery

18. Heymann; Achini Ranaweera, Seth Fraden

Brandeis University
"Xray transparant phase chip for protein crystalisation - part II: at the synchrotron"
Using microfluidic COC-PDMS hybrid devices, we can surf the phase diagram to decouple protein crystal nucleation and growth by constantly modulating protein supersaturation reversibly. We collect xray diffraction data form crystals grown in chip to solve protein structures.

19. Jang; Simon G Mochrie

Post Doctoral Associate
"Dynamic Signatures of Cadmium Sulfide Nanoparticles in Poly(styrene-b-isoprene) Block Copolymer Matrix"
One of the most exciting properties of block copolymer (BCP) is their ability to self-assemble into a variety of nanostructures. The incorporation of nanoparticles into self assembled BCP to create nanocomposite materials has a variety of potential applications, including semiconductor and optical applications. Here, we describe our recent progress in creating and characterizing BCP-nanoparticle nanocomposites using small angle x-ray scattering (SAXS), x-ray photon spectroscopy (XPCS), electron microscopy (EM), and surface force microscopy (SFM). An essential component of our nanocomposite materials are poly(styrene-b-2 vinyl pyridine) micelles, henceforth S2VP micelles, decorated with cadmium sulfide (CdS) nanoparticles, which are attached to the 2 vinyl pyridine blocks. In toluene and similar solvents, the styrene blocks from the shell and the 2 vinyl pyridine blocks, together with their attached CdS nanoparticles, form the core. To investigate the temperature-dependent structure and dynamics of BCP decorated with these micelles, we have chosen to incorporate S2VP micelles with core-embedded CdS nanoparticles into bulky poly(styrene-b-isoprene), henceforth SI, under the hypothesis that the micelles, which has polystyrene shells will preferentially located in the polystyrene rich region of the SI structure, and may even self –organized in these regions, leading to a hierarchically- ordered material. To elucidate the nanostructure and dynamics of the resultant nanocomposites, we are in the process of carrying out a series of SAXS, XPCS, EM, and SFM.

20. Jawerth; Stefan Münster; David A. Weitz

Harvard Univeristy
"The origin of non-linear network mechanics in fibrin and collagen networks"
An important constituent of the extracellular matrix are complex, three-dimensional networks of stiff biopolymer fibers that impart mechanical integrity to tissues and act as structural scaffolds for the embedded cells. Two prominent examples are networks formed from the proteins collagen and fibrin; Collagen networks are the structural component of connective tissue, tendons, ligaments and bone, whereas fibrin networks present the structural scaffold of blood clots. The underlying principles that govern the mechanics of these networks remain poorly understood. We study the mechanical response of stiff fiber networks with a novel shear cell that allows direct observation of the 3D network structure and concurrent measurement of the resulting forces as a network undergoes shear using confocal microscopy. Using image analysis, we identify and track the 3D structure of the network during this process. We determine the average individual fiber strain, average angular change between fibers, the characteristic correlation length and the non-affinity of branch point motion as the network is sheared. In addition, we approximate the shear stress required to deform the network using a simple model of individual fiber behavior. Using these data, we distinguish between microscopic models of fiber behavior and identify the origin of many intriguing mechanical properties such as the effect of strain-stiffening.
Keywords: biopolymers; mechanics; fibrin; collagen

21. Jensen; Daniel Pennachio, David Weitz, Frans Spapen

Harvard University
"Why do colloidal single crystals grown on a template sometimes develop extended defects?"
Perfect colloidal single crystals can be grown extremely rapidly by high-flux sedimentation onto FCC (100) templates. However, after growth, extended defects (dislocations, stacking faults) can nucleate and grow if the crystal exceeds a critical thickness that depends on the lattice misfit with the template spacing. We report measurements of the density of these misfit dislocations as a function of crystal thickness, and compare these results with calculations from the Frank-van der Merwe theory, adapted to account for the variation of elastic constants and lattice spacing with crystal thickness in the colloid.
Keywords: colloid, crystal, dislocation, defect growth

22. Jensen; N. Michele Holbrook, M. A. Zwieniecki

Harvard University
"Physical Limits to Leaf Size in Tall Trees"
Leaf size in angiosperm trees vary by more than three orders of magnitude, from a few mm to over 1 m. This large variability is, however, only expressed in small trees and the observed leaf size range declines with tree height, forming well-defined upper and lower boundaries. An interesting question is: what limits leaf size in tall trees? The vascular system of plants that distributes energy forms one of the largest known continuous microfluidic distribution networks. We show that the observed limits to leaf size can be rationalized by hydrodynamic constraints imposed by intrinsic properties of this transport network.
Keywords: biofluids, microfluidics, vascular systems, plants

23. Jones; Jonathan Celli PhD, Will Hanna

UMass Boston
"Variation in tracer probe diameter measures different levels of viscoelasticity in collagen gels"
Pancreatic cancer is an aggressive, chemoresistant disease whose high fatality rate could be due in part to the rigid stroma which forms around its tumors. While using passive microrheology techniques on 3-D pancreatic tumor models in collagen gels to study this yet-to-be-understood connection between stromal rigidity and malignant, chemoresistant character, we noticed a large variation in measurement of viscoelastic modulus that is correlated to tracer probe diameter. In particular, it appears that there is an inverse exponential relationship between tracer probe diameter and measured elastic modulus value. This variation in measured elastic modulus value between bead sizes is not seen in samples of plain tissue culture media. Given the fact that collagen fibers form an inhomogeneous network within a gel, we hypothesize that the smallest beads settle into the densest regions of the fiber network, while the largest beads are pushed to the outskirts of the dense regions during collagen fiber formation; therefore, beads with smaller diameters measure a higher value of the elastic modulus while larger beads measure a lower value in the same gel. This probe diameter dependent variation has also been observed in associating polymers and may prove to be a useful tool for analyzing the effects of experimental cancer treatments on stromal stiffness gradients.
Keywords: pancreatic cancer, passive microrheology


Worcester Polytechnic Institute
"E ect of quantum dots on the isotropic to nematic and nematic to smectic-A phase transitions in nano compos- ites"
Modulated Di erential Scanning Calorimetry (MDSC) is used to investigate the weakly first-order isotropic to nematic (I-N) and the continuous nematic to smectic-A (N-SmA) phase transitions of the liquid crystal octylcyanobiphenyl (8CB) doped with well-dispersed quantum dots (QdS) as a function of Qd concentrations. Thermal scans were performed for all samples having Qd (CdS) weight percent from w = 0.3 to 3 wt% first on cooling and then heating under near-equilibrium conditions. The I-N transitions heat capacity peak first glows then decreases in magnitude with increasing Qd content leaving a maximum at w = 0.3%. The N-SmA heat capacity peak remains bulk-like for all samples. Both transitions temperatures shift lower monotonically by 3 K for w = 0.3%. The enthalpy of both transitions evolve in a nontrivial way, generally decreasing with increasing w. These results are discussed in terms of the predominate disordering e effects of the Qds.
Keywords: 8CB, Quantum dots,Calorimetry

25. Lin; T. Kodger, D.A. Weitz

Harvard University
"Dynamics of charged colloids in a nonpolar solvent in response to an electric field"
In nonpolar solvents, particle charging is often controlled through the addition of suitable surfactants, which form charge-stabilizing reverse micelles. By combining microfluidics and confocal microscopy, we directly visualize the dynamics of charged colloidal particles in a nonpolar solvent with reverse micelles in response to an external electric field; this enables us to probe the internal electric field as well as the charging properties of the particle solution. We discover some surprising particle behavior: despite a constant applied electric field, particle transport through the fluid is nonlinear and the apparent particle mobility decays in time; subsequently, the charged particles appear to diffuse freely within the bulk solution. We characterize this behavior and find that the charged reverse micelles play a significant role.
Keywords: charged colloids, nonpolar solvent, dynamics, electric field

26. MacMinn; John S. Wettlaufer and Eric R. Dufresne

Yale University
"Deformation and relaxation in a poroelastic model system"
Poroelastic effects, where fluid flow through a porous solid is coupled to elastic deformation of the solid, play an important role in many natural and engineering systems. Here, we use laboratory experiments to explore this nonlocal coupling between the fluid and the solid in the context of the paradigmatic problem of fluid injection into a quasi-two-dimensional porous medium.
Keywords: poroelasticity, porous materials, granular materials, fluid flow, elasticity

27. Magkiriadou; J.G.Park, Y.-S.Kim, G.Yi, V.N.Manoharan

Harvard University Department of Physics
"An Insight on the Absence of Red Structural Color in Disordered Colloidal Packings"
Random packings of colloidal particles can appear colorful when the average interparticle spacing is comparable to a visible wavelength. These colors arise from constructive interference of light scattered from neighboring particles. This mechanism is well-known and has been observed in nature numerous times. However, structural red is notoriously difficult to make and, to our knowledge, absent in nature. We report on a recent insight on the physical mechanism behind this absence, gained from a study of the interplay between interference and single-particle scattering in random colloidal aggregates.
Keywords: structural color, scattering, random packing, red

28. Mbanga; Christopher Burke, Donald W. Blair, Timothy J. Atherton

Tufts University
"Arrested of coalescence of emulsion droplets of arbitrary size"
With applications ranging from food products to cosmetics via targeted drug delivery systems, structured anisotropic colloids provide an efficient way to control the structure, properties and functions of emulsions. When two fluid emulsion droplets are brought in contact, a reduction of the interfacial tension drives their coalescence into a larger droplet of the same total volume and reduced exposed area. This coalescence can be partially or totally hindered by the presence of nano or micron-size particles that coat the interface as in Pickering emulsions. We investigate numerically the dependance of the mechanical stability of these arrested shapes on the particles size, their shape anisotropy, their polydispersity, their interaction with the solvent, and the particle- particle interactions. We discuss structural shape changes that can be induced by tuning the particles interactions after arrest occurs, and provide design parameters for the relevant experiments.

29. McDermott; I. Morrison, D. A. Weitz

"Flocculation Studies of Electrostatically Stabilized Colloids in Nonpolar Media"
High-volume-loading dispersions of carbon black in hydrocarbon oils are of interest both commercially, where they serve as coatings, pigment-based inks, and model systems for automotive soot; and scientifically, as they form heterogeneous, space-filling, weakly attractive gels with unique rheological properties. Common dispersants used to stabilize carbon in oils typically utilize both electrostatic and steric repulsion mechanisms. In this work, we study systems of a highly fractal and conductive black, stabilized using ionic dispersants to better understand and characterize the interparticle forces at play in these dispersions. By measuring the electroacoustic response of these dispersions, along with the dispersion rheology, we can correlate the breakup of the carbon black gels with the evolution of surface charging. The sensitivity of the carbon dispersions to solution ionic concentration and valence is observed, recalling some of the earliest analytical techniques in colloid science, including examination of the Schulze-Hardy Rule for nonpolar solvents.
Keywords: Electrostatics, Charges in Nonpolar Media, Electroacoustics

30. Mertz; Shiladitya Banerjee, Yonglu Che, M. Cristina Marchetti, Valerie Horsley, and Eric R. Dufresne

Yale University
"Intercellular adhesions organize cell-matrix mechanics"

31. Mughal; M.-Carmen Miguel, Adil Mughal, and Stefano Zapperi

Institute of Mathematics and Physics, Aberystwyth University Penglais, Aberystwyth, Ceredigion, United Kingdom
"Laminar Flow of a Sheared Vortex Crystal: Scars in Flat Geometry"
We consider the laminar flow of a vortex crystal in the Corbino disk geometry [1]. Laminar flow can be induced by thermal fluctuations melting the crystal, but also by shear stress after applying a large current at zero temperature. While the velocity profile is the same in the two cases, the underlying vortex structure is completely different. A vortex crystal in this geometry can flow in a laminar fashion whenever the appropriate curvature is established in the vortex lattice. This curvature requires the presence of geometrically necessary disclinations, which here migrate from the boundary to the bulk of the crystal in the form of current-induced grain boundary scars in flat geometry. We provide an estimate of the characteristic current needed to initiate such a laminar flow regime in the vortex crystal and show that the result is in good agreement with simulations. [1] Laminar flow of a sheared vortex crystal: Scars in flat geometry C. Miguel, A. Mughal and S. Zapperi, Phys. Rev. Lett. 106, 245501 (2011)
Keywords: Corbino disk, vortex crystal, Laminar flow

32. Ocko; L. Mahadevan

"Feedback, Control, and Phase Transitions In Active Porous Media"
All life exists in a world of gradients and physical flows that need to be regulated. A ubiquitous method of controlling these flows is through the arrangement of matter to block or enable flow, whether this matter is comprised of cells, organisms, or building material. Often this arrangement is achieved through feedback mechanisms. We consider the simplest possible model for regulation of flow through feedback mechanisms, and find that it exhibits very rich behavior, including channelization and wall-building transitions.

33. Opathalage; Michael Heymann, Seth Fraden

Brandeis University
"Tittle: Xray transparent COC-PDMS hybrid microfluidic device for Protein Crystallization - part I: Fabrication"
A patterned PDMS layer of 80 microns thickness is sandwiched between two thermoplastic COC sheets of thickness 25 and 50 microns, respectively by a chemical bond of two complementary silanes. The resulting thin device is used for room temperature X-ray diffraction studies of protein crystals grown on-chip with the goal of obtaining a high resolution structure by merging diffraction sets from multiple crystals.

34. Papanikolaou; C. S. O' Hern, M. D. Shattuck

Yale University
"The crossover from random close to random loose packings of frictional disks"
Mechanically stable packings of frictionless disks with contact interactions form through fast quenches at random close packing (RCP). However, for frictional particles with static friction coefficient μ greater than μ*, the packing density slides toward random loose packing (RLP) at large friction. We elucidate the crossover from random close to random loose packing through simulations of bidisperse disks using the geometric asperity (GA)[1] and Cundall-Strack (CS) friction models. We demonstrate that a change takes place in the structure of allowed mechanically stable packings in configuration space: From uncorrelated points at zero friction to linear and other low-dimensional structures at small friction to higher dimensional structures at large friction. Further, we use the GA model to study dynamical mechanical properties without ad hoc assumptions for sliding contacts, and we find that low-frequency vibrational modes with significant rotational content display a strong peak below μ*. Their rotational content drastically changes from co-rotating contacting particles for low friction to counter-rotating, gear-like, for μ greater than μ* and the groups of particles with gear-like dynamical contributions percolate at μ*. Finally, the very existence of the low-frequency vibrational peak gives rise to a change in the scaling of the static shear modulus with pressure compared to the frictionless behavior. [1] S. Papanikolaou, C. S. O' Hern and M. D. Shattuck, arxiv:1207.6010 (2012)
Keywords: jamming, friction, packing, random

35. Pegoraro; Ming Guo, Allen Ehrlicher, David A. Weitz

"Collective cell motion on deformable substrates"
During collective cell motion, mechanical coupling between cells is governed by cell-cell contact, but on deformable surfaces long-range interactions mediated by the substrate also play a role. These long-range interactions can lead to both spatial and temporal correlations during cell migration that extend over many cell lengths. To investigate this further, we study collective cell migration on different substrates. We find that substrate stiffness as well as surface adhesion properties both play a role in determining collective behavior.

36. Pessi; L. Arriaga, W. Duncanson, N. Carroll, I. Miroshnyk, S. Mirza, J. Yliruusi and D.A. Weitz

School of Engineering and Applied Sciences/Department of Physics, Harvard University; Department of Pharmaceutical Technology, University of Helsinki
"Microfluidic fabrication of polycaprolactone microspheres for enhanced protein drug delivery"
Enteric-coated microspheres hold great potential as oral drug delivery system for therapeutic proteins. In this study, we use microfuidic technology for developing a formulation template for fabrication of polycaprolactone microspheres for enhanced protein delivery. More specifically, the goal is to employ biphasic flow to produce double (W/O/W) emulsion droplets with ultra thin shells, with the shell thickness being a key parameter for drug-release control. Biphasic flow enabled the use of viscose organic solvents (ethyl acetate) in the middle phase and constant production of double emulsion droplets for encapsulation of a model protein (β-galactosidase). Dissolution studies that are currently in progress aim to assess the effect of the shell thickness on the protein release profile from the formulations.
Keywords: Protein drugs, polycaprolactone, microfluidics, drug release

37. Plyukhin

Saint Anselm College
"Relativistic Langevin equation"
There is currently no consensus on the form of stochastic and master equations describing a relativistic Brownian particle. The difficulties are many, both technical and fundamental. The former are related to the nonlinearity of relativistic Langevin equations, while the latter are those of constructing of a Lorentz-invariant dynamics of instantaneously interacting particles without explicit taking into account field degrees of freedom. I will outline a possibility and restrictions of a straightforward extension of the conventional Langevin phenomenology to the relativistic domain.
Keywords: Relativistic Brownian motion

38. Ren; Tomasz Glawdel, Caglar Elbuken

University of Waterloo
"Droplet "
Droplet-based microfluidics has drawn ever-increasing attention over the past decade because of its potential to be employed as an enabling technology for high throughput combinatorial testing. A typical droplet-based microfluidic device employs two immiscible fluids to generate gas bubbles or liquid droplets at the nanoliter scale. This talk is limited to the study of liquid droplets in microfluidic devices. In particular, I will focus on introducing the models that we developed for droplet generation in T-junction microchannels and the effect of surfactant dynamics on droplet generation. Following this, droplet trafficking in microfluidic junctions under asymmetric geometric and flow conditions will be briefly discussed. Finally, I will talk about the capacitance sensor and microwave sensor that we developed for measuring droplet size, speed and content. The uniqueness of our microwave sensor is that it is also capable of simultaneously heating up individual droplets without affecting the surrounding oil streams and chip materials.
Keywords: Droplets, Trafficking, Sensing

39. Schade; Dazhi “Peter” Sun, Miranda C. Holmes-Cerfon, Elizabeth R. Chen, Jonathan A. Fan, Oleg Gang, Vinothan N. Manoharan

Harvard University
"Metafluid engineering with colloidal self-assembly"
A suspension of tetrahedral clusters of gold nanospheres is predicted to exhibit tunable, exotic optical properties, including isotropic negative refraction. The challenge is assembling the clusters, and we examine how that may be accomplished using DNA-driven colloidal self-assembly. We experimentally investigate the structures that form when gold nanospheres cluster around smaller spheres. Our previous experiments using polystyrene microspheres indicate that a 90% yield of tetrahedral clusters is possible near a critical diameter ratio.
Keywords: DNA, colloid, self-assembly, cluster, metafluid

40. Schreck; Rob Hoy, Mark Shattuck, Corey O'Hern

Yale University
"Shear Reversibility in Model Granular Media"
Athermal particulate systems such as foams and granular media are out-of-thermal equilibrium and therefore must be externally driven using shear or vibration to explore different configurations. Of particular interest is being able to predict and control the structural and mechanical properties of athermal systems as a function of the driving mechanism. In this work, we show numerically how particle collisions in cyclically sheared hard sphere systems can lead to microreversibility. We map out the steady-state "phase diagram" as a function of packing fraction ($\phi$) and strain amplitude ($\gamma_{max}$), and identify "point-reversible" states at low $\phi$ and $\gamma_{max}$ in which particles do not collide over the course of a shear cycle, and "loop-reversible" states at intermediate $\phi$ and $\gamma_{max}$ in which particles undergo numerous collisions but return to their initial positions at the end of each shear cycle. Loop-reversiblity is a novel form of self organization that gives rise to non-fluctuating dynamical states over a broad range of packing fractions from contact percolation to jamming, i.e. $\phi_P=0.55$ to $\phi_J=0.84$ in two dimensions.
Keywords: Granular media, suspensions, micro-reversibility

41. Sharma; T. Post, C. Lefebvre, N. Farrar, D. Sharma, and K. Farah

"Effectiveness of a Lacrosse Helmet "
Concussions resulting from sports injuries have been gaining increased attention in the media. Failure to diagnose a concussion and allowing a player to return to full activities can cause harmful and permanent damage and may ultimately be lethal. When a player is allowed to reenter the game with a concussion, a subsequent concussion may result in Second Impact Syndrome. Second Impact Syndrome can be deadly because when the brain is already injured since the injured brain is at a much greater risk of being effected by a trauma than the normal brain. This secondary trauma causes an increased rate of swelling in the cranial cavity, which results in an increase in pressure on the brain. Head injuries can be prevented by wearing a properly functioning helmet. The goal in this undergraduate project was to test the amount of force that a lacrosse ball would exert on different conditioned heads: 1) an unprotected head, 2) a head with a properly worn helmet and 3) a head with a cracked and damaged helmet. The effectiveness of the above conditions was studied dropping a lacrosse ball from a fixed height onto different conditioned heads. The amount of force applied to the head with each level of protection was determined through video analysis using logger pro software. Results indicated that wearing a proper helmet reduced forces on the head up to 73% compared to the control wearing no helmet. A defective helmet reduced force up to 22%. Hence, athletes who play a contact sport should always check for defects in helmets that they wear to ensure adequate protection from concussive forces.
Keywords: Video Analysis

42. Sharma; A. Scrutchfield, J.Reich, A. Berking, D. Sharma and K. Farah

"The Physics behind a Horse Jumping "
This is an undergraduate project where we wanted to connect the concepts of force and acceleration with a fun-filled real world event which would result in an interesting way of applying physics at the undergraduate level. Horse jumping game plays a major role in many equestrian sports and in this project, the biomechanics of jumping were the focus of the research. In this project, our goal was to find the effect of height of horse jump on the force exerted on horse’s hooves when it landed. Logger pro software in conjunction with video analysis was used to determine the overall force. In the experiment, a rider jumped a horse over different height jumps keeping the other parameters the same i.e. approach tempo, horse, rider and environment. The force increased in a non-linear fashion, with a 10% increase in force when the jump height raised from 2 feet to 2.5 feet and a doubling in force as the jump was raised from 2.5 to 3 feet. These results indicate that care should be taken when training to ensure that horses are only jumped as much as needed to improve their technique.
Keywords: Video Analysis

43. Sharma; N. Carroll, M. Michel, D. Sharma, K. Farah

"Newtonian Box Jumps"
We found an interesting way of studying Newton’s 2nd law using logger pro software and a video analysis method. Box jumps are a form of plyometric exercise used to strengthen the lower extremities of the body. An athlete would use box jumps as an exercise when they are trying to build up power and explosiveness where they lift their body weight. The problem that we studied in this project is how the quality of shoes affects the box jumps and how the results can be related with the Newton’s law. To do this project we used three different types of shoes; 1) Nike Air Forces, 2) Nike basketball shoes, 3) and a pair of Nike running shoes. We recorded videos for several box jumps using a motion detector and logger pro software. Later, we analyzed the video recording to get the associated take off speed and acceleration of the various shoes and then found the force exerted on the various shoe types. We found that force varied with shoe type. Wearing lighter shoes during box jumping is the most convenient as it exerts less force and less effort. The type of shoe provides different support to the foot and ankle. For example, the Air Forces had thick laces and a strap that went around the lower aspect of the ankle whereas the basketball sneakers were a bit lighter but the shoes themselves went up past the ankle region, laced the entire way up, and had an ankle strap.
Keywords: Video Analysis

44. Sinha; James Wilking, Agnese Seminara, David Weitz, Michael Brenner

Harvard School of Engineering and Applied Sciences
"Mapping Physiological Heterogeneities in Biofilms"
Biofilms are bacterial colonies that form on surfaces. Even genetically identical cells within a biofilm can differentiate into multiple cell types. Microbiologists have identified some of the chemical triggers that control these differentiation pathways; however, at a whole-colony level, the relationship between the environment and the differentiation process is not well understood. To explore this connection, we study the growth of bacterial colonies on a nutrient-containing agar plate. We observe the populations of different cell types, which is simplified by the radial symmetry of the colonies. Specifically, we investigate the spatial and temporal distributions of cell types for different nutrient concentrations, and compare these to theoretical calculations of nutrient gradients. This same methodology could be applied to other differentiation pathways as well.
Keywords: biofilms, differentiation, physiological heterogeneity

45. Style; Y. Che, J. S. Wettlaufer, L. A. Wilen and E. R. Dufresne

"Young's law fails on soft substrates"

46. Uchida; Toshimitsu Kanai, Ho Cheung Shum, Takeaki Araki, David A. Weitz

Harvard University
"Novel defect structures in particle-coated nematic liquid crystal drops"
Confinement of solid particles to a spherical surface of a drop comprised of a liquid crystal should induce fascinating new behaviors due to the delicate balance between the intrinsic packing constraints of the particles and the inevitable orientational defects in the liquid crystal induced by the topological constraints of the boundary conditions. We show that spherical particles can be confined very near to the interface of a spherical nematic-liquid-crystal drop with tangential boundary conditions, and describe the remarkable behavior that ensues. Completely new behaviors are observed as the particle concentration increases: The particles transform into a hexagonal packing driven by the formation of a striking new lattice-like defect structure, and the marked modification of the overall liquid crystal boundary condition at the interface of the drop, resulting in formation of a hedgehog structure like a liquid-crystal drop with homeotropic boundary conditions.
Keywords: liquid crystal; colloid; self assembly

47. Uspal; H. Burak Eral, Patrick S. Doyle

"Shape asymmetric particles assemble under flow in quasi-two-dimensional microchannels"
We investigate the effect of shape on the dynamics of a rigid particle confined to a shallow, “quasi- two-dimensional” microchannel and driven by external flow. Theoretically, we show that the interplay of three viscous hydrodynamic mechanisms determines particle behavior. Via self-interaction, a particle can translate across streamlines and, if shape asymmetric, rotate into alignment with the external flow. Via interaction with its hydrodynamic image, a particle approaching a channel side wall will be reflected towards the centerline. Consequently, rod-like particles oscillate between side walls, while asymmetric particles reversibly and stably focus to the centerline. Experiments performed with continuous flow lithography semiquantitatively confirm our theoretical predictions. Our results can be extended to systems of multiple particles, allowing design of swarms that self-organize under flow. Moreover, we find intriguing connections with bulk sedimentation.

48. Wang; Aleksey Lomakin, George Benedek

"Pathological crystallization of human immunoglobulins"
Condensation of immunoglobulins can take place in human body, particularly in multiple myeloma patients who overproduce monoclonal antibodies. The condensation of antibody causes severe subsequent complications called cryoglobulinemia. We report a study of monoclonal IgG overexpressed by two patients with multiple myeloma. These two IgGs form crystals, and we measured their solubility lines. Depending on the supersaturation, we observed a variety of condensate morphologies consistent with those reported in clinical investigations. Remarkably, the crystallization can occur at quite low concentrations. This suggests that, even within the regular immune response to infections, cryoprecipitation of immunoglobulin can be possible.
Keywords: immunoglobulin, antibody, crystallization

49. Wang; Ryan McGorty, David M Kaz, Vinothan N Manoharan

Harvard University
"Contact line pinning on surface defects of colloids"
It was recently shown that micron-sized particles relax logarithmically into an oil water interface, not reaching equilibrium position on experimental timescales. A model that involves activated hopping of the three phase contact line over the particle's surface defects describes the phenomenon well. Here we probe whether the defects are surface charge groups, or surface asperities. We use digital holographic microscopy to image particles at high frame rates, with 2nm precision in all three dimensions.
Keywords: colloids, interface, molecular kinetic theory

50. Wilking; Vasily Zaburdaev, Michael De Volder, Richard Losick, Michael Brenner, David Weitz

Harvard University
"Wrinkly Biofilms: What Lies Beneath?"
Biofilms are multi-cellular communities of bacteria embedded in a self-produced extracellular matrix, which form on surfaces. Bacillus subtilis is a model bacterium for investigating biofilm formation. Wild-type B. subtilis biofilms grown on agar exhibit a characteristic, wrinkled structure. We explore the complex architecture of these biofilms and discover well-defined structural features that impact liquid flow and appear to play a role in enhanced transport.
Keywords: biofilms, nutrient transport, advection

51. Ziblat; Dave Weitz

"Elucidating lipid domains function by protein-lipid matching "
Cell membranes consist thousands of lipid species differing by chemical structure that segregate into distinct domains and selectively incorporate or exclude proteins. Their complex metabolism and transport are regulated in humans by ~1000 genes and impairment of one may lead to severe pathologies. Despite the critical role of lipid domains, very little is known of their interactions with proteins, due to the limited tools that exist nowadays. We attempt to provide a new research tool that determines out of the thousands possible lipid domains, to which domains a protein associates with.
Keywords: lipid domains, protein interactions, liposomes

52. Zieringer; C. Holtze, D. A. Weitz

Harvard University
"Copolymerization in Microfluidic Devices"
We use microfluidics to control the composition of copolymers.
Keywords: Copolymers, Microfluidics, Radical Polymerization

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