Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D34: Gels and Complex Fluids |
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Sponsoring Units: GSOFT Chair: George Thurston, Rochester Institute of Technology Room: 210A |
Monday, March 2, 2015 2:30PM - 2:42PM |
D34.00001: Diffusion of polyelectrolytes in polyelectrolyte gels Anand Rahalkar, Murugappan Muthukumar Using dynamic light scattering, we have investigated the diffusion coefficient of sodium poly(styrene sulfonate) in a matrix of poly(acrylamide-co-acrylate) gels. The diffusion coefficient of the probe polyelectrolyte exhibits a crossover behavior from a particle-diffusion to entropic-barrier dominated diffusion, as the molecular weight is increased. The effect of electrostatics, by varying the charge density of the matrix, on probe diffusion constant will be presented. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D34.00002: The origin of and conditions for clustering in fluids with competing interactions Ryan Jadrich, Jonathan Bollinger, Thomas Truskett Fluids with competing short-range attractions and long-range repulsions exhibit a rich phase behavior characterized by intermediate range order (IRO), as quantified via the static structure factor. This phase behavior includes cluster formation depending upon density-controlled packing effects and the magnitude and range of the attractive and repulsive interactions. Such model systems mimic (to zeroth order) screened, charge-stabilized, aqueous colloidal dispersions of, e.g., proteins. We employ molecular dynamics simulations and integral equation theory to elucidate a more fundamental microscopic explanation for IRO-driven clustering. A simple criterion is identified that indicates when dynamic, amorphous clustering emerges in a polydisperse system, namely when the Ornstein-Zernike thermal correlation length in the system exceeds the repulsive potential tail range. Remarkably, this criterion also appears tightly correlated to crystalline cluster formation in a monodisperse system. Our new gauge is compared to another phenomenological condition for clustering which is when the IRO peak magnitude exceeds $\sim$ 2.7. Ramifications of crystalline versus amorphous clustering are discussed and potential ways of using our new measure in experiment are put forward. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D34.00003: A single parameter description of aggregate morphologies in two-dimensions Tamoghna Das, Mahesh Bandi A morphological hierarchy of two-dimensional aggregates has been studied using molecular dynamics. Particulate aggregates resulting from the competition between short-range attraction and long-range repulsion show a transition from non-compact to compact to percolated `gel' structures as the competition varies at a constant temperature and density. A three-dimensional (3D) parameter space controlling the competition is mapped to a single dimensionless parameter $\Lambda$. A unique relation found between the reduced second virial coefficient $B_2^*$, computed for a large set of points in the 3D parameter space, and $\Lambda$ provides strong support for the proposed description. The observed morphologies were further quantified using an entropic measure $S_2$ of positional information. A simple scaling relation between $S_2$ and $\Lambda$ shows the promise of describing the static structures of aggregates in terms of geometry alone. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D34.00004: Dynamics across the morphological transition in two-dimensional aggregates Mahesh Bandi, Tamoghna Das Microscopic dynamics of two-dimensional aggregates have been studied by analysing simulated particle trajectories generated by molecular dynamics. Tuning the competition between the short-range attraction and long-range repulsion in a particulate system at fixed temperature and density results in a continuous non-compact to compact morphological transition. The finite-size aggregates, obtained by very slow cooling, show long-time sub-diffusive behaviour irrespective of their morphologies. By analysing the relative displacement fluctuations of particles with respect to their nearest neighbours, non-compact aggregates can be attributed to bonding between particles while caging is found to be responsible for compact clusters. These dynamical mechanisms are further illustrated by the self-displacement fluctuation of particles which show a continuous change from power-law to exponential behaviour across the non-compact to compact transition. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D34.00005: 1- and 2-particle Microrheology of Hyaluronic Acid Austin Sagan, Sarah Kearns, David Ross, Moumita Das, George Thurston, Scott Franklin Hyaluronic acid (also called HA or Hyaluronan) is a high molecular weight polysaccaride ubiquitous in the extracellular matrix of soft tissue such as cartilage, skin, the eye's vitreous gel and synovial fluid. It has been shown to play an important role in mechanotransduction, cell migration and proliferation, and in tissue morphodynamics. We present a confocal microrheology study of hyaluronic acid of varying concentrations. The mean squared displacement (MSD) of sub-micron colloidal tracer particles is tracked in two dimensions and shows a transition from diffusive motion at low concentrations to small-time trapping by the protein network as the concentration increases. Correlations between particle motion can be used to determine an effective mean-squared displacement which deviates from the single-particle MSD as the fluid becomes less homogeneous. The real and effective mean-squared displacements are used to probe the local and space-averaged frequency dependent rheological properties of the fluid as the concentration changes. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D34.00006: A Rate-Dependent Shear Transformation Zone Model of Shear Band Formation During Flow Adam R. Hinkle, Michael L. Falk Recent shear-experiments of carbopol gels have revealed the formation of a transient shear band before reaching the steady-state characterized by homogeneous flow. Analysis of this phenomenon using a rate-dependent effective temperature in the shear transformation zone (STZ) theory reveals that the observed fluidization proceeds via two distinct processes: A shear band initiates and broadens via disordering at the interface of the band. This is accompanied by spatially homogeneous fluidization outside of the shear band where the disorder of the gel grows uniformly. Experimental data are used to parameterize the STZ theory, and direct, quantitative comparison is made to measurements of the structural evolution of the gel. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D34.00007: Effect of short range hydrodynamic on bimodal colloidal gel systems Arman Boromand, Safa Jamali, Joao Maia Colloidal Gels and disordered arrested systems has been studied extensively during the past decades. Although, they have found their place in multiple industries such as cosmetic, food and so on, their physical principals are still far beyond being understood. The interplay between different types of interactions from quantum scale, Van der Waals interaction, to short range interactions, depletion interaction, and long range interactions such as electrostatic double layer makes this systems challenging from simulation point of view. Many authors have implemented different simulation techniques such as molecular dynamics (MD) and Brownian dynamics (BD) to capture better picture during phase separation of colloidal system with short range attractive force. However, BD is not capable to include multi-body hydrodynamic interaction and MD is limited by the computational resources and is limited to short time and length scales. In this presentation we used Core-modified dissipative particle dynamics (CM-DPD) with modified depletion potential, as a coarse-grain model, to address the gel formation process in short ranged-attractive colloidal suspensions. Due to the possibility to include and separate short and long ranged-hydrodynamic forces in this method we studied the effect of each of those forces on the final morphology and report one of the controversial question in this field on the effect of hydrodynamics on the cluster formation process on bimodal, soft-hard colloidal mixtures. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D34.00008: Transient yield in reversible colloidal gels: a micro-mechanical perspective Lilian Johnson, Benjamin Landrum, William Russel, Roseanna Zia We study the nonlinear response rheology of colloidal gels via large-scale dynamic simulation, with a view toward understanding the micro-mechanical origins of the transition from solid-like to liquid-like behavior during flow startup, and post-cessation relaxation. Such materials often exhibit an overshoot in the stress response during startup, but the underlying microstructural origin of this behavior remains unclear. The gels studied here comprise Brownian particles interacting via hard-sphere repulsion and short-range attraction of strength of O(\textit{kT}) that leads to formation of a bi-continuous network. The relatively weak bonds allow the network to restructure over time; our recent work defines the structural evolution and dynamics of such coarsening, and its impact on linear-response rheology. Here we investigate the role of particle attractions and evolving structure on the nonlinear response of the gel. Upon startup of an imposed strain rate, the transition from rest to steady flow is characterized by one or more ``overshoots'' in the shear stress. Experimental studies, in which the overshoots depend on gel age, strain rate, volume fraction, and attraction strength, suggest that the underlying microstructural origin is a two-step process of cage breaking and bond breaking. However, our detailed studies of the microstructural evolution during startup challenge this view. We present a new model of stress development, relaxation, and memory in reversible colloidal gels in which the ongoing age-coarsening process plays a qualitatively new role. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D34.00009: Delayed yield in reversible colloidal gels: a micro-mechanical perspective Roseanna N. Zia, Benjamin J. Landrum, William B. Russel We study via dynamic simulation the nonlinear response of a reversible colloidal gel undergoing deformation under applied stress, with a view toward elucidating mechanisms of macroscopic yield at the level of particle dynamics.~Under shear, such gels may flow then regain solidlike behavior upon removal of the stress. The transition from solidlike to liquidlike behavior is a yielding process that is not instantaneous but rather occurs after a finite delay. The delay length decreases as stress increases, but the underlying microstructural origin is not clear.~Recent experiments reveal two regimes, suggesting multiple yield mechanisms.~ Theories advanced to link gel structure to rheology aim to predict the ultimate state of a gel under an applied load.~While these hypothesize a competition between bond breakage and reconnection rates, no such particle-scale dynamics have been directly observed, and it is not clear these theories reconcile with ongoing structural evolution.~To study these behaviors, we conduct large-scale dynamic simulation to model structural evolution and particle transport in colloidal gels subjected to a step stress. A range of volume fraction, attraction strength, and stress is studied, with detailed connection between macroscopic response, microstructure, and particle dynamics. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D34.00010: Using dissipative particle dynamics to model micromechanics of responsive hydrogels Alexander Alexeev, Svetoslav Nikolov, Alberto Fernandez De Las Nieves The ability of responsive hydrogels to undergo complex and reversible shape transformations in response to external stimuli such as temperature, magnetic/electric fields, pH levels, and light intensity has made them the material of choice for tissue scaffolding, drug delivery, bio-adhesive, bio-sensing, and micro-sorting applications. The complex micromechanics and kinetics of these responsive networks however, currently hinders developments in the aforementioned areas. In order to better understand the mechanical properties of these systems and how they change during the volume transition we have developed a dissipative particle dynamics (DPD) model for responsive polymer networks. We use this model to examine the impact of the Flory-Huggins parameter on the bulk and shear moduli. In this fashion we evaluate how environmental factors can affect the micromechanical properties of these networks. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D34.00011: Separation and concentration of protein and microgel dispersions Rafael Roa, Gerhard Naegele Membrane ultrafiltration is a pressure driven process where Brownian particles, such as small colloids or nanoparticles, are concentrated. This process is of high importance for the separation and enrichment of protein and microgel dispersions, where convective-diffusive particle transport determines the permeate flux. The efficiency of the separation process is thus strongly dependent on particle hydrodynamic structure and boundary conditions, membrane properties, and particle interactions. We calculate the concentration polarization layer and the permeate flux at different operating conditions for cross-flow ultrafiltration of BSA proteins and for ionic and non-ionic microgels. We show that the proper specification of the concentration dependent dispersion transport properties and the inclusion of microgel permeability have a significant effect on the filtration behavior on concentrated systems. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D34.00012: Relaxation Mode Analysis and Scale-Dependent Energy Landscape Statistics in Liquids Zhikun Cai, Yang Zhang In contrast to the prevailing focus on short-lived classical phonon modes in liquids, we propose a classical treatment of the relaxation modes in liquids under a framework analogous to the normal mode analysis in solids. Our relaxation mode analysis is built upon the experimentally measurable two-point density-density correlation function (e.g. using quasi-elastic and inelastic scattering experiments). We show in the Laplace-inverted relaxation frequency z-domain, the eigen relaxation modes are readily decoupled. From here, important statistics of the scale-dependent activation energy in the energy landscape as well as the scale-dependent relaxation time distribution function can be obtained. We first demonstrate this approach in the case of supercooled liquids when dynamic heterogeneity emerges in the landscape-influenced regime. And then we show, using this framework, we are able to extract the scale-dependent energy landscape statistics from neutron scattering measurements. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D34.00013: Gelation and glass transition of particles with short-range attraction induced by adsorbing microgel Guangcui Yuan, Junhua Luo, Charles C. Han Mixed suspensions of large hard polystyrene microsphere and small poly(N-isopropylacrylamide) microgel is used as model systems to investigate the static and viscoelastic properties of suspensions which go through liquid to gel and to glass transitions. The microgels cause short-range attraction between microspheres through bridging and depletion mechanism whose strength can be tuned by the microgel concentration. Baxter's sticky hard-sphere model is used to extract the effective inter-microsphere interaction introduced by bridging or depletion of microgels despite the fact that the physical mechanisms of bridging attraction and depletion attraction are different at a molecular level. A new state diagram of gelation and even of glass transition was constructed by taking the bridges as a short-ranged attractive interaction With the help of the well-defined bridging bonds, some controversies regarding to the interference between two origins for ergodic to$_{\mathrm{\thinspace }}$non-ergodic transition in condensed system, i.e. cage effect and bond effect, were clarified. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D34.00014: Osmotic Pressure in Ionic Microgel Dispersions Alan R. Denton, Qiyun Tang Microgels are microscopic gel particles, typically 10-1000 nm in size, that are swollen by a solvent. Hollow microgels (microcapsules) can encapsulate cargo, such as dye molecules or drugs, in their solvent-filled cavities. Their sensitive response to environmental conditions (e.g., temperature, pH) and influence on flow properties suit microgels to widespread applications in the chemical, pharmaceutical, food, and consumer care industries. When dispersed in water, polyelectrolyte gels become charged through dissociation of counterions. The electrostatic contribution to the osmotic pressure inside and outside of ionic microgels influences particle swelling and bulk materials properties, including thermodynamic, structural, optical, and rheological properties. Within the primitive and cell models of polyelectrolyte solutions, we derive an exact statistical mechanical formula for the contribution of mobile microions to the osmotic pressure within ionic microgels. Using Poisson-Boltzmann theory, we validate this result by explicitly calculating ion distributions across the surface of an ionic microgel and the electrostatic contribution to the osmotic pressure. Within a coarse-grained one-component model, we further chart the limits of the cell model for salty dispersions. [Preview Abstract] |
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