Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E2: Suspensions: Rheology II |
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Chair: Itai Cohen, Cornell University Room: 101 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E2.00001: Direct visualization of particle scale internal stresses in a colloidal glass Itai Cohen, Neil Lin, Matt Bierbaum, James Sethna Bullet proof windshields, smart phone screens, and Prince Rupert’s drop are all examples of how internal stresses can dramatically affect the strength of glass. Imaging the way internal stresses are distributed and their evolution under an applied load remains prohibitively difficult. For example, work on disordered granular packs suggests that stress heterogeneity may extend down to the scale of a single particle. While resolving stresses at the single atom scale is not feasible, measurements of stresses at the single particle scale in colloidal glasses, a widely used model system for atomic glasses, can be achieved by using Stress Assessment from Local Structural Anisotropy (SALSA). This method relies solely on the particle configurations obtained via high speed confocal microscopy. Here, we use SALSA to visualize the three dimensional stress network in a colloidal glass. By placing the suspension under shear we determine the evolution of this network and how it alters the bulk mechanical behavior of the suspension. Our work constitutes a first step towards understanding how local variations in the stress networks of glasses can lead to the dramatic mechanical properties of tempered glass. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E2.00002: Active microrheology of Brownian suspensions via Accelerated Stokesian Dynamics simulations Henry Chu, Yu Su, Kevin Gu, Nicholas Hoh, Roseanna Zia The non-equilibrium rheological response of colloidal suspensions is studied via active microrheology utilizing Accelerated Stokesian Dynamics simulations. In our recent work, we derived the theory for micro-diffusivity and suspension stress in dilute suspensions of hydrodynamically interacting colloids. This work revealed that force-induced diffusion is anisotropic, with qualitative differences between diffusion along the line of the external force and that transverse to it, and connected these effects to the role of hydrodynamic, interparticle, and Brownian forces. This work also revealed that these forces play a similar qualitative role in the anisotropy of the stress and in the evolution of the non-equilibrium osmotic pressure. Here, we show that theoretical predictions hold for suspensions ranging from dilute to near maximum packing, and for a range of flow strengths from near-equilibrium to the pure-hydrodynamic limit. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E2.00003: Using Shear Reversal and Biaxial Shear Flows to Investigate Anisotropic Shear Thickening in Colloidal Suspensions Neil Lin, Ben Guy, Michiel Hermes, Chris Ness, Jin Sun, Wilson Poon, Itai Cohen Shear thickening is a ubiquitous phenomenon in suspension flow where an increase in shear rate gives rise to an increase in viscosity. Whether contact forces play a role in continuous shear thickening of colloidal systems where hydrodynamic contributions are thought to dominate remains highly controversial. By performing shear reversal experiments on silica and latex colloidal particles, we directly measure the hydrodynamic and contact force contributions to the suspension viscosity. We find that contact forces are not only present, but dominate the shear thickening response. Since there are no system-spanning force networks in our low-volume fraction suspensions, it is not clear whether the thickening is isotropic or biased resulting in an anisotropic viscosity. To answer this question we employ biaxial shear rheology to determine whether thickening in such suspensions is isotropic. We apply a primary dominant shear flow to thicken the suspension, and simultaneously measure the suspension viscosity along the orthogonal direction using a secondary weak flow. We report on the evolution of this orthogonal viscosity as the system is driven into the shear thickening regime. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E2.00004: Non-equilibrium depletion interactions: first things attract, then they repel Benjamin Dolata, Roseanna Zia Non-Equilibrium depletion interactions in colloidal dispersions are studied theoretically via a combination of asymptotic and numerical solutions of the Smoluchowski equation. A pair of probes at arbitrary separation is driven by an external force at arbitrary orientation through a suspension, deforming the surrounding microstructure. The degree to which the structure is distorted, and the shape of this deformation, depends on the separation between the probes, on the orientation of the pair to the driving force, and on the strength with which the probes are forced relative to the entropic restoring force of the suspension particles. The resultant non-equilibrium osmotic pressure gradients give rise to both drag and interactive forces between the probes. When the external force is zero, the depletion attraction of Asakura and Oosawa is recovered. When an external force is applied, the interactive force can lead either to attraction or repulsion, as well as deterministic re-orientation of the probes relative to the external force, depending on initial separation, orientation, and strength of forcing. The use of this model for interrogation of non-continuum and elastically networked materials is explored. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E2.00005: Effect of confinement induced structures on suspension rheology Meera Ramaswamy, Brian Leahy, Yen-Chih Lin, Itai Cohen Understanding the flow behavior of confined colloidal systems is important in many industrial settings ranging from inkjet printers to pharmaceuticals. Confined colloidal suspensions under shear demonstrate many fascinating responses including vorticity-aligned strings in colloidal liquids and buckled phases in crystals. Despite the extensive studies of these confinement induced structures, the interplay between these exotic structural responses and the suspension rheology remains poorly understood. Here, we use a confocal rheoscope to image the suspension particle configuration while simultaneously measuring its stress responses. The confocal rheoscope has two precisely-aligned parallel plates that can confine the suspension with a variable gap size ranging from 3 to 20 particle diameters, allowing us to measure the response of the system as a function of the gap size. Moreover, we alter the rheological properties of the sample by adding a small amount of dimers. The dimers undergo Jeffery orbits at large strains and deform the confinement induced structures of the spheres, leading to a viscosity change. We discuss the results of these experiments and their implications in the areas of micro and nanofluidics. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E2.00006: Brownian Particles Under Shear: Rheology \& Microstructure Somayeh Farhadi, Madhura Gurjar, Nathan Keim, Paulo Arratia We present 2D rheological experiments of dense suspensions of Brownian (1 $\mu$m) particles. The particles, which are purely repulsive, are adsorbed at an oil-water interface and are sheared periodically by a magnetized needle. The area fraction of the sample is kept fixed at approximately 40\%, which is above its glass transition. We measure the bulk rheology at low strain amplitudes while simultaneously track the particles in order to understand the microstructural contributions to yielding and plasticity in thermal systems. Previous studies on nonthermal colloids (of size 4-6 $\mu$m) identified a regime, below yielding point, where localized regions of space with reversible cycles undergo plastic deformations. For thermal systems, we anticipate to observe a transition from plastically reversible to irreversible states as the Péclet number (which characterizes the shear-induced to diffusion-induced displacements) is decreased. We also investigate the directional diffusion of particles by probing anisotropy of the diffusion matrix, which gives us information on how the thermal and convective effects add up for highly packed systems. [Preview Abstract] |
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