NECF Meeting Abstracts
49th New England Complex Fluids Workshop
Harvard University | Friday, December 2, 2011
Registration deadline: Wednesday, November 30, 2011
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Abstracts for Invited Talks and Sound Bites:Invited Talks
"2 > 1: Cooperative assembly in drug delivery"
Cell surfaces are heterogeneous and dynamic. Molecular level control over membrane surface chemistry allows leukocytes and cancer cells to alter their adhesion - having significant effects on disease progression. We will show that cells orchestrate the density and organization of proteins and lipids to govern adhesion and chemotaxis. Vehicles - engineered using liquid-solid phase coexistence – are designed to complement the organization of molecules on cells to achieve strong, cooperative binding. We will demonstrate that heterogeneity is a key biophysical phenomenon that may be used to direct cell behavior.
"Watching the motions of the cell wall synthesis machinery and underlying cytoskeleton in B. subtilis with high precision particle tracking"
Rod-shaped bacteria elongate by the action of cell-wall synthesis complexes linked to underlying dynamic MreB filaments, but how these proteins function to allow continued elongation as a rod remains unknown. To understand how the movement of these filaments relates to cell wall synthesis, we characterized the dynamics of MreB and the cell wall elongation machinery using high-resolution particle tracking in Bacillus subtilis. We found that both MreB and the elongation machinery move in linear paths across the cell, moving at similar rates (~20nm / second) and angles to the cell body, suggesting they function as single complexes. These proteins move circumferentially around the cell, principally perpendicular to its length. We find that the motions of these complexes are independent, as they can pause and reverse,and also as nearby complexes move independently in both directions across one surface of the cell. Inhibition of cell wall synthesis with antibiotics or depletions in the cell wall synthesis machinery blocked MreB movement, suggesting that the cell wall synthetic machinery is the motor in this system. We propose that bacteria elongate by the uncoordinated, circumferential movements of synthetic complexes that span the plasma membrane and insert radial hoops of new peptidoglycan during their transit.
Invited; Stephen Quake
"Precision Measurement in Biology"
Is biology a quantitative science like physics? I will discuss the role of precision measurement in both physics and biology, and argue that in fact both fields can be tied together by the use and consequences of precision measurement.
The elementary quanta of biology are twofold: the macromolecule and the cell. Cells are the fundamental unit of life, and macromolecules are the fundamental elements of the cell. I will describe how precision measurements have been used to explore the basic properties of these quanta, and more generally how the quest for higher precision almost inevitably leads to the development of new technologies, which in turn catalyze further scientific discovery. In the 21st century, there are no remaining experimental barriers to biology becoming a truly quantitative and mathematical science.
"Magnetic fields and soft matter – directing self-assembly of block copolymer, nanowire, and surfactant mesophases"
The ability to transform matter that displays novel physics and properties into useful materials and devices is indivisibly linked to the ability to reliably control structure on length scales of interest. This has been well advanced in hard materials where, for example, the growth of meters-long single crystals of silicon via the Czochralski (CZ) process is both indispensable and routine. By contrast, the generation of near single-crystal self-assembled soft matter with arbitrary orientations on length scales beyond 1 mm remains surprisingly challenging. Under appropriate conditions however, magnetic fields do offer the ability to direct self-assembly of soft matter systems over large length scales in a very straightforward manner, with potential applications in water purification and energy generation. Here I discuss the interaction of magnetic fields with various soft mesophases and the conditions which enable their directed self-assembly. Key points are addressed including degeneracy of alignment and overcoming interfacial effects. The role of magnetic fields on order-disorder transitions in block copolymers is examined using novel in-situ scattering studies of systems under high fields.
"Fully integrated target enrichment, DNA sequencing, and variant calling with emulsion microfluidics"
Great strides were made in the development of sequencing technologies over the past decade. Information is accumulating on individual’s genomes, and the genetic makeup of disease tissue is shedding light on genetic diversity and disease etiology. The lessons learned are increasingly translational, ranging from identification of disease causing mutations to informing of treatment options. Available technologies enable extremely high throughput sequencing, ranging from whole exomes to full genomes. However, sample preparation and sequencing time is measured in days, run costs in thousands of dollars, and data analysis in hours to weeks once a run is complete. These constraints limit adoption of DNA sequencing in the clinic. There is a need for sequencing technologies with minimal sample preparation, turnaround time, and per sample cost. To address this need we are developing a sequencing technology requiring only minutes of sample preparation time. Genomic DNA samples are injected into the device after which sample enrichment for the target DNA of interest, amplification, sequencing, and variant calling all occur on-board the instrument. In addition sequencing accuracy is high, sequence reads are long, and the run time are short.
Adams; Louise Jawerth, Tom Kodger, Shin-Hyun Kim, Vinothan Manoharan, David Weitz
Aptly named Cerberus emulsions by graduate student Louise Jawerth, these three component double emulsions are generated for the first time using microfluidic techniques. This sound bite will address the synthesis of these stable, non-spherical water-oil-water double emulsions and highlight the different configurations that are possible. As a side note, the name Cerberus comes from the Greek mythological dog that has three or more heads; unlike the two-headed Janus god.
"Adaptive Finite Element Methods for Complex Fluids"
Complex fluid systems, such as multiphase flow and magnetohydrodynamics (MHD) yield time-dependent nonlinear systems of partial differential equations that couple a fluid with other internal properties (magnetic field in MHD and phase function in multiphase flow). In this talk, I will briefly discuss the applications of various discretization methods and multilevel solvers to solve such systems of equations. The goal is to solve the systems as efficiently as possible, while approximating the physical properties of the system accurately. To accomplish this, tools such as nested iteration and adaptive refinement are used. Examples of solving various multiphase flow problems and, if time, MHD test problems using these methods will be given.
"Field-induced motion of a ferrofluid drop, direct numerical modeling"
Here, we numerically simulate the dynamic response of a sessile
ferrofluid droplet on a (nonmagnetic) substrate in the presence of an
external magnetic field. Axisymmetric shapes of sessile droplets in an
imposed magnetic field are computed either with a fixed-contact-angle
condition or with a fixed-contact-line condition. The objective of this work is to introduce a volume-of-fluid based approach for the modeling of the contact line of a ferrofluid sessile droplet with and without contact angle hysteresis when subjected to an applied magnetic field.
Keywords: ferrofluid, magnetowetting, drop
Amstad; Sujit S. Datta, Dave A. Weitz
"Pickering emulsions stabilized with core-shell nanoparticles"
Emulsions are omnipresent in our everyday life in food, cosmetic and pharmaceutical products. One of the major difficulties of polymer surfactant stabilized emulsions is limited long-term stability. Pickering emulsions are emulsions stabilized by nanoparticles. The enhanced long-term stability of Pickering emulsions compared to polymer surfactant stabilized emulsions renders them attractive for many different applications. The influence of the nanoparticle size, concentration and wettability on the emulsion stability and the drop size is well known for Pickering emulsions stabilized with hard sphere nanoparticles. However, knowledge about properties of Pickering emulsions stabilized with core-shell nanoparticles is scarce. I am using gold nanoparticles sterically stabilized with PEG-thiol and dodecyl-thiol to investigate the potential of core-shell nanoparticles to stabilize emulsions. I am investigating the influence of the gold nanoparticle core size, polymer shell thickness, polymer composition and nanoparticle concentration on the Pickering emulsion stability and emulsion drop size. I will present preliminary results on similarities and differences of emulsions stabilized with hard sphere nanoparticles, core-shell nanoparticles and polymers respectively.
Keywords: pickering emulsion, core-shell nanoparticles
"Multistable alignment of nematic liquid crystals on patterned surfaces"
Surface micro- and nano-patterning techniques offer the powerful ability to impose spatially-varying alignment boundary conditions on a liquid crystal film. Unlike conventional homogenous treatments, these systems typically have a rich variety of stable configurations and so are very promising for applications such as electronic paper. I will outline recent analytical work on determining the stable configurations of a nematic in contact with a checkerboard patterned surface where the squares alternately promote vertical and planar alignment.
Keywords: nematic, patterning, boundary conditions
Blair; Christian Santangelo, Jon Machta
University of Massachusetts Amherst
Boltyanskiy; Holger Kress, Alexia Belperron, Cecile Mejean, Charles Wolgemuth, Linda Bockenstedt, Eric Dufresne
"Mechanical Properties of the Lyme Disease Bacterium, B. burgdorferi"
Lyme disease is primarily caused by the spirochete, Borrelia burgdorferi. These spirochetes have a unique structure and an unusual swimming behavior that result in very interesting mechanical properties; elasticity and fluid mechanics are immensely important for bacteria’s motility and escape from immune cells. In this study we explore the physical properties of the spirochete membrane and the elastohydrodynamic coupling of the bacteria with the surrounding medium. We modeled the interaction of an immune cell with a spirochete by the coupling of antibody-coated microspheres with the bacterial membrane. We dragged micro beads across the spirochete surface and found the bacteria membrane to be fluid. Moreover we observed that upon bead attachment, spirochetes ballistically transport antibody-coated beads to one of their ends. This spontaneous unidirectional transport is observed only in the wild type spirochetes containing flagella. In mutant bacteria lacking flagella (flaB mutants) the movement of beads attached to the surface is merely diffusive. Subsequently we investigate how spirochetes interact with the surrounding fluid. As a first step in understanding this interaction, we study the elastic properties of B. burgdorferi. We measure the bending modulus of the cell body without flagella by exploiting two methods. In one method, using holographic optical tweezers we attach microspheres on flaB mutants and employ the three-point bending test. In the other method we oscillate one end of a flaB mutant and fit the resulting form of the bacterium to a predicted shape of an elastic filament driven in a viscous fluid.
Keywords: bacteria, membrane fluidity, elasticity
Carrillo; Andrey V. Dobrynin, Daniel Russano
University of Connecticut
"Effect of electrostatic interactions on lubrication in polymeric bottle-brush layers"
Many connective tissues, such as cartilage demonstrate excellent lubrication and wear characteristics. Cartilages in mammalian joints can withstand pressures of the order of ten atmospheres and have remarkably low friction coefficient in the range of 0.001-0.03. The surface of the cartilage is covered with bottle-brush-like polyelectrolyte layer consisting of glycoproteins. This brush layer, which faces a similar layer on the opposing cartilage, is sheared as two surfaces slide passing each other during joint motion. We have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates to understand the role of the electrostatic and hydrodynamic coupling between brush layers on the lubricating properties in biological and polymeric systems. Glycoprotein layers were modeled as two opposing layers of highly charged bottle-brush macromolecules composed of Lennard-Jones particles grafted to a substrate. Simulations have shown that charged bottle-brush systems have lower friction under shear and weaker dependence of the disjoining pressure on substrate separation than neutral bottle-brush systems.
Keywords: Polymer simulations, Polyeletrolyte brushes, bottle-brushes
Carroll; Dimiter Petsev
Harvard/The University of New Mexico
"Microparticles with Bimodal Nanoporosity Derived by Microemulsion Templating"
We show that hierarchically bimodal porous structures can be obtained by templating silica microparticles with a specially designed surfactant micelle/microemulsion mixture. Oil, water, and surfactant liquid mixtures exhibit very complex phase behavior. Depending on the conditions, such mixtures give rise to highly organized structures. A proper selection of the type and concentration of surfactants determines the structuring at the nanoscale level. Tuning the phase state by adjusting the surfactant composition and concentration allows for the controlled design of a system where microemulsion droplets coexist with smaller surfactant micellar structures. The microemulsion droplet and micellar dimensions determine the two types of pore sizes.
Keywords: Emulsion polymerization, porous materials, surfactant self assembly, microemulsion, microfluidics
"Biofilm Formation in Microscopic Double Emulsion Droplets"
In natural, medical, and industrial settings, there exist surface-associated communities of bacteria known as biofilms. These highly structured films are composed of bacterial cells embedded within self-produced extracellular matrix, usually composed of exopolysaccharides, proteins, and nucleic acids; this matrix serves to protect the bacterial community from antibiotics and environmental stressors. Here, we form biofilms encapsulated within monodisperse, microscopically-sized double emulsion droplets. The bacteria self-organize at the inner liquid-liquid droplet interfaces, multiply, and differentiate into extracellular matrix-producing cells, forming manifold three-dimensional shell-within-a-shell structures of biofilms, templated upon the inner core of spherical liquid droplets. By using microfluidics to encapsulate bacterial cells, we have the ability to view individual cells multiplying in microscopically-sized droplets, which allows for high-throughput analysis in studying the genetic program leading to biofilm development, or cell signaling that induces differentiation.
Keywords: biofilms, double emulsions, microfluidics, gene expression
Collins; Vinothan N. Manoharan
"Design Rules for DNA-Colloid Clusters"
We study bulk melting behavior of DNA-coated microspheres and the assembly of finite clusters of these particles. Experiments using 3 different species of beads, and 3 different highly specific DNA duplexes with similar melting temperatures, have lead us to a design method for DNA sequences that promote efficient isothermal exploration of configuration space, and a geometric rule describing the structure of the coarse-grained, N-particle energy landscape.
Keywords: DNA, colloid, cluster
"“Effect of Alignment on smectic A to nematic phase transition of the aligned octylcyanobiphenyl nano-liquid crystal”"
Liquid Crystals (LCs) exhibit a wide range of mesomorphic phases for long range of applications either in the bulk form or as compounds and mixtures. In the smectic LC devices, more attention has been paying to get smectic phase transition earlier with higher quality reachers are showing their interest in the laser beam steering and the optical shutter applications to know how fast the smectic phase transition can be reached. Our interest is to understand the smectic A to nematic (SmA-N) phase transition behavior in the regard of its faster response. This study shows the effect of alignment on the activated kinetics of the SmA-N phase transition of the bulk octylcyanobiphenyl (8CB) of magnetic field. A detailed thermal analysis were performed for the aligned 8CB and found a significant temperature shift in the transition peak towards higher temperature as ramp rate increases following Arrhenius behavior. This behavior gives the information of the energy dynamics of the molecular motion and rearrangement of 8CB molecules near the SmA-N transition. The presence of alignment brings faster response time, an increased energy dynamics with higher activation.
Keywords: Nano-Liquid Crystals, Thermal Analysis, Kinetics, Arrhenius Behavior, Smectic A to Nematic Phase transitions
"Overview of Current Research Projects"
Flow modeling of complex fluids:
Development and numerical implementation of rate equations to describe the microstructure, rheology, and non-newtonian fluid mechanics of various types of polymeric fluids. A challenge lies in the flow modeling of various types of polymeric liquids and their distinct microstructure, e.g. polymer blends, polymer nano-composites, or liquid crystals. Furthermore, non-isothermal effects in polymeric fluids deserve special interest since temperature is an important process parameter.
Design of novel food emulsions:
Interfacial design of oil droplets is adopted to tailor the properties of oil in water emulsions. Various interaction mechanisms between oil droplets are considered, e.g. electro-static repulsion/attraction, depletion, and bridging flocculation, which influence the complex rheological response of food emulsions. This project is carried out in collaboration with the Department of Food Science at the University of Massachusetts Amherst.
Advancing processing technologies for polymers:
Development of a spinning process for bi-component synthetic fibers with tailor-made mechanical properties. In this project, model experiments are designed and computational fluid dynamics calculations are developed in order to exploit physical instabilities that lead to fibers with a desired interfacial morphology. This project is carried out in collaboration with the Department of Advanced Fibers of the Swiss Federal Institute of Materials Testing.
Keywords: Microstructural and rheological modeling, food process engineering, polymer processing
Duncanson; Tom Kodger, and Dave Weitz
In the medical field and the oil industry, there is a growing interest in smarter microspheres designed to seek out specific interfaces and sites and be triggered to deliver payloads or enhance contrast. Microspheres with high porosity or core-shell structures are typically formed by blending liquids containing surface active compounds with gases, cosmetics, foods, or drugs under high-shear. The chaotic production conditions produce microspheres with broad distributions in sizes, porosities, and shell thicknesses which require further processing or filtering to obtain the desired structures. This highlights the need for greater control during microsphere fabrication. Microfluidics offers greater control over the size distribution of these emulsions relative to conventional preparation methods. The flexibility of microfluidics allows us to use a variety of materials including self-assembling materials and unique surfactants to make smarter materials for contrast enhancement as well as payload delivery. We fabricate squishy bubbles using sophisticated microfluidic technologies, self-assembling, and triggerable materials. These advanced techniques coupled with the benefits of the smart materials create new opportunities to not only produce simple bubbles, but also highly complex functional bubbles.
Keywords: Microfluidics, Bubbles
Ehrlicher; Fumihiko Nakamura, John Hartwig, Martin Pollak, Tom Stossel, David Weitz
"Molecular Mechanotransduction: how forces trigger cytoskeleton dynamics"
Mechanical stresses elicit cellular reactions mediated by chemical signals. Defective responses to forces underlie human medical disorders, such as cardiac failure and pulmonary injury. Despite detailed knowledge of the cytoskeleton’s structure, the specific molecular switches that convert mechanical stimuli into chemical signals have remained elusive. Here we identify the actin-binding protein, filamin A (FLNa) as a central mechanotransduction element of the cytoskeleton by using Fluorescence Loss After photoConversion (FLAC), a novel high-speed alternative to FRAP. We reconstituted a minimal system consisting of actin filaments, FLNa and two FLNa-binding partners: the cytoplasmic tail of ß-integrin, and FilGAP. Integrins form an essential mechanical linkage between extracellular and intracellular environments, with ß integrin tails connecting to the actin cytoskeleton by binding directly to filamin. FilGAP is a FLNa-binding GTPase-activating protein specific for Rac, which in vivo regulates cell spreading and bleb formation. We demonstrate that both externally-imposed bulk shear and myosin II driven forces differentially regulate the binding of integrin and FilGAP to FLNa. Consistent with structural predictions, strain increases ß-integrin binding to FLNa, whereas it causes FilGAP to dissociate from FLNa, providing a direct and specific molecular basis for cellular mechanotransduction. These results identify the first molecular mechanotransduction element within the actin cytoskeleton, revealing that mechanical strain of key proteins regulates the binding of signaling molecules. Moreover, GAP activity has been shown to switch cell movement from mesenchymal to amoeboid motility, suggesting that mechanical forces directly impact the invasiveness of cancer.
Keywords: Mechanotransduction, Cytoskeleton, Actin
Fung; Rebecca W. Perry, Vinothan N. Manoharan
Dept. of Physics, Harvard University
"Minimizing radiation pressure in digital holographic microscopy"
Digital holographic microscopy permits precise 3D tracking of the Brownian motion of colloidal sphere clusters. However, radiation pressure from the laser used for imaging can perturb measurements of the Brownian motion of strongly scattering colloidal clusters. We discuss the implementation of a pulsed laser system to overcome this problem.
Keywords: colloids, imaging, digital holography
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
Huber; Mark Terasaki
University of Connecticut Health Center
"A new type of membrane junction of interesting topology"
The existence of a new type of membrane structure in eukaryotic cells is argued. This membrane shape has an interesting topology and serves as a junction between the planar sheets of the endoplasmic reticulum.
Keywords: bilayer membranes, membrane-bound organelles, eukaryotic cells, endoplasmic reticulum
Invited; Prof. Jan Schroers
"Metallic glasses form like plastics"
Invited; Prof. Ilona Kretzschmar
City College of New York
"Reaction-driven Janus and patchy particle dynamics"
Invited; Prof. Craig Maloney
Carnegie Mellon University
"Diffusion in soft particle suspensions near jamming"
Invited; Prof. Daniel Blair
"How size and stran determine the strength of soft materials"
Invited; Prof. Michael Falk
Johns Hopkins University
"Plastic flow and failure in metallic glass/nanocrystal composites"
Jang; Simon G. Mochrie, Suresh Narayanan, Alec Sandy
Post Doctoral Associate
"Static structure and dynamic behavior of cadmium sulfide nanoparticles in block copolymer matrix"
Block copolymers (BCP) have attracted increasing attention in various applications due to their ability to self-assemble into periodic nanostructures. The incorporation of nanoparticles into self assembled BCP has many potential applications, such as in semiconductor and optical applications. Small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) are proven techniques to study the internal structure and dynamic behavior of nanocomposites. In this presentation, we present the current accomplishment of cadmium sulfide (CdS) nanoparticles in the BCP matrix from transmission electron microscopy (TEM) of individual CdS nanoparticles, SAXS and XPCS experimental results, and TEM images of CdS nanoparticle arrangement in the BCP matrix.
Keywords: Block Copolymer, X-ray
Jawerth; Stefan Muenster, David Weitz
"A direct observation of the mechanism underlying strain stiffening in biopolymer networks"
Fibrin networks form the structural scaffold of blood clots; their non-linear mechanical properties are crucial to stem the flow of blood at a site of vascular injury. A hallmark of these networks along with other biopolymer networks is strain stiffening: a stiffness that increases non-linearly as a network is strained. Deformations of the fibers and the network combine to control the mechanical properties of the bulk and must lead to the strain stiffening behavior of the networks; however, the details of this process are unknown. We study fibrin networks undergoing shear on a confocal microscope and compare this to bulk rheological measurements. We track individual fiber branchpoints as function of system strain. We characterize the non-affinity of the motion and show that the low strain, linear regime corresponds to highly non-affine motion while the high strain, nonlinear regime corresponds to affine motion. Moreover, we show that the non-linear bulk response can be well approximated by considering the fibers to be linear elastic elements with soft compressive behavior and, therefore, is a result of the topology of the network itself rather than nonlinearity of its constituents.
Keywords: fibin, biopolymer network rheology, strain stiffening
Kalakonda; S. SARKAR,G.S. IANNACCHIONE,E. GOMBOS,G. GEORGIEV
Worcester Polytechnic Institute
"Macroscopic Ordering of CNT in a Liquid Crys- talline Polymer Nano-Composite by Shearing"
We present a series
of complimentary experiments exploring the macroscopic alignment
of carbon nanotubes (CNTs) in a liquid crystalline polymer (isotactic
polypropylene - iPP) nano-composites as a function of temperature,
shear, and CNT concentration. The phase behavior of iPP+CNT, studied
by Modulated Differential Scanning Calorimetry, revealed the evolution
of the -monoclinic transition and its dynamics, which are dependent
on CNT content and thermal treatment. These results indicate that
the CNT nucleates crystal formation from the melt. Spectroscopic ellipsometry
reveals a change in the optical constants that are connected to
the ordering of CNTs when the iPP+CNT is sheared. This anisotropy
is also exhibited in measurements of the electrical and thermal conductivities
parallel and perpendicular to the shear direction. The amount
of order induced into the dispersed CNTs is relatively low for these low
concentration samples (< 5 wt%).
X Prefer Oral Session
Keywords: The phase behavior of iPP+CNT-studied by Modulated Differential Scanning Calorimetry-the CNT nucleates crystal formation from the melt
Kalakonda; Germano S. Iannacchione,Erin Gombos and Georgi Georgiev
"Macroscopic Ordering of CNT in a Liquid Crystalline Polymer Nano-Composite by Shearing"
We present a series of complimentary experiments exploring the macroscopic alignment of carbon nanotubes (CNTs) in a liquid crystalline polymer (isotactic polypropylene - iPP) nano-composites as a function of temperature,shear, and CNT concentration. The phase behavior of iPP+CNT, studied by Modulated Di_erential Scanning Calorimetry, revealed the evolution of the monoclinic transition and its dynamics, which are dependent on CNT content and thermal treatment. These results indicate that the CNT nucleates crystal formation from the melt. Spectroscopic ellipsometry reveals a change in the optical constants that are connected to the ordering of CNTs when the iPP+CNT is sheared. This anisotropy is also exhibited in measurements of the electrical and thermal conductivities parallel and perpendicular to the shear direction. The amount of order induced into the dispersed CNTs is relatively low for these low concentration samples (< 5 wt%).
Keywords: Calorimetry, interaction of CNT with iPP, nucleating agent causing the rise of iPP phase Tp, thermal conductivity and optical constants.
"Fabrication of fluid-filled microcapsules by electrospray"
The sorting and separation of particles based on their physical properties is well practiced in analytical sciences and this capability has been implemented successfully in microfluidic devices. By contrast, the ability to separate micron scale particles based solely on their mechanical properties remains out of reach. Such rigidity-based separations are particularly compelling in bio-analytical applications where they can provide quantitative indicators of cell deformability which may serve as an assay for cell health in certain diseases. In this talk, I will discuss our efforts to design a microfluidic system to separate particles based on deformability.
Our preliminary work towards this goal has focused on the use of electrospray for fabricating model fluid-filled capsules that feature thin elastic polymeric shells of different rigidities. I will present the effect of electrospray conditions on the size and morphology of microcapsules generated by polyelectrolyte complexation with various cationic/anionic polymer pairs.
Keywords: Microcapsules, Electrospray
"Measuring size and charge of single objects in fluids: a nanoscale Millikan Oil Drop experiment"
Leng; Axel Guenther
University of Toronto
"Mosaic Hydrogels: Dynamic tesselation and coding of soft materials"
Soft materials with a spatially non-uniform composition that is closely linked to their function are common in nature. Such materials often possess a hierarchical architecture with length scales from nanometers to bulk dimensions. Strategies employed to achieve spatial organization at micron to millimeter length scales often involve a sequence of processing steps or lack spatiotemporal control. In my talk, I will introduce a flowable format for the preparation of mosaic and coded hydrogels in a one-step process. The composition of the obtained planar materials can be dynamically altered in two dimensions; an ability that I have utilized to encode information ranging from a single word to an entire paragraph. Finally, I will report directionally dependent properties measured for different mosaic hydrogels.
Keywords: hydrogels, microstructure organization, coded materials
Lin; David Weitz
"Dynamics of charged particles in nonpolar solvent in response to an electric field"
In nonpolar solvent, surfactant molecules aggregate to form charge-stabilizing reverse micelles. This enables surface charging of colloidal particles suspended in nonpolar solvent. We investigate the dynamics of such charged particles in response to an externally applied electric field. By combining microfluidics and confocal microscopy, we directly visualize the transport of particles between two parallel electrodes. We use direct visualization to measure the electrophoretic mobility of each particle and determine the effect of added surfactant on the measured mobility. In addition, we find that the presence of surfactant has a significant effect on the transport dynamics of the charged particles.
Keywords: charged colloids, nonpolar solvent, electrohydrodynamics
Magkiriadou; Jin-Gyu Park, Vinothan N. Manoharan
Physics Department, Harvard University
"Disordered photonic materials: the quest for red color."
We study the occurrence of uniform structural color in amorphous colloids, a phenomenon based on Bragg-type interference due to short-ranged structural correlations in an otherwise random medium. All manifestations of this coloration mechanism, either artificially created in the lab or naturally found in birds, have so far yielded only blue and green colors. Why not red? Based on our understanding
of the underlying physics and supported by some calculations, I will present our speculations for this conspicuous absence and propose novel colloidal systems which, when self-assembled into amorphous configurations, could yield beautiful, angularly-independent structural red.
Keywords: structural color, self-assembly, colloidal glass, photonic crystal
Massenburg; David Weitz
"Understanding Filtration using Pore Clogging"
Nearly every application involving fluid relies heavily upon filtration, yet filter design is not well understood. These designs can be replicated in two dimensions using soft lithography to create microfluidic devices. These features, such as of pore size distribution, can then be tested by clogging with micro particles and cells. Here we apply these probabilistic studies to understand filter features such as pore size distribution and tortuosity.
Keywords: Microfluidics, clogging, jamming, filtration
McDermott; Ian Morrison, David Weitz
"Electrostatic and steric stabilization of carbon black dispersions in nonpolar media"
While the stabilization of colloidal particles in nonpolar media has applications in fields ranging from petrochemical and lubrication engineering to printing and electronic inks, the mechanisms and physics behind the stabilizing forces is not well understood. We are investigating both electrostatically and sterically stabilized dispersions of carbon black through rheology, electroacoustics, and dielectrophoretic responses, with an emphasis on developing a bulk measurement to quantify the relative importance of each mechanism.
Keywords: dispersion forces, electrophoresis, nonpolar
Mertz; Yonglu Che, Valerie Horsley, Eric Dufresne
"Scaling of Traction Stresses with Size of Cohesive Cell Colonies"
Keywords: Traction force microscopy, epithelium, tissue
Perry; Thomas G. Dimiduk, Jerome Fung, Vinothan N. Manoharan
Harvard University SEAS and Department of Physics
"Wiggling Colloidal Clusters"
Using digital holographic microscopy, we observe shape changes in non-rigid clusters of micron-sized colloidal spheres. The spheres interact via a short-range attractive depletion interaction. Even small clusters of three particles show a bending motion when only two of the possible three bonds are formed.
Keywords: Colloids, Dynamics, Imaging Techniques
Radhakrishnan; Patrick T. Underhill
Rensselaer Polytechnic Institute
"Flow induced structures in dilute polymer solutions"
Hydrophobic polyelectrolytes, associative block copolymers and worm-like micelles are observed to form flow induced structures. These systems are particularly challenging to model because of the separation of time and length scales between the formation of these structures and the scale of the attractive and repulsive interactions. We have developed a new coarse grained model of polymer solutions that show flow-induced structures in our Brownian Dynamics simulations. We have analyzed the importance of a balance of the rotational and elongational components in forming these structures using oscillatory shear and elongational flows.
Keywords: flow induced structures, Brownian dynamics, polymer solutions
Saha; Ani Nikova,Pradeep Venkataraman, Vijay T.John, Arijit Bose
University of Rhode Island
"Carbon black as emulsion stabilizers"
We create particle stabilized octane-in-water ‘Pickering’ emulsions by modifying the pH and/or varying the salt concentration of a surface-functionalized carbon black suspension in water, adding octane to this suspension and vortex mixing. Cryo-SEM images reveal that at ‘high’ carbon black concentrations in the aqueous phase, the particles arrange into multiple layers at the octane-water interfaces. At ‘low’ concentrations, stable emulsions are created with sub-monolayer coverage. The the hydrophilic/hydrophobic balance, impacts particle positioning at the oil-water interface. When napthalene, a model polycyclic aromatic hydrocarbon, is added to the octane, its partitioning into the aqueous phase is reduced dramatically from a control case with no carbon black. These particles have potential use for emulsifying crude oil subsequent to an oil spill.
Keywords: Carbon Black, Pickering emulsion, crude oil emulsification
Sarfati; Eric Dufresne
"Attractive forces between charged colloidal particles in an external electric field"
Schade; Miranda C. Holmes-Cerfon, Elizabeth R. Chen, Jonathan A. Fan, Vinothan N. Manoharan
"Toward negative refraction via colloidal self-assembly"
We want to create a material with exotic optical properties that don't occur naturally, such as a negative refractive index for visible light. It has been predicted that tetrahedral clusters of gold nanoparticles will produce the electromagnetic response needed for such a metamaterial, so colloidal self-assembly is a promising fabrication route. Our experiments and simulations reveal that bidisperse sphere mixtures exhibiting short-range interactions can form tetrahedral clusters in high yield, taking us one step closer to our goal. This occurs at a critical size ratio for the two components of the mixture due to a geometrical singularity.
Keywords: colloid, cluster, self-assembly
"Structure and Dynamics in Sensitive Polymer Networks"
Stimuli-responsive or “smart” polymer gels consist of crosslinked, swollen polymer networks that are able to react to changes in its environment, typically by drastic swelling or shrinking or by selective crosslinking or de-crosslinking. It is this responsiveness which makes such gels attractive for applications in various fields, including those in drug delivery, catalysis, sensing, and photonics. To impart these types of stimuli-sensitivity to a polymer gel, the system must either exhibit very sensitive thermodynamic interactions with its environment, or it must be crosslinked by transient non-covalent bonds which can easily be broken and re-associated. Both classes of sensitive polymer networks are well established nowadays, and their utility for many applications has been or is still being explored. However, the basic understanding of the relations between the structure, dynamics, and function of these valuable materials is still far from complete. This is unfortunate, since it is exactly this knowledge which is necessary to tune their properties with a view to optimize them for applications.
We study the fundamental relations between the structure, dynamics, and the functionality of sensitive polymer gels. We focus on both supramolecular networks that are crosslinked through transient, non-covalent bonds and on environmentally sensitive networks which exhibit a delicate thermodynamic interplay with their solvent. A particular aspect of our work is to explore how micro- and nanostructural imperfections and inhomogeneities in the gel affect the behavior of these networks. To study this effect, we use droplet-based microfluidics to fabricate micrometer-sized gel particles which then serve as objects to be studied. The use of microfluidics allows us to impart custom inhomogeneities to the gel particles, thus offering the possibility to study their impact. Besides studying single microgel particles, we also use many microgel particles as building blocks to create larger gel-type materials.
Keywords: Smart Materials, Polymer Networks, Microgels
Selimovic; Francesco Piraino, Hojae Bae, Ali Khademhosseini
Harvard Medical School / BWH
"Polyester-based microfluidic devices for cell culture"
Recently, microwells have emerged as a staple element of microfluidic devices for cell
culture. Typical fabrication methods such as photolithography, soft lithography, or
etching require expensive equipment and are often lengthy and expensive. Thus, we have
developed a convenient, fast, and affordable method for microwell fabrication based on
laser ablation of a polyester film. The microwell size was dependent on the laser speed
and power, and the well depth could be controlled by creating a stack of several layers
of film. A single device with hundreds of microwells can be manufactured in less than
five minutes and is biocompatible. We cultured murine embryonic stem cells and human
hepatoblastoma cells in polyester microwells of varying sizes for 9 days and showed that
cell aggregates were formed, were viable, and could be successfully extracted from the
device, indicating the usefulness of our polyester microwell platform for cell culture
applications. Since little technical skills is required, and only a single, affordable
piece of equipment (laser engraver) is required for this device fabrication method,
polyester device have great appeal for microwell and other microscale biological
Keywords: microwells, stem cells, polyester devices
shao; Chinedum Osuji
"Suspension rheology of polymer particles with thermosensitive deformability"
The rheology of particulate suspensions has been observed to be a strong function of particle deformability. For example, the shear thinning of red blood cell suspensions is shown to increase with an increase in the deformability of the cell membrane, with important implications for circulatory behavior [Magnus I. Gregersen, Science (1967)]. Likewise, shear thickening is nearly ubiquitous in concentrated suspensions of rigid particles but almost absent in suspensions of softer, more deformable objects [Howard A. Barnes, Colloids and Surfaces (1994)]. However, we still lack a comprehensive description of the role of particle deformability in suspension rheology. This is due in large part to the difficulty of systematically and independently varying particle rigidity, volume fraction, interaction strength and size. We report here on the preparation and characterization of a colloidal system that permits such variation. Oligomeric polystyrene with a molecular weight of 1200 g/mol and a glass transition temperature (Tg) of 35oC was emulsified in water in the presence of a reactive surfactant. The use of a reactive species allows the surfactant to be covalently attached to the particle, with the degree of stabilization tunable by the molecular weight, concentration and species of the surfactant. The Tg of the oligomer is close to room temperature and so given the absence of cross-linking, the elasticity of the particles can be changed over several decades simply by manipulating temperature in an experimentally convenient regime. For a fixed time-scale, below Tg, particles behave as near-rigid spheres, while, above Tg, they are soft yet viscous droplets with structural flexibility. Here I will discuss preliminary data regarding the structure and rheology of these model suspensions (emulsions).
Keywords: suspension, deformability
Sinha; Agnese Seminara, James Wilking, Michael Brenner, and Dave Weitz
"Development of Sub-millimeter Size Towers in Bacillus subtilis Biofilms "
Biofilms are highly-organized bacterial colonies, which contain different cell types, even if they are all the same species. Bacillus subtilis biofilms form sub-millimeter size towers distributed around the surface of the colony. These towers contain high concentrations of spore-forming cells at the top, to aid in the propagation of the biofilm. The physical mechanism for how these structures form is still not well understood. Understanding the arrangement of the different cell types over time is important to elucidate which processes are responsible for the growth of the towers. In the present work, we use confocal fluorescence microscopy to help explain this relationship between cell type and colony morphology.
Keywords: biofilm, confocal
Taktikos; Vasily Zaburdaev, Holger Stark
Harvard University, Technical University Berlin
"A model for chemotactic signaling of microorganisms"
Various microorganisms, such as bacteria or amoebae, use chemotaxis for signaling among individuals - a common communication mechanism that is responsible for the formation of microcolonies. Our modeling of microorganisms is based on the Langevin dynamics of active walkers with rotational diffusion of the velocity direction and includes a torque to align the velocity direction along the gradient of a self-generated chemical field.
To account for the finite size, each microorganism is treated as a soft disk whose velocity is modified when it overlaps with other particles. The repulsion mechanism is based on a harmonic interaction and a linear relationship between force and velocity.
We show that for an isolated walker the long-time dynamics is diffusive and derive analytic expressions for its diffusion coefficient. The stability of two-particle clusters is investigated in terms of a phase diagram which separates free particles, metastable states and bounded clusters. For sufficiently large chemotactic coupling, we observe the formation of symmetric clusters.
Nanyang Technological University
"Single Molecule Resolution Traction Force in Live Cells"
It is increasingly evident that mechanic cues affect a wide variety of cells and can sometimes override biochemical cues to control cell division, cell death and even specify stem cell differentiation lineage. To understand how cells interact physically with their surrounding matrix, it is imperative to investigate the spatiotemporal distribution of forces and molecular players as cells undergo contractile activity. We examine human mesenchymal stem cell contractility at high temporal and spatial resolution on soft and hard substrates.
Uspal; Patrick S. Doyle
" Soft, Hydrodynamically Coupled Particles in a Hele-Shaw Channel"
Control of flowing suspensions is central to many emerging microfluidic applications. For instance, manipulation of small clusters is important in the synthesis of functional particles. Via theory and simulations, we study small clusters confined in a microchannel with thin cross section and subject to an external flow. We show that many-body hydrodynamic interactions sustain long-lived bound states with complex dynamics. As these interactions are sensitive to confinement, we investigate modulation of channel geometry as a means to perform sequential operations in a continuous process. We also probe the effects of shape and elasticity via a Lattice Boltzmann/Lattice Spring code, finding spontaneous excitation of elastic waves ("flapping"), and that orientational effects of shape enrich behavior. Our results demonstrate phenomena that could be exploited for assembly of soft colloids in microchannels.
Keywords: hydrodynamics, nonlinear phenomena, fluid/structure interaction, confinement
Wang; Ryan McGorty, David M Kaz, Vinothan N Manoharan
School of Engineering and Applied Sciences, Harvard University
"Studying particle-interface dynamics with digital holographic microscopy "
The dynamics of colloidal particles as they approach and then meet an oil-water interface are only beginning to be characterised. We use digital holographic microscopy to capture holograms of colloidal particles as they approach an oil-water interface in real-time, at over 5000fps. This data is then analysed to reveal the trajectories and dynamics of the particles.
Keywords: colloids, holograms
Keywords: fibrin, biopolymer networks
Xu; Eric Dufresne
"The Effect of Surface Topography on Interface Stresses During Peeling"
Surface topography can have a large impact on the adhesive strength of soft interfaces. While previous experiments have revealed some of the underlying mechanisms, there has been no direct measurement of interface stresses during adhesive failure. We use traction force microscopy to measure the microscopic distribution of interface stresses during peeling. We focus on the relationship between local stresses and topography near the peeling front.
Keywords: adhesion, surface pattern, PDMS
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