Bulletin of the American Physical Society
80th Annual Meeting of the APS Southeastern Section
Volume 58, Number 17
Wednesday–Saturday, November 20–23, 2013; Bowling Green, Kentucky
Session EC: Soft Matter, Complex Fluids and Polymers in the Southeast, Biophysics: Biomechanics and Cellular Mechanics |
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Chair: Robert Cohn, University of Louisville Room: 2 |
Friday, November 22, 2013 8:30AM - 9:06AM |
EC.00001: Dynamics of Particles in Soft Matter Invited Speaker: Michael Rubinstein Can the properties of materials be deduced from the analysis of the trajectories of probe particles diffusing through them? The anomalous diffusion of a particle in complex media could be due to three fundamental reasons: (1) Viscoelastic response of the medium to the deformation imposed on it by the moving particle; (2) The particle could be attracted to some regions of heterogeneous medium and be temporary localized in these ``sticky'' regions; (3) The particle is repelled from some regions of the medium and has to go over different energy barriers in order to diffuse through this medium. Can one determine which of these fundamental reasons cause the anomalous diffusion? We propose a method of analyzing particle trajectories to answer this question and to determine the corresponding properties of complex media such as distribution of relaxation times or energy distribution of attractive regions. We use scaling theory to derive the time dependence of the mean-square displacement \textless r$^{2}$(t)\textgreater of a probe nanoparticle in polymer solutions and melts. We distinguish several qualitatively different cases depending on the size d of the particle in comparison to solution correlation length $\xi $ and tube diameter a for entangled polymer liquids. We also describe a hopping mechanism for diffusion of particles larger than mesh size of polymer solids (networks and gels). We solve activated hopping model in which particle experiences thermally activated jumps between neighboring wells of different energy depths. We find that the particle diffusion is ordinary Brownian (not anomalous) if the width of the distribution of well energies $\Delta U$ is smaller than thermal energy \textit{kT}. In the opposite case ($\Delta $\textit{U\textgreater kT}) we discover the surprising result that although jumps between neighboring wells are completely random and uncorrelated, the particle displacements during consecutive time intervals are correlated. The source of these correlations is that the particle can be located in the same well during both time periods. As the result, while the mean square displacement of the particle is still Brownian, the distribution of displacements is non-Gaussian and is almost exponential. [Preview Abstract] |
Friday, November 22, 2013 9:06AM - 9:42AM |
EC.00002: Small-angle Neutron Scattering and its Applications for Strongly Interacting Soft Matter Systems Invited Speaker: Yuri Melnichenko The first experimental studies of concentrated and thus strongly interacting polymer systems became possible due to the development of the small angle neutron scattering (SANS) technique combined with deuterium labeling of a fraction of the polymer. The technique opened an opportunity to extract information about size, shape, conformational changes as well as molecular associations of polymers in ``crowded'' environments, due to capability of separating the \textit{inter-} and \textit{intra-}chain contributions to the structure. Modern applications of SANS are numerous and it has been successfully used for investigations of the variety of systems, such as polymeric materials, biological macromolecules, colloids, confined fluids, etc. In this talk I will give examples of how SANS can be used to study strongly interacting soft matter systems, where the technique, in combination with deuterium labeling and high concentration method, can provide important and often unique information on the structure and thermodynamic properties and has helped to unveil universal aspects of the polymer behavior in polymer solutions, blends, polyelectrolytes, supercritical mixtures, and nanocomposites. I will also overview instrumentation and sample environments at SNS and High Flux Isotope Reactor, Oak Ridge National Laboratory available for users who are interested in studying structure and dynamics of soft matter materials. [Preview Abstract] |
Friday, November 22, 2013 9:42AM - 10:18AM |
EC.00003: Solid-liquid phase transitions of fire ant rafts and towers Invited Speaker: David Hu Fire ants, Solenopsis invicta, link their bodies together to form waterproof rafts, which in turn drip, spread, and coagulate, demonstrating properties of an active material that can change state from a liquid to a solid. This soft-matter phase transition permits the raft to withstand environmental forces such as raindrops and crashing waves. We present our overview of our combined experimental and theoretical work on ant rafts and towers. Particular attention is paid to rationalizing construction rates based upon individual behaviors and constraints of the ants. We also present preliminary work using plate-on-plate rheology of the ants, extracting the active components by comparison with the rheological behavior of a collection of dead ants. [Preview Abstract] |
Friday, November 22, 2013 10:18AM - 10:54AM |
EC.00004: Are soap films ideal 2D fluids? Invited Speaker: Eric Weeks Soap films are very thin, but their thickness is finite. To what extent can they be thought of as an ideal two-dimensional fluid, and to what extent does the three-dimensional world intrude? By following the Brownian motion of tracer particles embedded in the soap film, my lab learns about the flow field of the film. Our experiments show that the soap flow fields are indeed two-dimensional in character, but that they are never ideal. Furthermore, soap films that are ``too thick'' are even less ideal. [Preview Abstract] |
Friday, November 22, 2013 10:54AM - 11:00AM |
EC.00005: Break
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Friday, November 22, 2013 11:00AM - 11:36AM |
EC.00006: Bio-Inspired Sticky and Slippery Materials Invited Speaker: Noshir Pesika Through evolution nature has come up with solutions or alternatives to overcome challenges encountered by living organisms. This has led to the field of biomimetics, in which scientists are continually developing new technologies or improving current technologies by learning from nature. In this talk, I will present our recent work in developing a gecko-inspired adhesive and demonstrate how the judicious design of the surface microstructure gives rise to unique properties reminiscent of the natural gecko adhesive system; i.e., when the adhesive is sheared in one direction, it offers high adhesion and friction forces, but when sheared in the opposite direction, the adhesion and friction forces generated are lower. As another example, I will also discuss how surface texturing of a compliant polymer film, inspired by cartilage, can lower friction forces and potentially reduce surface wear. [Preview Abstract] |
Friday, November 22, 2013 11:36AM - 12:12PM |
EC.00007: Toroidal droplets of nematic liquid crystal: Generation, stabilization and twist Invited Speaker: Alberto Fernandez-Nieves We generate nematic droplets with handles and stabilize them against surface-tension-driven instabilities using a continuous phase with a yield stress. For toroidal droplets, the nematic spontaneously twists; this happens for all, slender and fat tori. The addition of handles is accompanied by the presence of defects in the order; there are two -1 defects per additional handle located in regions with saddle geometry. [Preview Abstract] |
Friday, November 22, 2013 12:12PM - 12:48PM |
EC.00008: Equilibrium and Dynamic Forces in Binary Colloid Mixtures Invited Speaker: John Walz The presence of nonadsorbed nanoparticles in a dispersion of larger ``microparticles'' can greatly alter the dispersion stability. The effects can be quite complex, as low concentrations of the nanoparticles can induce flocculation, while higher concentrations may actually promote stability. Not only are such binary mixtures found in nature, but nanoparticles can be used to carefully tune the interaction force between two microparticles for purposes of studying colloidal crystallization and melting, and can also be used to separate microparticles of different sizes or surface properties. In this talk I will describe a modeling and experimental study to better understand the nature of the forces in such systems. Direct measurements of the force between a single microparticle and a planar surface in a well-characterized system were obtained using colloidal probe atomic force microscopy. This includes both equilibrium forces, in which hydrodynamic interaction between the microparticle and surface was negligible, and forces measured at varying approach speeds to determine the impact of the nanoparticles on the separation-dependent mobility of the microparticle. The equilibrium measurements clearly show both attractive depletion forces and longer-range, oscillatory structural forces that result from ordering of the nanoparticles in the confined region between the microparticle and surface. The dynamic measurements shot that the effective viscosity experienced by the microparticle decreases at smaller separations as the average nanoparticle concentration in the gap region decreases. The impact of these forces on the stability of a dispersion of microparticles will also be presented. [Preview Abstract] |
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