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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

37th New England Complex Fluids Meeting
Harvard University | Friday, December 5, 2008
Registration deadline: Wednesday, December 3, 2008
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Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Evelyn N. Wang

Massachusetts Institute of Technology
"Tunable Nanoengineered Surfaces for Microfluidics and Energy"
Developing and utilizing nanostructures to control fluidic behavior has attracted significant interest for a variety of applications including thermal management, energy, and lab-on-a-chip. In particular, such nanostructures can be used to create superhydrophobic, superhydrophilic, and tunable surfaces for droplet/bubble manipulation and liquid spreading. Fundamental understanding of the interactions of the fluid with these highly complex surfaces is limited, but is in significant demand for technological development and innovations.

In this work, we investigate both the effect of spreading on superhydrophilic surfaces and droplet manipulation on superhydrophobic surfaces. We fabricated silicon nanopillars ranging from 200 nm to 800 nm in diameter and heights of approximately 5 µm; the spacings between the pillars ranged from 800 nm to 2 µm. For the superhydrophilic surfaces, we experimentally characterized liquid spreading on the nanostructures using diffraction limited microscopy and with an environmental scanning electron micrograph. We observed a multi-layer spreading effect due to the features on the side walls of the nanostructures. Simultaneously, we developed an energy-based model to understand the effect of pillar spacing, height, and diameter on spreading dynamics. For the superhydrophobic surfaces, we coated the nanostructures with a silane chemistry to achieve contact angles greater than 150 degrees with water droplets. We investigated the ability to dynamically control fluid-nanostructure interactions via voltage and current modulation. We demonstrated reversible droplet manipulation from a non-wetted state (>90 degrees) to a wetted state (<90 degrees) by electrowetting with a voltage applied across the droplet, and heating with a short pulse of current through the nanostructured substrate. The mechanism associated with the droplet reversibility was investigated with experimental techniques including high-speed imaging and infrared thermometry, and with model development.
Keywords: nanostructures

2. Invited Speaker; Robert J. Wood

Harvard University
"Robotic Flies"
We seek to elucidate how to apply biological principles to the creation of robust, agile, inexpensive robotic insects. However, biological inspiration alone is not sufficient to create robots that mimic the agile locomotion of their arthropod analogs. This is particularly true as the characteristic size of the robot is decreased: to create dynamic, highly articulated robotic insects, we must explore novel manufacturing paradigms, new forms of actuation and sensing, and alternative control strategies for under-actuated, nonlinear, computationally-limited systems. This talk will highlight research in the Harvard Microrobotics Lab aimed at creating a flying robotic insect the size of a housefly.
Keywords: Robotic Flies

3. Invited Speaker; Hera Vlamakis

Havard Medical School
"Cellular Differentiation in Bacterial Biofilms"
Many microbial populations differentiate from free-living planktonic cells into surface-associated multicellular communities known as biofilms.  The remarkable architectures of many different laboratory-grown biofilms suggest a high degree of cellular specialization within these communities. Bacillus subtilis is an ideal model system for the study of differentiation within a multicellular community because several developmental events are already well defined in dispersed cultures of this bacterium. Within a biofilm, motileB. subtilis cells differentiate into non-motile chains of cells that form parallel bundles held together by an extracellular matrix. These bundles eventually produce aerial structures that serve as preferential sites for sporulation. By analyzing strains harboring multiple cell-type specific promoter fusions we have visualized the spatial anatomy of three physiologically distinct cell populations within mature biofilms. Motile, matrix-producing, and sporulating cells localize to distinct regions within the biofilm and the localization and percentage of each cell type is dynamic. Mutants unable to produce extracellular matrix form unstructured biofilms that are deficient in sporulation. This suggests that formation of an architecturally complex structure is a pre-requisite for cellular differentiation.
Keywords: biofilms

4. Invited Speaker; Steven Becker

RainDance Technologies
"The Unique Power of Droplet-based Biological Research"
RainDance Technologies is a provider of innovative microdroplet-based solutions for human health and disease research. The speed and simplicity of the exciting new technology by RainDance enables researchers to design experiments in ways that were previously unaffordable or unimaginable.

The RainStormTM technology combines the simplicity of individual reactions with the speed of processing 10 million picoliter droplets per hour. Each droplet is the functional equivalent of a test tube and can contain a single molecule, reaction, or cell. The platforms four unique capabilities inspire scientists to rethink their approach to biomedical discovery.

  • Encapsulated samples in droplets are easily generated from and returned to standard lab workflows
  • Highly repetitive assays increase statistical power and improve data quality
  • Molecular libraries allow processing of a variety of compounds in droplets to create massive single-plex reactions
  • Small sample size enables near-native biology studies

  • Sequence Enrichment, the RainDance initial solution, represents a true paradigm shift forward in the targeted sequencing of the human genome, one of the fastest growing segments of the $1 billion DNA sequencing market. The solution allows researchers to isolate specific regions of the genome for subsequent sequencing. It will enable the high-resolution analysis of genetic variation between individuals and populations at a level unmatched by current methodology. The RainDance utilization of RainStorm droplets in this application will serve as a model for future droplet-based applications.

    5. Invited Speaker; Daniel Fletcher

    University of California at Berkeley
    "Microfluidic Jets for Medicine and Biology"
    Efforts to treat diseases and engineer biological systems often require precise control of fluids at the micron scale. Low-speed delivery of drugs by pumping and diffusion is one major application area, but high-speed liquid jets that can cut or deform tissue are of growing interest. This talk will discuss recent work building and using micron-scale liquid jets for applications in medicine and biology. In particular, I will discuss piezoelectric-driven microfluidic jets that are used to cut tissue and deliver drugs to specific depths, as well as jets that are used to encapsulate proteins within unilamellar vesicles to create cell-like devices. As their transient fluid dynamics and interactions with deformable biological materials become better understood, microfluidic jets may become a central tool for engineering biology.

    Sound Bites

    1. Amy Rowat; Jacy Bird, David A Weitz

    Dept of Physics/SEAS
    "Tracking single cell lineages using a microfluidic device"
    Populations of genetically identical cells exhibit variation during differentiation, development, and in response to environmental stress. In some cases, phenotypes persist across multiple generations of cells: protein levels that vary with periods longer than the typical generation time provide a non-genetic mechanism for inheritance on timescales faster than genetic mutations occur. To study protein levels in single cells over multiple generations, we have developed a microfluidic device to spatially pattern and grow lineages deriving from single cells. We construct pedigree maps showing temporal variations in protein levels for an entire cell lineage. With this simple system, we can compare the temporal expression patterns of different classes of proteins, including mechanisms for non-genetic inheritance. Our device also enables the study of any biological phenomena as a function of replicative age and cell genealogy.
    Keywords: microfluidics; biophysics; protein expression

    2. Barry; Zvonimir Dogic

    Brandeis University
    "Chiral Influences in a Self-Assembling System"
    Chirality can have far reaching consequences in self-assembling systems. By combining observations made at the molecular and continuum lengthscales, I will describe how weak chiral interactions in a system of model rod-like particles, the filamentous bacteriophage fd, influence self-assembly pathways and dictate final self-assembled structure.

    3. Basu; Germano S. Iannacchione

    Worcester Polytechnic Institute
    "Carbon nanotube dispersed liquid crystal: A nano electromechanical system"
    Electric field induced director orientation of a nematic liquid crystal(LC)+carbon nanotube(CNT) system reveals insights on switching behavior for this anisotropic composite. Once the field goes off, the LC+CNT system relaxes back to the original orientation through a mechanical rotation, revealing the intrinsic dynamics. LC molecules and CNTs cooperatively form local pseudonematic domains in the isotropic phase due to strong LC-CNT interactions. These field-responsive anisotropic domains do not relax back to the original orientation on switching of the field off, which could find potential applications in memory devices.
    Keywords: Liquid Crystals, Carbon nanotubes, Nanotechnology

    4. Campas; L. Mahadevan

    Harvard University
    "Tip growth of pollen tubes"
    Plant cells have the ability to remodel their shape while sustaining an internal turgor pressure that can reach values up to 10 atmospheres. It is recognized that this requires a tight and simultaneous regulation of cell wall assembly and mechanochemistry, but it remains unclear how it is played out. Here we study the interplay between growth and mechanics in shaping a walled cell, in the particularly simple geometry of pollen tubes, which grow via the assembly and expansion of cell wall at the apical region of a cylindrically shaped tubular structure. We model the observed irreversible expansion of the cell wall during growth as the extension of an inhomogeneous viscous fluid shell under the action of turgor pressure, fed by a material source in the neighborhood of the growing tip. Our theory allows us to determine the radius of the pollen tube and its growth velocity in terms of the relevant physical parameters, including the turgor pressure, the secretion rate and rheology of the cell wall material. We find that a single dimensionless parameter, which characterizes the interplay between cell wall assembly and expansion, is sufficient to qualitatively understand the observed variability in pollen tube shapes. Our description provides a framework to understand tip growing cells of different species, both in plants (pollen tubes, root hairs, et cetera) and fungi (hyphal growth), and can be generalized to other cell shaping mechanisms such as diffuse growth.
    Keywords: morphogenesis

    5. Chao; Alice Gast, Klavs Jensen

    "Sphingomyelinase-induced phase transformations and spatial morphology switches in model raft membranes"
    We study the regulation of lipid membrane heterogeneity by the enzyme called Sphingomyelinase (SMase). This enzymatic reaction has been suggested to reorganize the lipid membrane morphology for many cellular processes. The phase behavior induced by SMase is complicated by the composition change by the reaction, the enzyme’s selective binding to different phases, and the substrate’s accessibility and replenishment in different phases. We report a series of phase transitions induced by SMase in model raft membranes over biologically relevant time scales. We identify a reaction-induced and a solvent-mediated-like phase transformation that act to switch the systems between three stationary lipid membrane morphologies, which may assume different functional states. We further demonstrate how the physiological parameters can tune the spatio-temporal membrane morphology based on the identified phase transformation mechanisms.

    6. Coupier

    Université de Grenoble
    "Solute concentration profiles around vesicles in microchannels"
    Vesicles are closed lipid membranes, whose deformability gives them the possibility to migrate across the streamlines in a channel. When centered in the channel, a vesicle flows with an axisymmetric steady shape. We use a digital holographic microscope to measure instantaneously its optical thickness, from which we deduce the solute concentrations inside and in the vicinity of the vesicle. An unexpected gradient along the longitudinal axis is observed.

    7. Daniel Reeves; Keith Cheveralls, Jane Kondev

    Brandeis University
    "Biochemistry on a leash: regulation of biochemical reaction rates by flexible tethers"
    Bimolecular reaction rates in both synthetic and natural systems are highly regulated. One method of regulation, as illustrated by the actin capping protein formin, uses flexible tethers to confine ligand binding sites in the vicinity of the reactive site. We describe this mechanism with two approaches. The first approach models the tether explicitly as a semi-flexible polymer. The second approach uses a potential (e.g. hard wall potential), to constrain the motion of the receptor within a confining volume. We find steady state binding rates in both models theoretically, using a combination of analytic and numerical techniques, and compare them. We explore how the reaction rates are regulated by parameters of the model such as the length of the polymer tether, and use our findings to explain the key features of the formin system. Finally, we suggest other systems, both synthetic and natural, in which these mechanisms for regulating bimolecular reactions might be at play.
    Keywords: diffusion reaction tether bimolecular reaction rate

    8. Dipti ; Marina Ruths

    "Shear-induced Rearrangement of Gold Nanoparticles on Self Assembled Monolayers of p-Aminothiophenol"
    Self-assembled monolayers (SAMs) of p-aminothiophenol (ATP), formed on template-stripped gold substrates, were used to study the adsorption of single layers of gold nanoparticles (30 nm). A number of adsorbed gold nanoparticles in the form of nanoclusters can be seen on the surface after 24 hours immersion time in the gold suspension (1.28*10-10 M). As a nN force was applied on the nanoclusters with an AFM tip, a shift in the lateral position of the nanoparticles was observed. This shift increases and changes the shape of the nanocluster as the force increases on the nanoparticles. This shift can be explained in terms of a sliding-rolling model where some nanoparticles roll over their neighbor nanoparticles as the stress increases on the nanocluster.
    Keywords: Gold Nanoparticles, Lateral Shear, SAM, ATP, AFM

    9. Dipti

    "“A Solution of Backlight Bleed Drawback of LCDs can be seen by Long Range Ordered Nematic Domain of Liquid Crystal 8CB”"
    Most common liquid crystal device (LCD) uses nematic to isotropic phase transition of the liquid crystal to get more vibrant images and better contrast ratios in terms of how deep their blacks are. A contrast ratio is the difference between a completely on and off pixel, and LCDs can have "backlight bleed" where light (usually seen around corners of the screen) leaks out and turns black into gray. Completely on and off pixel can be related to the orientation of liquid crystal domain from nematic to Isotropic transition. This research focuses how long range ordered nematic domains can affect the nematic to isotropic phase transition of aligned 8CB liquid crystal and brings a quicker and early occurrence of nematic to isotropic phase transition with smaller wing tales.
    Keywords: : Liquid Crystal, Magnetic Field Effect on Phase Transition, LCDs, Backlight Bleed.

    10. Eah; Matthew N. Martin

    Rensselaer Polytechnic Institue
    "Charged Nanoparticles in Non-Polar Solvents"
    We present 2D monolayers of gold nanoparticles floating at the interface of a toluene drop. Our gold nanoparticles are extremely special as they form a monolayer and float at the air-toluene interface. This allows for the simple self-assembly of 2D monolayers on any substrate, and is not limited in any way by the substrate’s size or shape. Since the nanoparticles are extremely mobile at the air-toluene interface, we can observe the process of monolayer formation by eye. Several important features of this self-assembly method have been discovered, although it is still mysterious why our gold nanoparticles are negatively charged in non-polar solvents. Not only is our method fast and highly reproducible, it is also green, leaving no byproducts after synthesis. We will briefly present our synthesis procedure and self-assembly method, and show videos of nanoparticles floating at the air-toluene interface and self-assembly.
    Keywords: self-assembly, nanoparticle, air-liquid interface, chemical synthesis

    11. Eric Dufresne; Wilfried Engl, Kevin Wallenstein

    "Imaging Stress in Drying Colloidal Films"
    We use traction force microscopy to measure stresses in drying colloidal films. We'll show some preliminary images of the stress distribution at a delamination front and along an opening crack.

    12. Fu; Charles Wolgemuth, Thomas Powers

    Brown University
    "Theory of swimming filaments in viscoelastic fluids"
    Motivated by our desire to understand the biophysical mechanisms underlying the swimming of sperm in the non-Newtonian fluids of the female mammalian reproductive tract, we examine the swimming of filaments in the nonlinear viscoelastic upper convected Maxwell model. We obtain the swimming velocity and hydrodynamic force exerted on an infinitely long cylinder with prescribed beating pattern. Viscoelasticity tends to decrease the swimming speed. We discuss how these results affect what type of swimming motions can effectively propel microorganisms. We also use these results to examine the swimming of a simplified sliding-filament model for a sperm flagellum, in which hydrodynamic forces play a crucial role in determining beating patterns.

    13. Greg Huber

    UCONN and UCHC
    "Diffusion in a dynamic crowded space"
    An interesting power law is found to relate diffusivities in crowded spaces where the occluded clusters are time dependent.
    Keywords: diffusion, anomalous diffusion, cytoplasm, crowding

    14. Guanglai Li; Jay X. Tang

    Brown University
    "Accumulation of microswimmers near surface due to steric confinement and rotational Brownian motion"
    Microscopic swimmers display some intriguing features dictated by Brownian motion, low Reynolds number fluid mechanics, and boundary confinement. We re-examine the reported accumulation of swimming bacteria or bull spermatozoa near the boundaries of a fluid chamber, and propose a kinematic model to explain how collision with surface, confinement and rotational Brownian motion give rise to the accumulation of micro-swimmers near a surface. In this model, an elongated microswimmer invariably travels parallel to the surface after hitting it from any incident angle. It then takes off and swims away from the surface after some time due to rotational Brownian motion. Based on this analysis, we obtain through computer simulation steady state density distributions that reproduce the ones measured for the small bacteria E.coli and Caulobacter crescentus, as well as for the much larger bull spermatozoa swimming near surfaces. These results suggest strongly that Brownian dynamics and surface confinement are the dominant factors for the accumulation of microswimmers near a surface.
    Keywords: microswimmer, Brownian motion, accumulation near surface

    15. Haiyi; Mahadevan L

    School of Engineering and Applied Science, Harvard
    "The shape of leave"
    The undulating morphology of leaves is now accepted as a consequence of differential growth of the underlying tissue. Various qualitative and quantitative aspects of the buckling patterns may thus be ascribed to the distribution of non-uniform growth in the lamina, and have been demonstrated in normal and mutant leaves, as well as in physical models thereof. To understand the different modalities that arise quantitatively, we construct a mathematical model for the stability of an initially flat elastic ribbon with gradients in growth. Using a combination of analysis, numerical simulation, and experimental observations, we map out the phase space of possible shapes for these growing ribbons. In general, we find that as the relative growth strain is increased, the ribbon-like structure first switches to a catenoidal shape and then undulating edges. Our framework allows us to delineate the few macroscopic parameters that control the morphology of elongated leaves and flower petals and helps to explain the large variety of observed shapes.
    Keywords: leaf; growth; wrinkle

    16. Hyeran Kang; Dhananjay Tambe, Vivek Shenoy and Jay X. Tang

    Brown University
    "Mechanics of biomimetic systems propelled by actin comet tails"
    The motility of intracellular bacterial pathogens such as Listeria monocytogenes is driven by filamentous actin comet tails in a variety of trajectories. Here, we present the in vitro study on the actin-based movements using spherical beads of different sizes coated with VCA protein, a partial domain of N-Wasp, in platelet extracts. Long term two-dimensional trajectories of the spherical beads motility show characteristic difference than those observed for bacteria, which have both elongated shape and asymmetric expression of the polymerization inducing enzyme. The trajectories also vary sensitively with the bead size and shape. These results provide a useful test to our new analytical model including the rotation of the bead relative to the tail.
    Keywords: actin comet tails, long term 2D trajectory

    17. Jang

    "Polymer Nanotubes"
    Polymer nanotubes and nanowires have been widely studied because of their promising mechanical, thermal, electrical, and optical properties. The nano-scale membrane template method has been adopted to synthesize the nanotubes and nanowires. Using layer-by-layer assembly, the sequential deposition of cationic and anionic polyelectrolytes on nano-scale membrane templates is used to build the polymer nanotubes. In the present work, we focus on the fabrication of polyethylene oxide nanotubes which will have uniform microstructure. The optimal fabrication conditions will be studied: deposition time, temperature, pH, and ionic strength. Physical, electrical, and optical techniques will be used to characterize the properties of the nanotubes.
    Keywords: Nanotubes

    18. Johannes Soulages; J. Hostettler, G. H. McKinley

    "New opposed-nozzle fixture for measuring the extensional properties of low viscosity liquids."
    We describe the development of a new opposed jet device for measuring the apparent extensional viscosity of low-viscosity fluids at high deformation rates. The design builds upon the original opposing-jet device of Cathey & Fuller that was commercialized as the RFX instrument by Rheometrics (now TA Instruments). In contrast to the RFX stand-alone system, the new opposed-nozzle fixture can be mounted onto a controlled strain rheometer such as the ARES or ARES-G2. It consists of two opposed jets mounted on rigid arms and terminated with interchangeable disposable syringe needle tips. The first arm is directly connected to the force rebalance transducer (FRT) sensor of the ARES while the second one is attached to a three-axis translation stage for accurate positioning and alignment of the two opposing nozzles. The test fluid is forced out under pressure as two free jets through the opposed needle tips by the action of a twin syringe pump. The resulting stagnation flow generates an approximately uniform biaxial extension field and the resulting torque acting on the arm connected to the FRT is recorded as a function of time for each imposed volumetric flow rate. In this initial feasibility study, particular attention is paid to the coupling arrangement between the tubing from the twin syringe pump and the nozzle arm connected to the FRT so that a minimum residual torque resistance is guaranteed. Different size nozzles and moment arms are investigated to explore the operating envelope and stability boundaries of the FRT. Apparent extensional viscosity data are presented for a glycerol/water mixture.
    Keywords: Extensional rheometry; elongational viscosity; opposed nozzles

    19. Kanai; P.J. Lu, E. Sloutskin, D.A. Weitz

    SEAS, Harvard University
    "Crystallization of Charged Colloids in Nonpolar Solvents Enabled by Reverse Micelles"
    We report that adding micelles yields strong electrostatic repulsion enough to crystallize the particles in nonpolar solvents. The micelles function both to add charge to particles and to screen the electrostatic interaction among particles. This enables wide-range control of electrostatic interaction, leading to the observation of a reentrant behavior with respect to the micelles concentration.
    Keywords: Charged colloids, Crystallization, Micelles, Confocal microscopy

    20. Kaoui; Chaouqi MISBAH

    CNRS - Universite de Grenoble I
    "Modeling and simulation of vesicle dynamics"
    We study dynamics and deformation of a vesicle (a closed phospholipid membrane) under Poiseuille and shear flows. For this purpose we use simulations (boundary integral and lattice-Boltzmann methods) as well as analytical calculation (small deformation theory). The boundary integral method is suitable to simulate dynamics of a vesicle suspended in infinite fluid, therefore we use it to study the lateral migration of a vesicle placed in unbounded Poiseuille flow. On the other hand, the lattice-Boltzmann method is suitable to simulate a vesicle flowing in confined geometries (e.g. a micro-channel). For testing our code, we compute the known vesicle equilibrium shapes and we obtain the known tank-treading motion of a vesicle under shear flow. The small deformation theory is used to derive two dynamical equations governing the orientation and the deformation of a vesicle. These equations are used to obtain a phase-diagram summarizing all the known dynamical modes (tank-treading, tumbling and vacillating-breathing) that a vesicle exhibits under shear flow.
    Keywords: Vesicle, simulation, modeling, Poiseuille flow, shear flow

    21. Katherine Humphry; David Issadore, Keith A. Brown, Lori Sandberg, Robert M. Westervelt, and David A. Weitz

    Department of Physics, Harvard University
    "Microwave Dielectric Heating of Drops in Microfluidic Devices"
    We present a technique to locally and rapidly heat aqueous drops in microfluidic devices with microwave dielectric heating. Water absorbs microwave power more efficiently than polymers, glass, and oils due to a lossy dielectric response at GHz frequencies. We demonstrate microwave dielectric heating in a microfluidic device that integrates a flow-focusing drop maker, drop splitters, and metal electrodes that locally deliver microwave power from an inexpensive and commercially available 3.0 GHz source and amplifier. The relevant heat capacity of the system is a single thermally isolated aqueous drop with volume ~30 pL, enabling very fast thermal cycling. We measure characteristic drop heating times as short as 15 ms for temperature increases as large as 30°C above the base temperature of the microfluidic device. Lab-on-a-chip implementations of biological and chemical assays that require rapid and local control of temperature, such as PCR amplification of DNA, can benefit from this new technique.
    Keywords: microfluidics, microwave heating, lab-on-a-chip

    22. Kolinski; Aussilous, P. Mahadevan, L

    School of Engineering and Applied Sciences, Harvard
    "Rolling Rucks in Rugs"
    The static stability of ripples, or rucks, in an elastic sheet is analyzed numerically and experimentally. For a ripple on a horizontal plane, a non-linear scaling of height vs. applied length change is demonstrated, where height scales as [applied length change]^(1/3). We proceed to show the existence of stable ruck states on a plane at finite angle of inclination. The dynamics of the ruck follow naturally; at steeper angles of inclination, we observe a critical angle for the onset of motion. The shape of the ruck in motion will be shown in a video clip. We observe a terminal velocity at all angles of inclination, and show that inertial air drag is responsible for the terminal velocity.
    Keywords: ripples, elastic sheet

    23. Lin; Amy C. Rowat, Harald Herrmann, Chase C. Broedersz, Frederick C. MacKintosh, Eleanor Millman, and David A. Weitz

    Harvard University
    "Elastic Properties of Vimentin Networks"
    We measure the mechanical properties of in vitro networks of the intermediate filament protein vimentin by rheometry. Vimentin networks are highly elastic even for small volume fractions of protein and exhibit dramatic stiffening with strain. We find that divalent ions such as Ca2+ and Mg2+ act as effective cross-linkers in the vimentin network. The observed linear and nonlinear elastic responses at intermediate strains can be explained quantitatively by affinely stretching the entropic fluctuations of single semiflexible filaments; at high strains, enthalpic stretching of the individual filaments contributes to the observed nonlinear response. To assay the degree of affine strain, we image the microscale strain field of the network under shear. We find that the degree of nonaffinity is small and depends on the concentration of both filament and divalent ions while it is insensitive to the applied strain. This result supports our interpretation of bulk measurements that the nonlinear response to strain is due to biopolymer affine stretching.

    24. Meng; Adeline Perro, Vinothan N. Manoharan

    Physics & SEAS, Harvard University
    "Optical Properties of Core-shell Colloidal Particles "
    We synthesize core-shell colloidal particles that consist of small polystyrene cores and large shells of poly(NIPAM-co-AAc). When the shell is index-matched to water at room temperature, the scattering is dominated by Rayleigh scattering from polystyrene cores. As we change the solvent condition, the shell starts to shrink and scatter light. Both the scattering cross section and the forward scattering of the particles increase, characteristic of Mie scatterers. We use optical microscopy, static light scattering, and turbidimetry to study this optical transition.
    Keywords: Core-shell Colloids, Hydrogel, Light Scattering, Turbidimetry

    25. Merrill; Sunil K. Sainis and Eric R. Dufresne

    Yale University
    "Pairwise Additivity in Colloidal Electrostatics"


    26. Millman; Jerome Fung, Kamilia Kaczor, Vinothan Manoharan

    Harvard University
    "Dynamics of Virus Assembly"
    Although much is known about the structure of viral capsids, very little is known about the dynamics of their assembly. We plan to study the dynamics of single capsid shell formation through a combination of scattering and surface plasmon resonance spectroscopy.

    27. Morrison; Introduction to Soft Matter Class of '08

    "Are clouds soft matter? Is the physics squishy?"
    From the Soft-Matter wiki at SEAS. Entry on "Lightning" There is a great book on preview in Google Books called the Microphysics of Clouds and Precipitation that has a great chapter on cloud electricity. Section 18.4 of the chapter talks about weakly charged clouds, and how the charges seem to be distributed over the volume of the cloud. One would expect that base of the cloud, since it is a poor conductor, would be negative, while the top would be full of positive ions; this actually ends up not being experimentally true in some cases, though this is not discussed in the book that I could find. Dr. Morrison did mention that depending on the nucleating particles there could be a charge variance in clouds of different geographical regions: has anyone found a reference for this? The chapter then goes on to discuss the individual particles in the cloud, and how this distribution is varied in thunder clouds versus fair weather clouds. This was an interesting graph about the electric charges on differently sized drops and types of clouds, although not all of the supporting text is available online. Looking over the book, it appears to make the case for clouds being a form of Soft Matter. Anyone have any thoughts on it?
    Keywords: Dust, drops, dispersions and discharge

    28. Morrison

    L. Mahadevan
    "Reversibility in Microfluidic Systems"
    Microfluidic systems are of great interest for a number of applications, ranging from chemistry on the nanoliter scale to information encoding. Depending on the architecture of the system, the flow of bubbles can be rather complex. A simple theoretical model for these complex microfluidic constructs treats the system as a circuit obeying Kirchhoff's rules, with each bubble adding a unit resistance to the channel it occupies. We propose a simple method to experimentally determine the resistance of an individual bubble experimentally, by passing a train of equally spaced bubbles through a single loop region made of two channels of equal length. We show that the sequence may be exactly recoverable if the train is passed through the loop, and the flow is reversed. The reversibility of the bubble train depends on the input spacing, and the range of spacings that can be recovered is determined.
    Keywords: Microfluidics

    29. Nelson

    Harvard University
    "Gene Segregation in Microorganisms"
    Gene Segregation in Microorganisms David R. Nelson Neutral mutations optimally positioned at the front of a growing population wave can increase their abundance via a "surfing" phenomenon. When initial variants are present in roughly equal proportions, gene segregation somewhat similar to spinodal decomposition occurs at the moving population front. Experimental and theoretical studies of this effect will be presented, using bacteria and yeast as model systems.
    Keywords: bacteria, spinodal decomposition

    30. Raad; Marjan Rafat, Debra T. Auguste

    Brown University and Harvard University
    "Fabrication of reversibly adhesive fluidic devices by magnetic adhesion"
    Fluidic devices are typically made by fabricating polydimethyl siloxane (PDMS) for irreversible bonding to a glass substrate by plasma treatment. However, this method limits the range of materials for fluidic device fabrication. We present a simple and inexpensive method to fabricate fluidic devices without plasma treatment. We use magnets to reversibly adhere PDMS to substrates. This approach enables fluidic devices to be fabricated with materials other than PDMS and glass, such as hydrogels. Moreover, our method can be used to fabricacte composite devices as well as three-dimensional scaffolds and hydrogel fluidic devies for tissue engineering.
    Keywords: fluidic devices, reversible adhesion, magnets

    31. Randy Ewoldt; Gareth H. McKinley, Anette E. Hosoi

    "Controllable adhesion using field-responsive yield stress fluids"
    A field-responsive 'smart' fluid, which exhibits a field-dependent microstructure corresponding to dramatic changes in material properties, can enable externally-tunable adhesion. Such adhesive properties of smart fluids are most commonly tested in simple shear deformation, but practical applications also require the material to support normal loads. We experimentally study normal loading of non-Newtonian magnetorheological fluids confined between rigid surfaces as the external magnetic field, the fluid dimensions, and the surface roughness are varied. The peak adhesive force and the mode of failure are controlled by the field-responsive nature of the magnetorheological fluid forming the adhesive layer. We demonstrate that magnets can be used for tunable and reversible adhesion to non-ferromagnetic surfaces.
    Keywords: reversible adhesive, yield stress fluid, rheology, magnets

    32. Reyssat; L. Mahadevan

    SEAS, Harvard University
    The scales of pine cones open and close in response to environmental humidity fluctuations. We study the dynamics of these motions, from the cellular scale to the whole pine cone. The underlying bilayer structure that is responsible for motion may be reproduced to build very simple biomimetic devices, which have applications ranging from sensors to oscillators, and artificial flowers.
    Keywords: pine cones, hygroscopic motion, biomechanics

    33. Romanowsky; Adam Abate, Jeremy Agresti, David Weitz

    Harvard University
    "Valve-based flow focusing for drop formation"
    We have demonstrated modulation of the size and formation frequency of drops in a microfluidic flow-focus device, using single-layer membrane valves to provide local control of the channel geometry. This control is independent of the flow rates of the fluids and can be modulated from a steady state up to hundreds of Hz. We have also made double emulsions with a controllable number of inner drops, adjustable independently of volume fraction. These valves may find application for prototyping devices, for synchronizing drop trains, and for improving drop uniformity over time despite flow rate drift.
    Keywords: microfluidics, flow focus, drop, valve, modulation

    34. Sainis; Frank Vollmer

    Rowland Institute at Harvard
    "Colloidal Physics using Microresonator Cavities "
    Microscopic resonator cavity modes have sharp spectral features that are extraordinarily sensitive to any changes in the local environment and the surface of the resonator. We report on recent measurements in our group that could offer probes of colloidal physics on the nanometer length scale.
    Keywords: Colloidal Physics, Microresonators

    35. Schneider

    University of Augsburg & the Center of Nanoscience Munich
    "Driven Microluidics in bulk and soft interfaces"
    Acoustic driven flow is used to mimic blood flow in our microcirculatory system. We unravel the principles of self organized blood clotting and the initial steps of cell adhesion. From Bulk to Interfaces Ultra large giant unilamellar vesicles U-GUVs are designed to study the impact of shear stress on the hydrodynamics inside the membrane plane and the lateral transport along one dimensional domain boundaries.
    Keywords: microfluidics, acoustic driven flow, lipid membranes

    36. Seila Selimovic; Frédéric Gobeaux, Seth Fraden

    Brandeis University
    "Microfluidic PhaseChip for Exploration of Concentration / Temperature Phase Diagrams"
    We have developed a PDMS PhaseChip consisting of a storage layer for 1000 independent aqueous droplets and a control layer containing a linear gradient of salt solutions. Because of the difference in chemical potential of water in the two layers water passes through the PDMS membrane, giving us excellent control over the solute concentrations inside the droplets. We operate the device on a robotic stage with a spatial temperature gradient. Thus, we can simultaneously conduct a large number of different concentration / temperature experiments to map out a phase diagram. This is exemplified on a PEG/salt system.
    Keywords: microfluidics, phase diagram

    37. Sharma; (Mohan Srinivasarao)

    Massachusetts Institute of Technology (Georgia Institute of Technology)
    "Breath-figure-templated assembly of ordered arrays of pores in polymer films"
    Breath figures formed on evaporating polymer solutions exposed to the blast of humid air involve growth and self-assembly of water drops that are non-coalescent. The hexagonally close packed, nearly monodisperse drops, eventually evaporate away, leaving a polymer film with ordered array of pores. We provide the first quantitative attempt aimed at the elucidation of the mechanism of this breath-figure-templated assembly. The dynamics of drop nucleation, growth, noncoalescence and self-assembly are modeled by accounting for various transport and thermodynamic processes. The theoretical framework developed in this study allows one to rationalize and predict the structure and size of pores formed in different polymer-solvent systems under given airflow conditions.
    Keywords: Breath figures, noncoalescence, porous polymer films,

    38. Sigdel; Germano S. Iannacchione

    "Calorimetric Study of the Isotropic–Nematic and the Nematic–Smectic-A Phase Transitions in Carbon Nanotubes and Liquid Crystal Composites:"
    A high-resolution calorimetric study of the isotropic to nematic (I-N) and the nematic to smectic-A (N-SmA) phase transitions of carbon nanotubes (CNTs) and liquid crystal octyl-cyanobiphenyl (8CB) composites (8CB+CNTs) as a function of CNTs concentration is reported. Scans were performed on heating and cooling for all samples (0.5–6 wt% of CNTs) over a wide temperature range well above and below the two transitions in pure 8CB. Both the I-N and the N-SmA transitions evolve in character and have their transition temperatures shift lower as the wt% of CNTs increases. For intermediate wt% values, new transitions features are observed, which suggest new phase ordering of the CNTs within the liquid crystal host.
    Keywords: Calorimetry, Liquid crystal, carbon naotubes

    39. Sigolene Lecuyer; Kumaran Ramamurthi, Howard A. Stone, Richard Losick

    Harvard University
    "Geometric cue for protein localization in a bacterium"
    The cues driving localization of proteins in cells or bacteria are frequently mysterious. We bring evidence that the membrane protein SpoVM recognizes a geometric cue in Bacillus subtilis bacteria. In vivo experiments show that SpoVM localizes to a particular patch of the inner membrane in sporulating Bacillus subtilis: the convex surface of the developing spore. Our in vitro experiments strongly support the hypothesis that this localization is driven by geometry rather than biochemical recognition. When purified SpoVM is incubated with polydisperse micrometer-sized DOPC vesicles, we observe that the protein preferentially adsorbs on smaller vesicles, of diameter similar to the size of the bacterial spore (∼ 1 μm). Using fluorescent GFP-tagged SpoVM, we quantify the amount of adsorbed protein by confocal microscopy. Our results, when interpreted using existing protein adsorption models, suggest the existence of a cooperative adsorption mechanism for high enough membrane curvature, which involves the formation of small clusters of proteins.
    Keywords: lipid vesicles, membrane protein, bacteria

    40. Sloutskin; P. J. Lu and D. A. Weitz

    SEAS and Physics Department (Harvard)
    "Non-spherical crystal nuclei are stabilized by entropy"
    Early nuclei of hard sphere crystals, forming in a colloidal suspension, have non-compact and non-spherical shapes. The balance between fluid-solid interfacial tension and conformational entropy determines the morphology of these nuclei, and alters their size distribution. The size distribution, deviating from early theoretical predictions, may be responsible for the experimental nucleation rates, which are higher by orders of magnitude compared to predictions of classical nucleation theory.
    Keywords: nucleation, colloids, confocal microscopy

    41. Thomas Angelini; Michael Brenner, David Weitz

    SEAS, Harvard
    "Matrix Production in Response to Nutrient Depletion in Bacillus Subtilis Biofilms"
    In Bacillus Subtilis biofilms, a transition occurs in the rate of extra-cellular polysachharide (EPS) production. At early stages of development, when the biofilm is thin, a low level of matrix is expressed. At later stages, when the biofilm has thickened, EPS production is dramatically increased. This transition could be a response to nutrient depletion, as there must be a critical biofilm thickness, above which nutrients cannot diffuse into the center of the biofilm before being consumed by cells at the edge. Here we quantify biofilm size and shape during the early stages of Bacillus Subtilis biofilm growth, simultaneously monitoring matrix expression levels. We show that the critical biofilm size scales with nutrient concentration as expected by a simple nutrient depletion model.
    Keywords: biofilm

    42. Underhill

    "Mechanism for collective behavior in suspensions of swimming microorganisms"
    Large collections of swimming microorganisms exhibit collective behavior including large scale fluid structures, mixing, and swimming velocities larger than a single organism can produce. These phenomena should have important implications for cellular processes such as chemotaxis, quorum sensing, and biofilm formation. We have performed large scale computer simulations of > 100,000 organisms including multi-body hydrodynamic interactions. The hydrodynamic interactions are sufficient to produce collective behavior. We have also shown the mechanism by which the orientational correlations between swimmers lead to collective behavior.
    Keywords: swimming microorganisms, hydrodynamic interactions

    43. Veatch

    Cornell University
    "Critical fluctuations in biomembranes"
    Multicomponent lipid bilayer membranes can contain two coexisting liquid phases, named liquid-ordered and liquid-disordered. Recently, we demonstrated that large (micron-scale) and dynamic critical fluctuations are found in ‘simple’ ternary bilayer membranes prepared with critical compositions. Remarkably, robust critical behavior is also found compositionally complex biomembrane vesicles isolated directly from living cell plasma membranes. Current work focuses on characterizing lateral heterogeneity in intact cells, and investigating possible functional implications of critical behavior in membrane biology.
    Keywords: lipid raft, liquid-ordered, cholesterol

    44. Vlahovska; Gerrit Danker, Chaouqi Misbah

    "Vesicles in Poiseuille flow"
    Blood flow in the microcirculation depends crucially on the migration of red blood cells towards the flow centerline. We develop an analytical theory that sheds light on the migration mechanisms. Our results identify the ratio between the inner and outer fluid viscosities as the main controlling parameter; the membrane bending resistance is of less importance. At low viscoisty ratios, the vesicle deforms into a tank-treading shape, which is an ellipsoid if the vesicle is far-away from the flow centerline. The resulting migration is always towards the flow centerline, unlike other soft particles such as drops. Above a critical viscosity ratio, the vesicle tumbles and cross-stream migration is suppressed. The theory predicts a surprising coexistence of two types of shapes at the centerline, a bullet-like and a parachute-like shape.

    45. Wong; Ken Halvorsen

    Harvard University, The Rowland Institute at Harvard
    "Beyond the frame rate: Measuring high-frequency fluctuations with light intensity modulation"
    Power spectral density measurements of any sampled signal are typically restricted by both acquisition rate and frequency response limitations of instruments, which can be particularly prohibitive for video-based measurements. We have developed a new method called Intensity Modulation Spectral Analysis (IMSA) that circumvents these limitations, dramatically extending the effective detection bandwidth. We demonstrate this by video-tracking an optically-trapped microsphere while oscillating an LED illumination source. This approach allows us to quantify fluctuations of the microsphere at frequencies over 10 times higher than the Nyquist frequency, mimicking a significantly higher frame rate.
    Keywords: particle tracking, signal processing, optical tweezers

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