Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B59: Rheology and Flow of Particulate Matter IIFocus

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Sponsoring Units: GSOFT GSNP Chair: Ryohei Seto Room: BCEC 257B 
Monday, March 4, 2019 11:15AM  11:27AM 
B59.00001: Fragility in shear jamming of dense frictional suspensions Ryohei Seto, Abhinendra Singh, Omer Sedes, Morton M Denn, Bulbul Chakraborty, Jeffrey Morris Particulate systems often undergo fluidsolid transition. The packing fraction that corresponds to this particular transition is often found to be protocol dependent. In case of frictional particulate systems, starting from a forcefree random configuration at φ > φ_{sj}^{μ }and subjected to shear by constant stress, some rearrangements (i.e., flowing) may occur before getting jammed. Such shear jammed states may flow again by changing the direction of the applied stress; such states are termed "fragile." We quantify this fragility with a stress reversal simulation and identify the point where the fragility vanishes as the isotropic jamming point φ_{j}^{μ}. We also present a shearjamming phase diagram of stressdependent suspension model with a criticalload friction model. 
Monday, March 4, 2019 11:27AM  11:39AM 
B59.00002: Uncovering Instabilities in the Spatiotemporal Dynamics of a ShearThickening Cornstarch Suspension Brice SaintMichel, Thomas Gibaud, Sebastien Manneville We explore unsteady dynamics in a dense cornstarch suspension by coupling long rheological measurements under constant shear stresses to ultrasound imaging. We demonstrate that unsteadiness in DST results from localized bands that travel along the vorticity direction with a specific signature on the global shear rate response. These propagating events coexist with quiescent phases for stresses slightly above DST onset, resulting in intermittent, turbulentlike dynamics. Deeper into DST, events proliferate, leading to simpler, Gaussian dynamics. We interpret our results in terms of unstable vorticity bands as inferred from recent model and numerical simulations. 
Monday, March 4, 2019 11:39AM  11:51AM 
B59.00003: Shear jamming, DST and fragile of frictional granular materials under oscillatory shear Michio Otsuki, Hisao Hayakawa The mechanical response of twodimensional frictional granular 
Monday, March 4, 2019 11:51AM  12:27PM 
B59.00004: Using orthogonal shear and acoustic excitation to tune thickening and jamming in suspensions Invited Speaker: Itai Cohen Running fast enough over a tub of thickening cornstarch suspension and not sinking is one of the most entertaining science demonstrations garnering millions of viewers every year. As stress is applied to the suspension, the cornstarch particles come close enough to form force chains that are capable of supporting the weight of a person. This thickening, while fun for demonstrations, causes problems for efficient processing of industrial suspensions and garners a high cost. Previously we showed that by applying fast orthogonal perturbations, the viscosity of suspensions under shear can be tuned and decreased by over an order of magnitude. Here, we show that impact driven jamming can be eliminated using a similar technique. Specifically, we conducted a series of impact and extensional flow experiments while cyclically oscillating the bottom plate of our rheometer about the vertical axis. We observe a drastic reduction in the normal forces when the oscillations are turned on. These experiments have the potential to alter suspension processing methods used in numerous industries. 
Monday, March 4, 2019 12:27PM  12:39PM 
B59.00005: Dynamics of Localized Stresses in Shearthickening Cornstarch Suspensions Vikram Rathee, Daniel Blair, Jeffrey S Urbach Dense colloidal and cornstarch suspensions are common examples of shear thickening suspensions. Spatially resolved surface stresses in a model colloidal suspension using boundary stress microscopy (BSM) revealed dynamic regions of substantially increased local stresses that appear intermittently and move in shear direction. We report BSM of cornstarch suspensions and also find large stress heterogeneities, but with very different dynamics. Above critical stress, the heterogeneous regions propagate along the vorticity direction, perpendicular to shear direction, while only at higher applied stresses we do observe propagation in the shear direction similar to the behavior observed in colloidal suspensions. 
Monday, March 4, 2019 12:39PM  12:51PM 
B59.00006: Connecting shear jamming and discontinuous shear thickening Ian Madden, Erik Luijten Shear stress can induce jamming at packing fractions below the static jamming packing fraction. Here we investigate shearjamming transitions in a model of twodimensional shearjammed discs, observing a transient shearjamming behavior corresponding to fragile states before the shearjamming transition. The existence of these states below the onset of shearjamming was first discovered by Bi et al. (Nature 480, 355–358 (2011)), and further investigation uncovered the related phenomenon of discontinuous shear thickening (Nature 532, 214217 (2016)). We use molecular dynamics simulations to connect theory to experiment, revealing discontinuous shear thickening and transient shearjamming to be intimately related. Using this knowledge, we obtain guidance for the design of an experimental system that can reproduce our result. 
Monday, March 4, 2019 12:51PM  1:03PM 
B59.00007: Microscopic signatures of yielding in concentrated nanoemulsions under largeamplitude oscillatory shear Robert Leheny, Michael C. Rogers, Kui Chen, Matthew J Pagenkopp, Thomas G Mason, Suresh Narayanan, James Harden Yielding occurs when a solid is stressed beyond its elastic limit. Signatures of yielding at the nanotomicroscale are irreversible changes to the material's structure. In amorphous solids, the intrinsic disorder makes identifying these microstructural changes difficult. We describe xray photon correlation spectroscopy experiments on a series of concentrated oilinwater nanoemulsions subjected to in situ largeamplitude oscillatory shear in which we characterize the shearinduced droplet dynamics associated with yielding [1]. The dynamics include irreversible rearrangements among the droplets that occur in some regions of the nanoemulsions during a given shear cycle and residual strainlike displacements in those regions that do not rearrange. We observe a powerlaw distribution in the size of regions undergoing rearrangement. The values of the onset strains for rearrangement correlate with the concentrationdependent macroscopic yielding behavior. Specifically, they occur below the strains at which the nanoemulsions become effectively fluidized and, except for the lowest concentration sample in the study, significantly above the threshold strain for nonlinear rheological response. 
Monday, March 4, 2019 1:03PM  1:15PM 
B59.00008: Traction force rheology of colloidal polycrystals and glasses Zsolt Terdik, David A Weitz, Frans A Spaepen Micron sized colloidal particles in a solution can be assembled and ordered into densely packed polycrystals or disordered glasses. Due to the large size and slow dynamics of colloidal particles, confocal microscopy can be used to directly measure the 3D structure of dynamics of colloidal solids. Measuring the stress response, in addition to visualization, is a challenge because large colloids (which are essential for confocal microscopy) and thermal interaction energies necessarily give rise to solid phases with exceptionally small elastic constants on the order of 10100 mPa. We introduce a new technique, traction force rheology, to directly measure the mechanical response of colloidal polycrystals and glasses while simultaneously visualizing the microstructure. The method consists of a bilayer of colloidal solid sitting atop a well calibrated soft polymer gel of comparable shear modulus. The composite bilayer is sheared and the shear stresses are inferred from the displacement of embedded tracer particles in the gel. To complement the direct stress measurements, we visualize, in 3D, the time evolution of the complex microstructure including dislocation and grain boundaries in colloidal polycerystals and flow defects in the colloidal glass. 
Monday, March 4, 2019 1:15PM  1:27PM 
B59.00009: Using oscillatory shear to tune the viscosity of shear thickening suspensions Meera Ramaswamy, Abhishek Shetty, Itai Cohen When concentrated colloidal suspensions are under stress, their viscosity can increase by over an order of magnitude. These shear thickening suspensions have many interesting technological applications but are also extremely difficult to process in an industrial setting due to the strong dependence of the flow properties on the shear rate. Previous work has shown that the viscosity of these suspensions can be tuned by applying fast orthogonal perturbations to the system. This behavior, however, was only demonstrated for a few values of applied stress and one suspension volume fraction. We are using a custom built attachment to a standard Anton Paar rheometer, to extend these previous measurements to various volume fractions, applied stresses, and system sizes. In this talk, I will describe our results which show that dethickening due to orthogonal shear is volume dependent, and a nonmonotonic dependence on the primary applied strain rate. Understanding these trends has consequences for applications involving shear thickening fluids ranging from 3D printing to the processing of cement. 
Monday, March 4, 2019 1:27PM  1:39PM 
B59.00010: Multiparticle finite element model of highly deformed hydrogel assemblies Craig Maloney, Ahmed Elgailani Abstract: Packings of hydrogel particles, like any granular material, become rigid when volumetrically confined. At low confinement near the onset of rigidity, linear elastic contact mechanics should provide a good description of the interparticle forces. At higher confinement, the particles become strongly deformed and linear elastic contact mechanics no longer provides a reasonable description of the interparticle forces. Here, we report on simulations using a multiparticle finite element technique, employing the FloryRehner constitutive law, to model the full nonlinear elastic deformation of all particles in the packing. We show that the shear modulus (μ) of the packing depends strongly on confining pressure (p) with a crossover from a lowpressure regime where μ ~ p^{1/2 }to a highpressure stronglyfaceted regime where μ ~ p^{1/4} where interstitial space has essentially vanished and the facet geometry no longer evolves with pressure. 
Monday, March 4, 2019 1:39PM  1:51PM 
B59.00011: Yielding Versus Jamming: Critical Scaling of Sheared SoftCore Disks Jacob Thompson, Abe Clark Using discrete element simulations, we study critical behavior for yielding of assemblies of softcore repulsive disks over a range of dimensionless pressures P. We isotropically compress the disks in a shearperiodic (LeesEdwards) geometry and then perform quasistatic simple shear. After each shear strain step, we relax the potential energy and dilate or compress the grains to maintain fixed pressure P and then evaluate the shear stress τ. We find that the number density of mechanically stable (MS) states and the strain between MS states obey finitesize scaling consistent with a diverging length scale ξ ~ ΣΣ_{c}^{}^{ν}, where Σ=τ/P. We observe two distinct values of ν: one during the initial stress buildup, ν ≈ 1.7, and another characterizing the slips during steady state shear, ν ≈ 1.1. The critical stress Σ_{c} increases as P is decreased and approaches a constant in the lowP limit, Σ_{c }≈ 0.1. However, the critical behavior (including the values of scaling exponents) is otherwise independent of P over several orders of magnitude, including well above the jamming transition. Our results show that critical scaling behavior associated with yielding is distinct from jamming, which may explain similarities among nonlocal flows of granular materials, emulsions, and other soft materials. 
Monday, March 4, 2019 1:51PM  2:03PM 
B59.00012: Structual evolution of amorphous systems during large scale deformation Ethan Stanifer, M. Lisa Manning Granular and amorphous materials deform plastically via localized structural rearrangements, although it remains unclear how microscopic structure and material preparation control such events. To address this question, many tools have been developed that use features of the linear response or dynamical matrix to predict the locations of localized rearrangements using structural information alone. However, these methods become less predictive across an avalanche, where stress fluctuations generated by one localized rearrangement can trigger other rearrangements resulting in a largescale structural change that is not captured by the linear response at the beginning of the avalanche. Therefore, we develop a method to study the linear response of a system during an avalanche. Specifically, we use dimensionality reduction to project the Hessian and forces into the space orthogonal to the minimization direction, and other unstable directions. We extend existing tools for identifying structural defects using this reduced Hessian and study how the population of structural defects evolves during an avalanche with a goal of developing a statistical description of structural evolution during largescale mechanical instabilities. 
Monday, March 4, 2019 2:03PM  2:15PM 
B59.00013: Colloidal drops under extreme stress Srishti Arora, Michelle R Driscoll We investigate the nonNewtonian behavior of dense colloidal suspensions by impacting a millimetric size droplet onto a solid surface. This method provides a unique platform to gain insight into suspension flow at high stresses and velocity gradients, a regime that is nontrivial to access using conventional rheometric techniques. Here, we present a comprehensive study of colloidal drop impact. We measure the maximal spread of the impacting colloidal drop while varying impact velocity and particle volume fraction. We find that the extent of spreading decreases with increasing volume fraction or decreasing impact velocity. Moreover, there exists a critical volume fraction below which the colloidal drop spreads radially into a flattened disc as commonly observed for Newtonian fluids. Above this critical volume fraction, no inertial spreading is observed, suggesting the onset of a jamming transition inside the colloidal drop. Furthermore, we observe a variety of elastic behaviors which manifest above the critical volume fraction and are controlled by impact velocity. 
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