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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

55th New England Complex Fluids Meeting
UMass Amherst | Friday, June 7, 2013
Registration deadline: Thursday, June 6, 2013
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Abstracts for Invited Talks and Sound Bites:

Invited Talks

Sound Bites

1. Alipour; Charles W Wolgemuth

Boston University, UCHC
"Passive cell-cell interactions in elastic media"
Crawling eukaryotic cells exert dipole-distributed traction stresses on the external environment, pulling backwards at the front of the cell and forward at the rear of the cell. We propose a model to understand cell movements through the ECM, by considering a dipole-crawler moving through an isotropic, linear elastic medium. This model captures the major features of the deformations that are induced by motile cancer cells in collagen. In addition, the model suggests that the deformations that are induced in the matrix can provide a mechanism by which distal cells can interact with one another through matrix-mediated interactions. This analysis suggests a mechanism that may be relevant for the collective migration of cells during cancer metastasis and other processes where numerous cells move through the ECM.
Keywords: cell migration, dipole stress

2. Barnes; A.D. Dinsmore

UMass Amherst
"Scanning Probe Measurements of Surface Potential on Insulating Surfaces "
Using Scanning Kelvin Probe Force Microscopy (SKPFM), we measure with nanoscale resolution the pattern of surface charge due to contact-charging on a glass surface. Recent work has shown that contact charging creates a random ‘mosaic’ of static charges on the surface of two materials that are brought into contact and then separated, challenging old assumptions of one object charging uniformly positive and the other uniformly negative [Baytekin et al, Science 333, 308 (2011).]. These experiments are challenging and the results are scarce; we are therefore investigating a new, robust experimental approach. We use glass substrates with two parallel gold electrodes. The surface potential in the glass region, between the two electrodes is a sum of the known potential from the electrodes and the potential due to unknown static surface charge. Scanning with multiple values of electrode potentials and subtracting the electrode potential leave us left with the residual surface charge of interest and a built-in check of reproducibility. Preliminary results show that the surface charge is constant after 4 hours. This technique may lead to more widespread measurements of contact electrification and a powerful probe of this poorly understood process. This work is supported by a gift from the Xerox University Affairs Committee and by the NSF-funded UMass MRSEC on Polymers (DMR-0820506).
Keywords: contact electrification, static charges, KPFM

3. Baykal; Igor A. Kaltashov, Paul L. Dubin

UMass Amherst
"Counterion condensation on heparin oligomers"
We examined the electrophoretic behavior of chain-length monodisperse heparin oligomers. Their mobilities varied inversely with the logarithm of the contour length L, for L from 3 to 10 nm and reached an asymptotic limit for L> 20 nm. The generality of this effect was indicated by similar behavior for oligomers of poly(styrenesulfonate). A recent theory of polyelectrolyte end effects (Manning, G. S. Macromolecules 2008, 41, 6217.), in which chain termini exhibit reduced counterion condensation was found to account quantitatively for these results. A qualitative explanation for the anomalously high value of µ of native heparin, 10-20% higher than those seen for synthetic polyelectrolytes of higher linear charge density is suggested on the basis of similar junction effects (Manning, G. S. Macromolecules 2008, 41, 6217.), which reduce counterion condensation at the interfaces of regions of high and low sulfation.
Keywords: heparin oligomers, counterion condensation, end effects

4. Bruss; Gregory M. Grason

University of Massachusetts Amherst - Polymer Science and Engineering
"Crystalline and straight VS defective and twisted"
Cohesive assemblies of filaments are a common structural motif found in diverse contexts, ranging from biological materials such as fibrous proteins, to artificial materials such as carbon nanotube ropes and micropatterned filament arrays. We show that the non-linear influence of twist on the interaction energy between filaments derives from two distinct geometric features of twisted bundles: (i) the geometrical frustration of inter-filament packing in the bundle cross-section; and (ii) the evolution of the surface geometry of bundles with twist, which dictates the cohesive cost of non-contacting filaments at the surface. Twist-induced packing frustration in the bundle core gives rise to the appearance of topological defects, excess 5-fold disclinations, while the evolution of filament contact at the surface of the bundle generically favors twisted geometries for sufficiently long filaments. Our analysis of both continuum and discrete models of filament bundles shows that, even in the absence of external torque or intrinsic chirality, cohesive filaments universally favor twisted ground states above a critical (length/radius) aspect ratio and below a critical stiffness threshold.
Keywords: filament bundles, self-assembly, crystalline defects, packing, geometric frustration

5. Burke; Badel Mbanga, Donald Blair, Timothy Atherton

Tufts University, UMass Amherst
"Hard Sphere Packings on Spheroidal Surfaces"
Arrested coalescence is of fundamental interest for the structure and stability of emulsions, including explosives, food products and oil. It occurs when colloidal particles adsorbed on a fluid-fluid interface become jammed due to a reduction in the interfacial surface area. In this work we study packings of spherical particles bound to a spheroidal surface in order to investigate the effects of curvature variation, as is present in arrested systems. We observe that defects are attracted to regions of higher Gaussian curvature, as expected from theories of analogous systems. Additionally the formation of scars (chains of defects) is studied with respect to varying particle number and spheroidal aspect ratio. It is seen that at aspect ratios close to one there is a clear onset of scar formation at a minimum particle number, while for higher aspect ratios this transition is softened. Finally, a rich variety of packings are found which belong to nontrivial subgroups of the spheroidal symmetry group; we characterize these as a function of aspect ratio and number of particles.
Keywords: packing, defects, curved interfaces

6. Chilukuri; Cynthia H. Collins, Patrick T. Underhill

Rensselaer Polytechnic Institute
"Structure and dynamics of swimming microorganisms in planar Poiseuille flow."
Using numerical simulations, we study the structure and dynamics of dilute suspensions of flagellated swimming microorganisms (for example, E. coli) in confinements under an imposed Poiseuille flow. We find that the distribution of the microorganisms across the channel is influenced by the flow and the dispersivity along the channel follows a non-monotonic trend with increasing Peclet number (unlike that of passive tracers whose dispersivity continually increases). By using a scaling approach that largely depends on the swimmer orientations (at low Peclet numbers) and the fluid flow (at high Peclet numbers), we show that the predicted values of dispersivity compare well with the dispersivity measured by tracking each organism.
Keywords: microorganisms, dispersion

7. Dias Kassuga; Jonathan Rothstein

University of Massachusetts Amherst
"Buckling of particle-laden interfaces"
Particle-laden interfaces have been shown to have very interesting physical behavior, such as being able to resist compressive and shear stresses, and helping stabilize emulsions and foams. We study the buckling of an oil-water interface populated by micron-sized latex particles using a Langmuir trough. Histograms show that the wavelength distribution has two peaks, which indicates that there occurs a cascading phenomenon similar to that observed in thin solid sheets. This behavior is affected by successive compression cycles due to rearrangement of the particles on the interface.
Keywords: Microparticles, buckling, liquid-fluid interfaces

8. Dinsmore; Wei He, Nesrin Senbil, B. Davidovitch

UMass Amherst Physics
"Capillary Forces on Spheres at Crowded Interfaces"
We measure capillary forces on particles at fluid interfaces in order to assess the key parameters that yield effective stabilizing particles. In our experiments, a particle is attached to a cantilever, which is used to pull the particle perpendicular to the interface. Simultaneously, we image from the side to measure the cantilever’s deflection and thus the pulling force, as well as the height of the particle and the shape of the interface. We focus on the effects of the contact angle and the presence of extra free-floating particles. We find that Young-Dupre contact angle holds for PDMS coated glass sphere in a pulling up process, and free-floating particles at the interface reduce the work required to remove a particle, suggesting that the surrounding particles reduce the effective interfacial tension, similar to surfactants. We also find that increasing mean curvature has the same effect of adding free-floating particles. The results may be helpful in formulating oil dispersants. This work is supported by the Gulf of Mexico Research Initiative (GoMRI) through the Consortium for the Molecular Engineering of Dispersant Systems (C-MEDS).
Keywords: contact angle; capillary;Pickering

9. Dressler; Martin Becker, Laura Gottardo, Urlich Heck, Rudolf Hufenus, Manfred Heuberger

UMass Amherst, DHCAE Krefeld, Empa St. Gall
"Design of a production process for bi-component fibers based on CFD simulation"
To design a spinneret for the production of bi-component fibers with a shear thickening core and a shear thickening sheath, we perform core-annular flow experiments of immiscible liquids using non-standard injection needles. Furthermore, we use Computational Fluid Dynamics (CFD) simulations to track the complex interface of a liquid jet moving in a co-flowing ambient fluid. Simulations have been carried out for immiscible Newtonian liquids at room temperature and high temperature polymers.
Keywords: polymer processing, fiber spinning, non-newtonian fluid mechanics

10. Elaz; Maria L. Kilfoil, Brian Griffin, Kevin Smith

Umass Amherst
"Universality of the Onset of Anaphase on the Mitotic Spindle"
Within a cell, force generators affect the motions and one process in the cell where their presence is evident is mitosis, a highly regulated process that must be completed with high fidelity. We show that there is universality in the motions of the mitotic spindle in living budding yeast cells at the transition from metaphase to anaphase by analyzing fine relative motions within the cell. When a perturbation is made to the system -- by deleting force generators, or motors, from the cell -- this onset may no longer be universal and exhibit slower motions with greater cell-to-cell variation. By analyzing this behavior we illustrate the role of motors and other components in the regulation of mitotic spindle motions.
Keywords: Biophysics, cell mechanics, mollecular motors, cell cycle transitions, mitotic spindle

11. Gehrels; Ben Rogers, Vinothan Manoharan

Harvard University
"DNA Mediated Colloidal Self-Assembly: Linker Design"
DNA has become a valuable tool for creating specific, temperature dependent interactions between colloidal particles. By correctly designing the linking method and strength of binding, systems can be designed with arbitrary complexity and precisely tuned thermodynamic properties.
Keywords: DNA, Colloids, Self-Assembly

12. Goldfain; Tom Dimiduk, Vinothan Manoharan

Harvard University
"A Point Source Digital Holographic Microscope"
Digital holographic microscopy enables three-dimensional imaging of objects with sub-micron resolution at time scales limited only by the frame rate of a high-speed camera. In this microscopy technique, coherent light is scattered off a sample and the resulting interference pattern is recorded with a camera. From the interference pattern, the light wave is numerically propagated backward to determine the amplitude and phase of the wave throughout the volume between the camera and the sample. I have built a point-source holographic microscope, in which a laser beam emerges from a pinhole as a spherical wave and scatters off a sample, creating a hologram that is then reconstructed to yield an image of the sample.
Keywords: Digital Holographic Microscopy, 3D Imaging

13. Gramlich; Leslie Conway, Jennifer L. Ross

University of Massachusetts - Amherst
"Microtubule Defects Inhibit Kinesin-1 Motility"
We observe microtubule defects inhibit kinesin-1 motor motility. We created defect sites by end-to-end annealing microtubules with different protofilament radii. We also observed a correlation between preferred motor binding and defect location.
Keywords: Kinesin-1 Motility Microtubule

14. Khandavalli; Jonathan P. Rothstein

UMass Amherst
"The Effect of Nanoparticle Colloidal Dispersions Rheology on Liquid Transfer during Gravure Printing"
Roll-to-roll patterning of small-scale features on a rapidly moving web is an industrially important process with a broad range of commercial applications both old and new. In gravure coating, an engraved roller with a regular array of shallow cavities/cells is used to pick up fluid from a reservoir. It is then passed through a flexible doctoring blade in order to meter off excess fluid before printing the fluid onto a flexible substrate. We will present an experimental investigation into the effect that complex fluid rheology can have on the dynamics of liquid transfer from an idealized gravure cell to a flat rigid substrate. In this talk we will investigate both the impact of viscoelasticity and shear thickening on the ink transfer process in gravure coating. The viscoelastic liquids used are a series of high molecular weight PEO solutions in water. The shear thickening systems used in the study are colloidal dispersions of silica nanoparticles in PPG. The effect of shear thickening and shear thinning behavior of these colloidal nanoparticle dispersions in the gravure printing will be discussed while comparing with Newtonian and viscoelastic fluids. A simplified process was developed which imposes an extensional flow using a modified filament stretching rheometer in which one of the endplates is replaced by a plate containing a single truncated conical gravure cell. The deformation and stretching of the resulting liquid bridges, the motion of the contact line within the gravure cell and the total amount of fluid removed from the gravure cell are studied as a function of the imposed stretch rate, the fluid rheology, the geometry of the gravure cell, the gravure cell configuration. The performance of viscoelastic, shear thickening and shear thinning nanoparticle dispersions will be compared directly to Newtonian fluids.
Keywords: shear thickening, extensional rheology, gravure printing, nanoparticles, fumed silica

15. Kilfoil; Kevin Smith, Brian Griffin

UMass Amherst
"The Mechanics and Fluctuation Spectrum of the Metaphase Mitotic Spindle"
The mechanics of cells is strongly affected by molecular motors that generate forces in the cellular cytoskeleton. We show the spectrum of fluctuations of the cytoskeleton of living cells comprised of the mitotic spindle during metaphase, a semiflexible network driven out of equilibrium by molecular motors exerting transient tensile stresses. To attempt to understand the low-frequency fluctuation enhancement, we compare the results to theoretical predictions of a two-fluid model of a low-density semiflexible network that uses simple assumptions regarding the motor activity in the system.
Keywords: cell mechanics, rheology, force generation, biopolymer assemblies

16. Magkiriadou; Jin-Gyu Park, Young-Seok Kim, Gi-Ra Yi, Vinothan N. Manoharan

Department of Physics, Harvard University
""Inverse Glasses" with Structural Colors: The End of the Quest for Red?"
When a colloidal glass is illuminated, the short-ranged spatial correlations between neighboring particles can give rise to constructive interference for a particular wavelength. Unlike the structural colors arising from Bragg diffraction in colloidal crystals, the colors of these colloidal glasses are independent of angle due to the disordered, isotropic microstructure. We therefore call them "photonic glasses". A similar coloration mechanism is found in the feathers of certain birds. Notably, there have been no known examples of red photonic glasses either in nature or in typical colloidal systems. Using scattering theory we have shown that the absence of red can be explained by the wavelength-dependence of the single-particle scattering cross-section, which can override the interference condition set by the structure. However, if the refractive index contrast in the glasses is inverted, such that instead of colloidal particles in a medium of lower refractive index there are air cavities in a medium of higher refractive index, our theory and preliminary data suggest that angle-independent, structural red color is possible.
Keywords: colloidal glass, structural color, red

17. Prasad; Christian Santangelo, Gregory Grason

University of Massachusetts, Amherst
"Sphere packings in random two-phase mixtures."
We have implemented computational models of bi-disperse "jammed" sphere packing to analyze contact distributions in two-phase sphere mixtures. Our aim is to identify key parameters that govern local and global contact structures in these particle networks of random sphere packing assemblies. We observe that by varying radius and number ratio of these spheres, the local arrangement of neighbors around a sphere changes. We are interested in building simple analytical theories of network structures using mean field models of sphere packings, based on the influence of these parameters on the arrangement of spheres in our assemblies.

18. Rathi; David M. Ford

University of Massachusetts Amherst - Department of Chemical Engineering
"Colloidal Stability of Nanoparticles"
The stability of colloidal suspensions has been widely studied with traditional DLVO theory based on electrostatic repulsion and van der Waals attraction interactions. The DLVO theory works well for large particles and low salt concentrations but fails to accurately predict the quantitative behavior for nanoparticles and biologically relevant salt concentrations due to ion-specific interactions, solvent effects, and steric effects . Also, the assumptions such as linearization of the Poisson-Boltzmann equation are questionable in the range of interest. The current study focuses on modifying the existing the theory to take into account previously neglected interactions for the systems of technological interest, such as ligand-stabilized gold nanoparticles in biological media.
Keywords: colloidal interactions, DLVO, Nanoparticles

19. Rizzo; Cristian Staii, Timothy J. Atherton, James D. White, Elise M. Spedden

Tufts University
"A Stochastic Study of Axonal Growth Rates"
Description of neuron growth behavior is essential in elucidating the environmental factors that prompt the formation of neural networks. However, the staggering number of physical and chemical guidance cues that influence axonal growth prohibits understanding of growth behavior from a purely mechanistic perspective. Using a phenomenological approach, we record the distribution of axonal growth rates at several time points, and describe observed behavior in terms of an effective potential and diffusion coefficient. In so doing, we establish a method by which the effects of environmental stimuli on axon growth may be systematically quantified.

20. Ross; Joshua Pringle, Amutha Muthukumar, Amanda Tan, Leslie Conway

University of Massachusetts Amherst
"Microtubule Gliding with Crosslinkers"
The anti-parallel microtubule cross-linker, MAP65, was added to filament gliding assays and shown to reduce velocity and transiently bind between anti-parallel gliding microtubules. When microtubules were pre-bundled with MAP65, condensed regions were formed that resemble cellular organizations of mitosis.
Keywords: Kinesin, motor, active matter

21. Senbil; Wei He,Benny Davidovitch,Anthony Dinsmore

Umass Amherst
"Role of Contact-Line Boundary Conditions on Interfacial Particles"
Capillary interactions are crucial in natural and technological settings. We are studying capillary interactions of millimeter-sized spheres. In particular, we focus on the case where one of the two spheres is Young-Dupre sphere and the other one does not maintain a constant contact angle, as is commonly assumed. The latter one is attached to a thin, rigid rod to control the height. Equilibrium separation is observed between these two spheres rather than a monotonic behavior. Similar results are expected for shapes which has pinned contact line such as disks. We explain our results with a model where analogy to electrostatic charges is made. The trend of our experiments agrees well with our prediction. The results may point the way to describing interactions among spheres or discs that do not have a uniform contact angle. This work was supported by the NSF through grant CBET--0967620
Keywords: capillary,contact angle,interfaces

22. Sokolov

Department of ME, BME, Tufts University
"Self-Assembly of Nanoporous Particles: Liquid Crystal Stage Defines Their Shape"
Growth of even simple crystals is a rather hard problem to describe because of the non-equilibrium, kinetic nature of the process. Recently a synthesis of extraordinary curved nanoporous silica colloidal shapes, such as rods, discoids, spheres, tubes and hollow helicoids has been reported. These particles demonstrate an example of shapes of high complexity, similar to one observed in the biological world. Furthermore, these structures are the natural examples of the assembly of nanostructures into larger scale objects, which is one of the most important tasks of modern nanotechnology. In this respect, the ability to control curved shapes portends a variety of applications and new technologies where nanostructure and geometry determine function (for example, slow drug release, encapsulation of fluorescent molecules, lab-on-a-particle, etc.). The understanding of fundamentals of the shaping mechanism can also shed light on the formation of various shapes in biological world (what is called morphogenesis). Here I demonstrate the evidence that the liquid crystal stage of the particle morphogenesis is defining part. Moreover, this process can be described as an equilibrium process driven by the thermalization of free energy. Apart from visual resembling between the result of simulations and the observed shapes, a quantitative proof will be presented. In particular, we found that the distribution of the free energy obeys the Boltzmann distribution, which is predicted by the equilibrium thermodynamics. Several examples of application of the synthesized particles of different morphology will be given.
Keywords: Self-Assembly, Free energy, Nanoporous Particles

23. Spedden; Cristian Staii

Tufts University
"Temperature Response of the Neuronal Cytoskeleton"
Neurons display an increase in elastic modulus with a decrease in ambient temperature. Through a combination of Atomic Force Microscopy–based elasticity mapping and bright field/fluorescence microscopy we demonstrate that the dominant mechanism by which the elasticity of neurons changes in response to temperature is the contractile stiffening of the actin components of the cytoskeleton. With a drop in temperature from 37°C to 25°C the measurable elastic landscape moves from tubulin-dominated to actin dominated. Disruption of myosin II activity suppresses this temperature-dependent response.
Keywords: AFM

24. Thyagarajan; Raghuram Thyagarajan, Ray M. Sehgal and David M. Ford

Umass Amherst - Department of Chemical Engineering
"Thermodynamic and Kinetic Models for Self-Assembly of Colloidal Particles"
Self-assembly of colloidal particles into crystals is an emerging area of scientific interest that has applications in manufacturing of photonic crystals and other meta-materials. A thorough understanding of the thermodynamics of these systems is essential to building defect free crystals. We have been investigating small (10-100 particle) systems that employ either a depletion attraction potential or externally applied electric field as the mechanism to promote assembly. We model the self-assembly process using coarse-grained representations that can capture the dynamics and equilibrium properties of the colloidal clusters. For achieving the coarse-graining, we first used diffusion mapping to find the relevant number of “slow” dimensions in which the system evolves. We then identify suitable observables or order parameters that strongly correlate with diffusion map co-ordinates. So far, two order parameters, namely radius of gyration Rg, and crystallinity C6 have provided a satisfactory description of the crystallization process for one model system and 2-D stochastic models have been built in the space of these order parameters. We are currently investigating refinement of these models and generalizations to other assembly processes.
Keywords: diffusion mapping,order parameters

25. Wang; Thomas G Dimiduk, Vinothan N Manoharan

Harvard University, SEAS and Physics
"Digital holographic microscopy of asymmetric particles"
Digital holographic microscopy has been established as a fast, 3D imaging technique. The Mie solution for how the spheres scatter light can be fit to data to obtain the best fit object position to <1nm precision. An exact Mie solution, however, does not exist for arbitrary objects. Here we explore the possibility of using a Discrete Dipole Approximation to model how light scatters from asymmetric particles, and fit these scattering patterns to data.
Keywords: holography, colloids

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