Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session S09: Externally Driven and Active Granular MatterFocus
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Sponsoring Units: DSOFT GSNP Chair: Alberto Fernandez-Nieves, University of Barcelona; Ramon Planet, University of Barcelona Room: Room 132 |
Thursday, March 9, 2023 8:00AM - 8:36AM |
S09.00001: Interplay between ordering and mobility in a system of self-propelled grainsNarayanan Menon Invited Speaker: NARAYANAN MENON Self-propelled particles can locally break rotational symmetry along two distinct axes. The symmetry of the propulsion identifies one axis, and the other is the symmetry of the pair interaction, which could be for example, set by the shape of the particle. In biological systems and in most synthetic systems, these axes are aligned. A 2-dimensional vibration-fluidized system of macroscopic grains affords us a flexible model system for peeling these axes apart and exploring how ordering in a dense system of grains may be promoted or disrupted by self-propulsion. In-plane grain motion is driven by collisions with the floor and ceiling in the vertical direction. The collisional noise is rectified to produce directional motion in the horizontal plane by anisotropic features on the top and bottom surfaces. Thus, the shape of the particle can be kept fixed, while the anisotropy of its motion is varied. We show two examples using square tiles where this flexibility allows us to explore qualitatively new behaviour. First, we explore the phase behaviour of square grains in the presence of self-propulsion. Orientational and translation ordering transitions in the dense phase are qualitatively altered as the activity of the grains is tuned. To our knowledge, there are very few experimental systems in which the phase behaviour has been mapped out in this fashion. Second, we show that the melting of a crystallite of grains in noisy conditions can be strongly affected by the relative orientation of the symmetry of the self-propulsion and the symmetry axes of the packing. Endowing the grains with activity not only modifies typical time-scales of metling, but can introduce qualitatively new routes to melting that were not available in the corresponding equilibrium system. |
Thursday, March 9, 2023 8:36AM - 8:48AM |
S09.00002: A density-independent glass transition in designer active granular matter PRAGYA ARORA, Rajesh Ganapathy, Ajay Kumar Sood Dense assemblies of active particles are thought to undergo dynamical arrest akin to supercooled equilibrium liquids. However, how activity modifies the approach to a glassy state continues to be debated. Addressing this question is essential even as a growing body of evidence suggests that dense assemblies of biological cells share hallmark traits of equilibrium glass physics. By designing synthetic active matter systems that capture certain key features of living ones, we systematically investigate active glassy dynamics in this model system. This allowed us to explore the interplay of nature and the degree of activity, shape and topological defects on glassy slowing down. We observed non-trivial relaxation mechanisms unique to active glasses. We anticipate that our experimental strategy will help prune theoretical predictions of active glasses. |
Thursday, March 9, 2023 8:48AM - 9:00AM |
S09.00003: Fire-ant columns Alberto Fernandez-Nieves, Caleb J Anderson We study fire-ant columns, an active version of passive granular columns, and find that, despite the inherent activity of the ants and their natural tendency to rearrange, the ants develop force-chain structures that help support the weight of the column. Hence, the apparent mass at the bottom of the column saturates with added mass in a Janssen-like fashion, reminiscent of what is seen in passive-grain columns in wide containers. Activity-induced rearrangements within the column, however, lead to changes in the force-chain structure that slightly reduce the supportive nature of the force-chains, and fluctuations in the apparent mass that scale like the law of large numbers. We capture the experimental findings in simulations based on Janssen’s model but that include a fluctuating friction force with the walls to incorporate activity and affect the force-chain structure of the ant column. |
Thursday, March 9, 2023 9:00AM - 9:12AM |
S09.00004: Liquid-state properties and jamming dynamics of athermal persistent active matter Chandan Dasgupta, Suman Dutta, Pinaki Chaudhuri, Madan Rao To investigate the effects of non-thermal particle-scale forcing in dynamic arrest, we have studied, using Langevin simulations, a two-dimensional athermal binary mixture of active Brownian particles with persistent self-propulsion forces. In the limit of infinite persistence time, this system exhibits a jamming transition as the strength of the active force is decreased. The homogeneous liquid state obtained at large values of the active force exhibits unusual properties: the average kinetic energy and the width of the distribution of the kinetic energy increase with increasing system size and a length scale extracted from spatial correlations of the velocity field also increases with system size without showing any sign of saturation. We also investigate how this active liquid approaches a force-balanced jammed state when the self-propulsion force is removed or reduced to a small value. We show that the jamming proceeds via a three-stage relaxation process whose timescale grows with the magnitude of the active force and the system size. We relate the dependence of the jamming time on the system size to the large correlation length obtained from velocity correlations in the liquid state. |
Thursday, March 9, 2023 9:12AM - 9:24AM |
S09.00005: Tune it your way: activity induced, stress controlled bifurcation of the rheology of a dense 3D Quincke rotor suspension Edward Ong, Itai Cohen, Abhishek M Shetty We experimentally investigate how activity can be used to tune the rheology of a dense suspension of Quincke rotors in 3D and connect our observations to changes in the 3D microstructure. Under a constant stress, the suspension viscosity shows a clear bifurcation in response to a constant electric field being applied, depending on the magnitude of the applied stress. For low stresses, the viscosity increases with increasing electric field strength, eventually reaching a jammed state for larger field strengths. This is associated with the local densifications of particles into strong percolating structures that can withstand the shear, induced by the random rotation of the particles. For high stresses, the viscosity decreases with increasing electric field strengths, and this is associated with the shear induced degeneracy of the direction of rotation in this large shear limit. Our study reveals how exactly activity interacts with shear in 3D to result in this novel bifurcation mechanism for tuning the suspension flow. |
Thursday, March 9, 2023 9:24AM - 9:36AM |
S09.00006: High-strength, fully dense granular crystals Francois Barthelat, Ashta Navdeep Karuriya Typical granular materials are far from optimal in terms of mechanical performance: Random packing leads to poor load transfer in the form of thin and dispersed force lines within the material, inhomogeneous jamming, and strain localization. In addition, localized contacts between individual grains result in low stiffness, low strength and brittleness. Here we take a new approach where we consider granular materials as materials by design. We vibrated millimeter-scale 3D printed grains with rhombic dodecahedral or truncated octahedral shapes to assemble them into fully dense FCC and BCC granular crystals. These granular crystals are up to 10 times stronger than traditional randomly packed spheres, and they can display a rich set of mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, cross-slip, shear-induced dilatancy, geometric hardening, micro-buckling. To capture some of these mechanisms we developed a multiscale model that incorporate local friction between grains, resolved shear and normal stresses on available slip planes, and prediction of compressive strength as function of loading orientation. The predicted strength is highly anisotropic and agrees well with triaxial compression experiments. |
Thursday, March 9, 2023 9:36AM - 9:48AM |
S09.00007: Mobile superstructures with switchable behavior via redistribution of vibrated particle clusters Kyungmin Son, Ho-Young Kim Confining active particles in a free-to-move scaffold is an emerging strategy to create superstructures capable of directed motion and shape change. Many active particle ensembles with a low number density have been proposed, which can deform by internal stress or external stimuli; however, structures with particles close to the jamming condition have been rarely studied. Such densely packed systems are structurally robust and thus advantageous to operate in high-stress environments. Here, we present rigid particle structures consisting of a mobile boundary enclosing a few active polar particles and randomly vibrated nonpolar particles. By tuning the shape and number of polar particles, the superstructures exhibit diverse behaviors ranging from random, rotational to directional motion. We show that particle packing density and mechanical properties of the scaffold additionally modulate dynamic properties – the magnitudes of polarity and chirality – of these structures. Our study paves the way for significant regulation of cluster behavior with small changes in the components of dense systems. |
Thursday, March 9, 2023 9:48AM - 10:00AM |
S09.00008: Base states and transitions of an acoustically levitated cluster of anisotropic particles Qinghao Mao, Brady Wu, Bryan VanSaders, Heinrich M Jaeger Anisotropic particles are important building blocks for the self-assembly of complex structures. However, little is known about the energy landscape of small clusters made of anisotropic particles and even less for clusters formed through underdamped motion in the presence of nonthermal fluctuations. Acoustic levitation, a method of suspending sub-millimeter objects against the force of gravity in air, is an ideal tool to investigate assembly under these conditions. We levitate low-aspect ratio rods of diameter 40-50μm in the nodal plane of an acoustic standing wave, where geometry-dependent secondary scattering forces cause anisotropic attraction and assembly rods into dense clusters. By tuning the driving frequency of the acoustic trap off-resonance, we introduce stochastic nonthermal forces arising from hydrodynamic coupling in the system. Activating and cooling the rod cluster with such forces allows us to study the cluster's underdamped structural transformations, which we compare with molecular dynamics simulations of thermally formed anisotropic particle clusters. |
Thursday, March 9, 2023 10:00AM - 10:12AM |
S09.00009: Hydrodynamic Coupling Melts Acoustically Levitated Crystalline Rafts Brady Wu, Bryan VanSaders, Melody X Lim, Heinrich M Jaeger Particles interacting hydrodynamically often exhibit chaotic trajectories, even for as few as three particles. Such interactions can agitate the degrees of freedom of the particles, producing configuration dependent athermal fluctuations. Acoustic levitation, where granular particles are levitated with intense ultrasound, is an ideal platform to study such systems. Here, we acoustically levitate and assemble a granular raft, where particles form an open lattice with tunable spacing. Oscillating air flows between neighboring particles in the levitated raft, establishing a hydrodynamic coupling even when particles are at rest. The hydrodynamic coupling gives rise to spontaneous excitations in the lattice, in turn driving intermittent particle rearrangements. Under the action of these fluctuations, the raft transitions from a predominantly quiescent, crystalline structure, to a two-dimensional liquid-like state. We show that this transition is characterized by dynamic heterogeneity and intermittency, as well as cooperative particle movements, that produce an effectively `cageless' crystal. These findings shed light on fluid-coupling driven excitations that are difficult to isolate and control in many other hydrodynamic systems. |
Thursday, March 9, 2023 10:12AM - 10:24AM |
S09.00010: Granular Dynamics in Spectral Acoustic Holograms Mia C Morrell, Julianne Lee, David G Grier Acoustic traps use forces exerted by structured sound waves to transport granular materials along specified trajectories in three dimensions. The structure of the acoustic force landscape is governed by the amplitude and phase profiles of the sound's pressure wave. These profiles can be controlled through deliberate spatial modulation of monochromatic waves, by analogy to holographic optical trapping. Alternatively, spatial and temporal control can be achieved by interfering a small number of sound waves at multiple stationary frequencies to create acoustic holograms based on spectral content. We demonstrate spectral holographic trapping by projecting acoustic conveyor beams that move millimeter-scale objects along prescribed paths. Spectrally-rich sound waves projected by just two sources can implement a superposition of stationary and dynamic force landscapes whose Moire interaction drives trapped particles through dynamical states analogous to those in sliding charge density waves and the recently-discovered wave-driven oscillator. |
Thursday, March 9, 2023 10:24AM - 10:36AM |
S09.00011: Flow induced rigidity percolation in shear thickening suspensions Emanuela Del Gado, Abhay Goyal, Nicos Martys Discontinuous shear thickening (DST) is associated with a sharp rise of a suspension's viscosity with increasing applied shear rate. A key signature of DST, highlighted in recent studies, is the very large fluctuations of the measured stress as the suspension thickens. A clear link between microstructural development and the dramatic increase of the stress fluctuations has not been established yet. To identify the microstructural underpinnings of this behavior, we perform simulations of sheared dense suspensions. By analyzing particle contact networks, we identify a subset of constrained particles that contribute directly to the rapid rise in viscosity and the large stress fluctuations. Indeed, both phenomena can be explained by the growth and percolation of constrained particle networks---in direct analogy to rigidity percolation. A finite size scaling analysis confirms this is a percolation phenomenon and allows us to estimate the critical exponents. Our findings reveal the specific microstructural transition that underlies DST. |
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