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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


56th New England Complex Fluids Meeting
Worcester Polytechnic Institute | Friday, September 20, 2013
Registration deadline: Wednesday, September 18, 2013

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Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Gericke; Arne Gerick

WPI
"How phosphoinositide lipds control protein function"
TBA
Keywords: 

2. Germano; Germano S. Iannacchione

WPI
"Liquid crystal and protein colloidal dispersions"
TBA
Keywords: 

3. Kudrolli; Arshad Kudrolli

Clark University
"Pearling and arching instabilities in granular suspensions"
We will discuss the formation of structures as a result of rapid solidification of a granular suspension on a liquid absorbing substrate which may be important to fabricate granular structures at rates and scales where molds are difficult to use. The granular suspension - consisting of a dense mixture of non-Brownian particles and a Newtonian liquid - emerges from a nozzle either as a droplet or a jet depending on the flux rate, nozzle diameter, and the material properties of the suspension. We demonstrate that in the droplet regime, pearl like structures result due to the rapid imbibition of the liquid resulting in a cluster of grains which are held together by wetting liquid bridges. These pearls can delicately balance on each other resulting in tower-like structures with a designed cross-section. While coiling and weaving patterns can be observed similar to viscous Newtonian fluids in the jetting regime, the non-Newtonian nature of granular suspensions can lead to novel structures which are mechanically stable. On a translating substrate, we find spontaneous formation of periodic arches besides other periodic and aperodic structures.
Keywords: 

4. Tang; Jay X. Tang

Brown University
"How does a motile bacterium cope with complex fluids and interfaces?"
Microbes inhabit planet earth over billions of years and have adapted to diverse physical environments, particularly at or near interfaces. A common aquatic bacterium, Caulobacter crescentus, is a species of choice by microbiologists to study cell differentiation and genetic regulation. From the lens of fluid physics, I will discuss our study on the motility of C. crescentus swarmer cells in both viscous and visco-elastic media, their accumulation near solid surfaces, and their behavior at an air/water interface. Specifically, I will describe both physical effects and surface chemistry that cause the motile cells to get trapped near the air/water boundary, and strategies the cells can employ to free themselves. The broader goal of our study is to elucidate interfacial microbial functions through comprehensive analysis of factors. By understanding the mechanisms involved, which the microbes have figured out through the course of evolution, we hope to gain useful insights for environmental and biomedical applications involving diverse species of motile bacteria.
Keywords: 


Sound Bites

1. Arpag; Göker Arpağ*, Shankar Shastry**, William O. Hancock**, Erkan Tüzel*

*Department of Physics, WPI, **Department of Bioengineering, Penn State University
"Experimental and computational investigation into cooperative cargo transport by mixtures of kinesins from different families"
Transport of intracellular cargo often involves multiple motor types, either having opposite directionality such as during bidirectional transport of vesicles, or having the same directionality but different speeds. While significant progress has been made in characterizing motors at the single- molecule level, predicting their ensemble behavior is still challenging. To uncover the force-dependent properties of diverse kinesins and to understand how diverse kinesins attached to the same cargo coordinate their movement, we carried out microtubule gliding assays using pairwise mixtures of motors from the kinesin-1, 2, 3, 5 and 7 families. To match their processivities and ensure identical binding to the glass substrate, the motors were fused to the dimerization domain and coil-1 of kinesin-1, and the neck linkers were adjusted to have a uniform length of 14 amino acids. Uniform motor densities were used and microtubule-gliding speeds were measured as the ratio of fast motors varied from 0 to 1. Depending on the motor pair, velocity versus motor fraction curves varied from convex up to nearly linear to convex down. These findings were recapitulated using a coarse-grained computational model of gliding assays. The simulations incorporate force dependent velocities and dissociation rates from the literature along with mechanical interactions between motors bound to the same microtubule. The simulations also suggest that the motor compliance plays a minimal role in the observed gliding speed compared to observations in quantum dots. The gliding assays combined with the modeling allows us to test hypotheses regarding the characteristics of diverse kinesins under predominantly axial load, avoiding the large normal forces inherent in optical tweezer experiments.
Keywords: Cargo transport, kinesins

2. Basu; Chris Ford, Aviv Regev, Dave Weitz

Harvard University, Broad Institute
"Controlled host-pathogen interaction using droplet microfluidics"
We study host-pathogen interaction in a controlled environment using a droplet-based microfluidic setup. Candida albicans and murine macrophage cells are used for this purpose. We use a combination of fluorescent reporter assays and fluorescence-based sorting techniques to identify especially virulent sub-populations of c. albicans cells as well as optimal conditions under which the fungal infection can be successfully curbed. The sorted cells of interest, which include both c. albicans and macrophage cells are studied exhaustively using transcriptomic sequencing techniques.
Keywords: candida albicans, macrophage, emulsion microfluidics

3. Evans; Roie Shlomovitz, Thomas Boatwright, Michael Dennin, Alex Levine

UCLA, UMass Amherst
"Submerged particle interface microrheology: solving "the case of the missing modulus""
Microrheological studies of phospholipid bilayers and surfactant monolayer systems present a particularly useful avenue for studying the flow properties of fragile, complex fluid systems over a large dynamic range. Unfortunately, in many cases microscopic particle tracking methods disagree with macroscopic flow methods by several orders of magnitude. This "missing modulus" has been speculated to originate in the heterogeneity of the monolayer under study, as well as the unknown boundary conditions and uncertainty in particle position intrinsically associated with interaction between the tracer bead and the monolayer. In this talk we discuss an alternative method for performing microrheology experiments, where the tracer bead is submerged a known depth beneath the monolayer. Despite a weaker coupling between the surface and the bulk, we demonstrate both experimentally and theoretically that submerged particle interface microrheology recovers the missing modulus in several representative monolayer systems.
Keywords: Langmuir monolayers, membrane microrheology

4. Kingsley; James Kingsley*, Stephan Koehler**, Erkan Tuzel*

*Department of Physics, Worcester Polytechnic Institute; **Department of Physics, Harvard University
"Swimming with Noise"
Swimmers on the macroscopic scale face many challenges. Examples include micron scale bacteria that use chemotaxis to obtain food or spermatozoa that win the race to fertilize an egg. One major issue at these small scales is to move efficiently in the presence of Brownian noise exhibited by the surrounding molecular fluid. Here, we use a novel particle-based fluid simulation technique—known as Stochastic Rotation Dynamics (SRD)—to study swimming at nano scales. We perform case studies of a minimalistic swimmer, and identify different regimes of transport. At long times thermal fluctuations dominate, resulting in random walks, and at short times, the swimmer’s motion recapitulates the directed motion of a macro scale swimmer in a viscous fluid. Our results have implications for nano-scale systems involving transport ranging from biology to robotics.
Keywords: particle-based methods, Purcell swimmer, MPCD, SRD

5. Marshall; Izabela Stroe, Germano Iannacchione

Worcester Polytechnic Institute, Dept. of Physics
"Experimental Studies of Liquid Crystal and Protein Interactions"
The fundamental interactions between proteins and liquid crystals were studied using 4-Cyano-4-pentylbiphenyl (5CB) and equine heart myoglobin in varying concentrations. Polarizing Light Microscopy was used to capture images of pure and mixed components, depicting a caging process of the 5CB by the myoglobin as it competes for interaction with water. Dielectric Relaxation Spectroscopy provided evidence of an inverse relationship between the hydrated-myoglobin/5CB ratio and charge carrier diffusion barrier energy.
Keywords: Hydrated Protein Dynamics, Liquid Crystals, DRS, PLM, 5CB, Myoglobin

6. McDermott; Ian Morrison, Dave Weitz

Harvard University
"Self-Preserving Carbon Black Aggregation in Nonpolar Media"
Charge stabilization of colloids is nonpolar solvents is an important but poorly understood physical phenomenon. The necessity of charge-stabilizing surfactants both in solution and on the particle surface presents two complications for proper understanding of electrostatic stability: the discretization of solution charges in inverse micelles, and the presence of a (potentially stabilizing) steric barrier on the particle. When measuring the stability of electrosterically stabilized polydisperse nonpolar carbon black dispersions in response to varying concentrations of a simple oil electrolyte, we observe short-term aggregation that appears self-limiting, with a metastable particle size that is electrolyte concentration-dependent. The scaling of this concentration dependence give insight on the nature of charge-stabilization in a nonpolar system with discrete solution charges.
Keywords: colloidal dispersions, carbon black, colloidal aggregation

7. Morse; Jay X Tang

Brown University
"Surface Dependent Swimming Patterns of Caulobacter Crescentus"
When suspended in bulk homogenous fluid, a swimming bacterium is a roughly symmetric system as the cell body and helical flagella rotate in opposing directions around the same axis. However, the presence of a solid surface or fluid-fluid interface breaks this symmetry and allows for phenomena such as accumulation and curved swimming trajectories. It has been previously shown that swimming bacteria tend to dwell near solid surfaces for extended periods of time and follow circular trajectories. We are currently studying the effects of a gas-liquid interface on the swimming dynamics of Caulobacter Crescentus. While a clean interface can be treated as a free surface, chemical adsorption can alter surface properties that in turn influence bacterial swimming. Depending on surface composition, accumulation and circular swimming patterns can be observed.
Keywords: Bacerial Locomotion, Surface Rheology

8. Tang; Michael Morse, Guanglai Li

Brown University
"How does a motile bacterium cope with complex fluids and interfaces?"
Microbes inhabit planet earth over billions of years and have adapted to diverse physical environments, particularly at or near interfaces. A common aquatic bacterium, Caulobacter crescentus, is a species of choice by microbiologists to study cell differentiation and genetic regulation. From the lens of fluid physics, I will discuss our study on the motility of C. crescentus swarmer cells in both viscous and visco-elastic media, their accumulation near solid surfaces, and their behavior at an air/water interface. Specifically, I will describe both physical effects and surface chemistry that cause the motile cells to get trapped near the air/water boundary, and strategies the cells can employ to free themselves. The broader goal of our study is to elucidate interfacial microbial functions through comprehensive analysis of factors. By understanding the mechanisms involved, which the microbes have figured out through the course of evolution, we hope to gain useful insights for environmental and biomedical applications involving diverse species of motile bacteria.
Keywords: bacterial motility, viscoelasticity, mechanical instability, hydrodynamics, chemotaxis

9. Thomas; Rachel Brunetti, Qi Wen

Department of Physics, Worcester Polytechnic Institute
"Measuring the Viscoelasticity of Living cells using Atomic Force Microscopy"
While it is known that cells respond to the mechanical properties of their environment, limited information is available regarding how such stimuli are transduced. The cytoskeleton is a critical component used by cells to sense substrate stiffness, and information on the cell viscoelasticity as a function of substrate stiffness may lead to a larger composite model of the mechanism through which cells respond to external mechanical stimuli. 3T3 fibroblasts were grown on polyacrylamide gels of varied stiffness and an atomic force microscope (AFM) was used to perform dynamic micromechanical tests. The AFM first indents a cell and then applies a 10 Hz sinusoidal indentation. The storage, E’, and loss modulus, E’’, were calculated from the amplitude and phase offset between the force and resulting indentation curves. Both E’ and E’’ increase with substrate stiffness. We also found that the ratio of E’’ to E’ decreases as substrate stiffness increases, which agrees with our hypothesis that cells become more solid-like as the extracellular matrix becomes stiffer. Additionally, the value of E’ was higher in vimentin null cells, suggesting that cells change their cytoskeletal composition to adapt to the loss of vimentin.
Keywords: cell, viscoelasticity, AFM, microindentation

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