Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M18: Rheology & Phases of Complex Colloidal Systems |
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Sponsoring Units: GSNP Chair: Alberto Fernandez-Nieves, Georgia Institute of Technology Room: 403 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M18.00001: Random-Walk Trajectories of Probe Particles in Viscoelastic Complex Fluids Manas Khan, Thomas G. Mason Trajectories of tracer spheres in complex fluids can exhibit exotic patterns that have interesting temporal and spatial dependence. In passive particle-tracking microrheology, measured trajectories can often be converted into linear viscoelastic properties of the complex fluids. To better portray the diversity in potentially observable trajectories, we have created a random walk simulation for spheres in viscoelastic complex fluids. In a simple case, for a Maxwell-Voigt fluid, a tracer bead is modeled as a harmonically bound Brownian particle in a potential well that itself diffuses over longer time-scales. We also show trajectories for a complex fluid having a wide distribution of relaxation times, as described by a generalized Maxwell fluid, and for a different complex fluid having a significantly anisotropic viscoelasticity along orthogonal spatial directions. This generalized approach enables us to generate trajectories for a wide range of complex fluids within the limit of linear viscoelasticity, and these trajectories, when viewed at different sampling times and total observation times, provide insight into experimentally measured particle-tracking microrheology measurements. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M18.00002: Inertial effects in viscoelastic materials and their implication in passive microrheology Tsutomu Indei, Jay Schieber, Andr\'{e}s C\'{o}rdoba We review our recent series of works about inertial effects of soft viscoelastic materials on the particle diffusion in the materials, and their ramifications for one- and two- bead passive microrheology. Firstly we focus on the effects of particle inertia, especially on the oscillation of the particle's mean-square displacement (MSD). This is the resonance oscillation between the inertial motion of the particle and the elastic components of the viscoelastic materials. Secondly we discuss the material inertia, focusing on the so-called Basset force of the viscoelastic bodies. The kinetic energy of the particle is dissipated not only due to the Stokes drag but also through the Basset force as the radiational propagation of the shear wave excited by the particle motion. The resonance oscillation of the MSD tends to decrease due to the Basset force. The Basset force is characterized by the wave length $\Lambda $ and the penetration depth $\Delta $ of the shear wave. At high frequencies, the Basset force becomes important when $\Lambda $ is less than the particle size (for single-bead microrheology) or less than the distance between two particles (for two-bead microrheology). On the other hand, at low frequencies, the Basset force is effective when $\Delta $ is larger than the sample size. Finally we show several examples of microrheological analysis taking account of these inertial effects. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M18.00003: Nanoparticle Salts: Structure, Rheology and Ion Transport Yu H. Wen, Lynden A. Archer Above a critical volume fraction associated with nanoscale particle spacing, interactions between tethered molecules (charged or uncharged) significantly affect particle-particle interactions and hence suspension rheology. We report on the structure, rheology, and ion transport of nanoparticles cofunctionalized with tethered salts and neutral molecules. Contrary to uncharged counterparts, charged particles in a low dielectric medium are shown to exhibit soft glassy rheology behaviors at low particle loadings, due to electrostatic bridging of salts. In addition, tethered molecules in a nanochannel created by particle crowding are conceptually similar to entangled polymers in a tube and, as a result, concentrated particle suspensions share a similar plateau modulus with entangled polymer melts. Our findings suggest that particle interactions can be fine-tuned using tethered salts of different charge densities, and geometrical confinement on tethered molecules produces topological constraints analogous to those in entangled polymers. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M18.00004: Liquid-Gel-Liquid Transition and Shear-Thickening in Mixed Suspensions of Silica Colloid and Hyperbranched Polyethyleneimine Guangcui Yuan, Huan Zhang, Charles C. Han The rheological property of mixed suspensions of silica colloid and hyperbranched polylethyleneimine was studied as functions of particle volume fraction, ratio of polymer to particle, and pH value. A mechanism of liquid-gel-liquid transition for this mixed system was proposed based on the amount and the conformation of polyelectrolyte bridges which were able to self-arrange with solution environments. The equilibrium adsorbed amount ($C_{\mathrm{p}}$*) for a given volume fraction of particles is an important concentration ratio of polymer to particle denoting the transition of irreversible and reversible bridging. For mixed suspensions at equilibrium adsorbed state ($C_{\mathrm{p}}\approx C_{\mathrm{p}}$*), the adsorption-desorption of polymer bridges on the particles can reversibly take place, and shear thickening is observed under a steady shear flow as a result of rapid extension of bridges when the relaxation time scale of extension is shorter than that of desorption. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M18.00005: Dynamics of Cubic Colloids John R. Royer, George L. Burton, Daniel L. Blair, Steven D. Hudson There have been significant advances in the synthesis of anisotropic particles, however little is known about how shape and directional interactions influence particle dynamics in a suspension. We address this issue by studying both the bulk rheology and micro-scale particle dynamics in suspensions of hollow, silica cubes. These cubes are particularly well-suited for studying the role of anisotropy since they are mono-disperse, readily dyed and index-matched for confocal imaging, and can be synthesized in bulk quantities. Using confocal microscopy to image dilute, quiescent suspensions of cubes, we find the long-time diffusion coefficient decreases with packing density as $D_\infty/D_0 \simeq 1-3.1\phi$, differing from the standard hard-sphere slope of -2.1. Similarly, small-volume viscometry reveal a higher intrinsic viscosity for the cubic particles, demonstrating that the particle shape has a significant impact on the suspension dynamics. We further investigate these shape-effects using confocal-rheometry to characterize shear-induced diffusion in these cubes. Using depletion, we also investigate the role of attractive, directional interactions, tuning the interaction strength by varying the depletant size and concentration. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M18.00006: Using colloids to model worm-like micelles Kazem V. Edmond, Tess W.P. Jacobson, Stefano Sacanna, Andrew D. Hollingsworth, David J. Pine We measure the viscosity of self-assembling chains of colloidal particles using a Zimm viscometer, a custom built Couette apparatus. Our microscopic particles are shaped like contact lenses or bowls, specially fabricated to fit inside one another so that they readily form chains in the presence of a depletant. Careful tuning of the interaction strength in a suspension of particles induces the formation of long chains. Shearing this material can twist, stretch, and break the chains, causing the material to exhibit unique rheological properties. We anticipate that these colloidal chains will model the behavior of worm-like micelles. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M18.00007: Transient formation of bcc crystal in suspensions of pNIPAM-based microgels Urs Gasser, Juan-Jose Lietor-Santos, Andrea Scotti, Oliver Bunk, Andreas Menzel, Alberto Fernandez-Nieves In suspensions of soft and deformable microgel particles, both the colloidal and the polymeric degrees of freedom are relevant for phase behavior at high concentrations. Crystal structures different from those formed by hard spheres (HS) are expected to form at high concentrations. However, our and other group's experimental work have shown that the crystal structure is comparable to that found in HS. Here, we present a small-angle X-ray scattering study of crystal growth in a system of slightly charged and swollen microgels of poly(N-isopropylacrylamide) co-polymerized with acrylic acid. As in HS, we find that random hexagonal close packed crystal grows in all samples and slowly transforms towards the face centered cubic lattice, which appears to be the equilibrium structure, as in HS. However, at intermediate volume fractions, a body centered cubic crystal phase appears, which is not stable and disappears as the samples age. This behavior is expected for fuzzy particles with a steric repulsion [P. Ziherl and R.D. Kamien, Phys. Rev. Lett. 85, 3528 (2000)]. This suggests that our observations could be related to the predictions of this model for fuzzy particles. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M18.00008: Crystals and liquids of ionic microgel particles: Osmotic pressure and phase coexistence Miguel Pelaez-Fernandez, Anton Souslov, L. Andrew Lyon, Paul M. Goldbart, Alberto Fernandez-Nieves We quantify the phase behavior of suspensions comprised of swollen, ionic microgels while measuring the system osmotic pressure. Surprisingly, the osmotic pressure of dilute suspensions is much larger than that expected for an ideal gas. Furthermore, we find that the width of the liquid-crystal phase coexistence region increases as the microgels are made softer; this is true in terms of a generalized volume fraction, $\zeta =$\textit{nV}$_{0}$, with $n$ the particle density and $V_{0}$ the dilute microgel volume. We will discuss the role played by the ions in our observations and compare with expectations based on computer simulations. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M18.00009: Dissipative Particle Dynamics simulation of colloidal suspensions Safa Jamali, Arman Boromand, Joao Maia DPD as a mesoscale method was firstly proposed to study dynamics of suspensions under flow condition. However the proposed method failed to capture shear properties of suspensions because it lacked: first a potential to reproduce lubrication forces and second a clear definition for the colloid surface. Recently we reported a modified DPD method which defines colloidal particles as particles with hard core and a dissipative coat. An additional lubrication force was introduced to include the short-range hydrodynamics that are not captured in original DPD. The model was found to be able to reproduce shear properties of suspensions for a wide range of different systems, from monodisperse to bimodal with different volume fractions, compositions and size ratios. In present work our modified DPD method is employed to study both equilibrium and flow properties of colloidal suspension. Zero shear viscosity of suspension is measured using Green-Kubo expressions and the results are compared to theoretical predictions. Furthermore, structure formation in suspensions is studied in respect to energy landscape of the fluid both at rest and under flow. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M18.00010: Inertia and dissipation mechanism in jammed soft-particle suspensions Arka Roy, Kamran Karimi, Craig Maloney Suspensions of soft particles exhibit a remarkable bifurcation at the random close packing volume fraction, fc. There is a yield stress above fc but not below, and the flow curves at various f have been shown to collapse onto a universal scaling function near this point. Particle-level models take contact deformation into account to model elastic forces and treat the drag forces in the very dense regime where long-range hydrodynamic interactions are thought to be negligible - with varying levels of sophistication: from ``pair-drag'' formulations that apply a lubrication calculation to the film at contact to simple ``mean drag'' approaches where the mobility matrix is diagonal. We show that, in simple shear, these two model give consistent results for the shear modulus, yield stress, and single-particle diffusivity as functions of f but only in the quasi-static regime. They show dramatically different behavior in the rate-dependent regime. In particular, the diffusion constant scales as the shearing rate to a non-trivial power with the power depending on the damping mechanism. Furthermore, we explore a ``granular'' regime where the inertia of the particles is no longer negligible and the finite rate behavior shows a complex interplay between inertial and dissipative timescales. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M18.00011: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M18.00012: Comparison of the Physical Aging Behavior of a Colloidal Glass after Shear-Melting and Concentration Jumps Xiaoguang Peng, Gregory McKenna We have prepared a thermosensitive core-shell PS-PNIPAM/AA particle system and have investigated the aging response of its colloidal dispersions subsequent to both shear-melting and temperature (concentration)-jump perturbations using sequential creep experiments to probe the response of the system. The experiments were performed in the vicinity of the glass concentration or temperature as evidenced by the strongly varying relaxation time with decreasing temperature. The colloidal glass displays aging behavior after both types of perturbation, but the kinetics of the aging are different, demonstrating that the structural changes induced by the mechanical perturbation are different from those induced by the temperature or concentration jump. We find that time-aging time superposition is valid in both cases and that the aging rate, as measured by the double logarithmic slope of the aging time shift factor vs. aging time, decreases with increasing temperature, similar to what is seen in aging of molecular glasses. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M18.00013: Structural bistability in quasi-hard-discs under adaptive circular confinement Ian Williams, Erdal C. Oguz, Paul Bartlett, Hartmut Loewen, C. Patrick Royall The behaviour of materials under spatial confinement is dramatically different from that in the bulk. The exact nature of behavioural modification in confined systems is strongly dependent on the boundary enclosing the system with soft walls inducing different phenomena than similar hard walls. Here we present a quasi-two-dimensional colloidal model system confined by an adaptive circular boundary defined using holographic optical tweezers. The adaptive boundary is deformable, enabling mechanical measurements of pressure and leading to the observation of a novel structural bistability between concentric particle layering and locally hexagonal configurations at high density. These findings are reproduced in analogous Monte Carlo simulations. Additionally, shearing the confined system drives the this bistability resulting in the observation of a novel oscillatory state characterised by periodically self-similar structural organisation. Under varying conditions, both shear melted and rigid-body-like flow behaviour is observed. [Preview Abstract] |
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