Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X48: Statistical and Nonlinear Physics IFocus

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Sponsoring Units: GSNP Chair: Mark Shattuck, City College of New York Room: BCEC 251 
Friday, March 8, 2019 8:00AM  8:12AM 
X48.00001: Interlocking of Sheared Nonconvex Hexapods Yuchen Zhao, Jonathan Barés, Robert P Behringer Packings of nonconvex or elongated particles can form freestanding structures like walls or arches, due to particle interlocking effects leading to geometric cohesion. Interlocking effects have been studied for various particle shapes and aspect ratios, but the microscopic origins of the stabilization of rigid structures remains unclear. We report on experiments on hexapods that consist of three orthogonal spherocylinders, whose centers are bonded at one origin. The diameter and lengths of the spherocylinders are 3, 10, 20 and 30 mm, respectively. We subjected aggregates of spheres or hexapods to quasistatic direct shear and developed novel techniques to measure structural information such as particle positions, orientations, and bending. For spheres and 10 mm hexapods, we observed plasticity phenomena consistent with the MohrCoulomb model. For 20 and 30 mm hexapods, however, we observed strainstiffening during shear. By analyzing Xray microcomputed tomography data collected during the shearing process, we found that the onset of strainstiffening is associated with particle bending and that particles that bend significantly tend to be aligned with the compression direction of the shear. 
Friday, March 8, 2019 8:12AM  8:24AM 
X48.00002: Study of Electrical Percolation in CrO_{2}/Cr_{2}O_{3} nanoparticle composite system Shiva Pokhrel, Brendon Waters, Ehab H Abdelhamid, Zhi Feng Huang, Boris Nadgorny Percolation is a random probabilistic process that triggers a phase transition in disordered systems. In this work we investigate the classical percolation behavior in a CrO_{2}/Cr_{2}O_{3} halfmetal/insulator composite system. Cr_{2}O_{3} was obtained by annealing of CrO_{2} powder in air; composite samples with varying volume fraction were prepared by mixing Cr_{2}O_{3} with CrO_{2}. The percolation threshold and the power law scaling exponent near the threshold were identified by studying the changes in the electrical resistance of the pellets with different volume fractions. Experimental results are compared with theoretical calculations, which were carried out via the combination of mechanical contraction method and Monte Carlo simulations for this binary composite system of nonoverlapping hard spherocylinders. 
Friday, March 8, 2019 8:24AM  8:36AM 
X48.00003: Robotic active matter on a deformable surface generates an analog gravity system Shengkai Li, Yasemin Ozkan aydin, Charles Xiao, Gabriella H Small, Jennifer Rieser, Pablo Laguna, Daniel Goldman Many analog gravity models of general relativity (GR) have been developed (e.g. in fluids and BoseEinstein condensates) with goals to probe GRlike phenomena in the laboratory. One common analog is to study the dynamics of freely moving objects (like marbles) on twodimensional curved or elastic sheets. Such systems have issues which prevent them from functioning as analog gravity models, including dominance of Earth’s gravity over metric dynamics, the inability to model timelike curvature effects, and dissipation which limits persistent dynamics. Here, we circumvent these issues by developing an exact analog gravity system based on an active matter system: a robot car driving on a deformable membrane. We observe qualitative GRlike features including circular and precessing orbits around a central depression. We extract a dynamical system that describes our experiments. Remarkably it is the selfpropelled aspects of the car that allow a formal mapping to Einstein’s equations in 2+1 dimensions, thus creating an accurate GR model whose parameters can be tuned to mimic different astrophysical situations (e.g. Schwarzschild metric around a black hole). 
Friday, March 8, 2019 8:36AM  8:48AM 
X48.00004: Abstract Withdrawn

Friday, March 8, 2019 8:48AM  9:00AM 
X48.00005: Dendritic crystal growth of ammonium nitrate from aqueous solution Andrew Dougherty Dendritic crystal growth is an important example of nonequilibrium pattern formation that involves both nonlinear and noisedriven effects. The resulting largescale structures are sensitively dependent on relatively small effects, such as surface tension, and on small anisotropies in those quantities. In this work, we report new results for the dendritic growth of ammonium nitrate from supersaturated aqueous solution. This system has been studied previously by van Driel et al.[1] and shown to exhibit several different morphologies, including both steady state dendritic growth and a state with persistent tipsplitting behavior. Specifically, we present new measurements of the tip radius ρ, growth speed v, and sidebranch spacing λ, along with initial estimates of the product Dd_{0}, where D is the chemical diffusion constant and d_{0} is the capillary length. We then compare the resulting esimate of σ^{*} = 2 d_{0}D/vρ^{2} with the values for other materials and with theoretical expectations. 
Friday, March 8, 2019 9:00AM  9:12AM 
X48.00006: Twisted states in lowdimensional hypercubic lattices Young Sul Cho, Seungjae Lee, Hyunsuk Hong Twisted states with nonzero winding numbers have been observed in a ring composed of sinusoidally coupled identical oscillators. In this presentation, we consider finitesized ddimensional hypercubic lattices, namely square (d=2) and cubic (d=3) lattices with periodic boundary conditions. For identical oscillators, we observe new states where the oscillators belonging to each line (plane) for d=2 (d=3) are phase synchronized with nonzero winding numbers along the perpendicular direction. We note that these states can be reduced into twisted states in a ring with the same winding number if we regard each subset of phasesynchronized oscillators as one single oscillator. For nonidentical oscillators, we observe similar patterns with slightly heterogeneous phases in each line (d=2) and plane (d=3) for random configurations. 
Friday, March 8, 2019 9:12AM  9:24AM 
X48.00007: Drainage through holes drives Arctic sea ice melt ponds to the critical percolation threshold Predrag Popovic, Mary C Silber, Dorian S Abbot During the summer, vast regions of the Arctic sea ice are covered by meltwater ponds that significantly lower the ice reflectivity and accelerate melting. Ponds develop over the summer melt season through an initial stage of rapid growth followed by drainage through macroscopic holes. Recently, we showed that ponds after drainage resemble percolation clusters near a critical percolation threshold. Here, we explore the physical mechanism behind this previouslyunrecognized constraint on pond evolution. We show that organization towards the percolation threshold is a consequence of pond drainage through macroscopic holes. The threshold sets the upper limit and scales the pond coverage throughout its evolution after the beginning of drainage. Furthermore, we show that, after rescaling, pond coverage fraction as a function of number of open holes follows a universal curve. This curve governs pond evolution during and after pond drainage, which allows us to formulate an equation for pond coverage evolution that captures the dependence on ice properties. 
Friday, March 8, 2019 9:24AM  9:36AM 
X48.00008: Disconnectivity graphs in planted spinglass problems Katja Biswas, Helmut Katzgraber A disconnectivity graph is a simplified representation of a highdimensional energy landscape consisting of lowenergy pathways between different minima in the landscape. As such, one can use disconnectivity graphs to obtain insights into the accessibility of different minima in the energy landscape, and therefore make predictions for the computational effectiveness of different optimization methods. In this talk different disconnectivity graphs for different planted spinglass problems are presented, and their similarities and differences discussed. 
Friday, March 8, 2019 9:36AM  9:48AM 
X48.00009: Floating elastic membrane in rotation : shape of interface and wrinkling instability Lucie Domino, Doireann O'Kiely, Dominic Vella, Antonin Eddi When rotated at constant speed, the surface of a liquid takes the well known shape of a parabola. If the surface of the liquid is covered with a thin elastic membrane, the force balance is modified, which changes the obtained shape. We study the influence of the elastic properties of the membrane on this shape, and compare with experiments. We also report the apparition of a buckling instability at the edges of the floating membrane : radial creases appear and grow closer to the center as rotation speed is increased. 
Friday, March 8, 2019 9:48AM  10:00AM 
X48.00010: Geometric Stiffening and Softening of an Indented Floating Thin Film Monica Ripp, Vincent Démery, Teng Zhang, Joey Paulsen Thin sheets can behave nonlinearly while the local material response is purely linear. We separate two distinct mechanisms for geometric nonlinearities by studying the normalforce response of an indented polymer film on a liquid bath, using experiments, simulations, and theory. First, we show that the force reaches a plateau at large indentation, causing the effective spring constant of the system to soften, and we use a simple geometric model to capture this behavior at large slopes [1]. Then, we map out the full phase diagram at small slopes, supporting recent predictions [2] that the system stiffens out of the typical linear response for small deflections. Our results provide a prototypical example of how multiple distinct geometric nonlinearities may arise under monotonic driving. 
Friday, March 8, 2019 10:00AM  10:12AM 
X48.00011: Simulating the dynamics of actuated thin elastic sheets Silas Alben, Robert D Deegan, Alex A. Gorodetsky We develop a method to simulate the dynamics of thin elastic sheets under timedependent external or internal forces, e.g. contact forces, temperature change, or chemical reactions within the sheet. The dynamics are difficult to simulate efficiently due to numerical stiffness resulting from the inplane stretching term in the elastic energy. We present a semiimplicit time discretization that allows the dynamics to be simulated stably with essentially no timestep constraint, allowing for fast simulations. We show examples for thin sheets, bilayers, and models of active gel materials, using different spatial geometries and different types of spatial discretizations. 
(Author Not Attending)

X48.00012: Damage propagation in a model cohesive granular medium MarieJulie Dalbe, Nicolas Vandenberghe, Emmanuel Villermaux, Cédric Bellis Understanding how a crack propagates in a heterogeneous medium is essential in numerous fields, since this process can lead to catastrophic events. The opening of a crack can lead to the emission of an elastic wave, which can interfere with the rest of the material, and provide energy to open new cracks. In particular, in a brittle material, the critical stress leading to rupture is different from one experiment to another. This is usually imputed to the presence of defects, which are weaker points in the material, making crack propagation easier. The interaction of the elastic waves emitted by a crack opening, and existing defects is still not fully understood. 
Friday, March 8, 2019 10:24AM  10:36AM 
X48.00013: Mechanism of largescale flow reversals in turbulent thermal convection Yin Wang, PikYin Lai, Hao Song, Penger Tong We report a new kind of convective instability for turbulent thermal convection in a closed thin disk cell. It is found that the convective flow stays over a long steady “quiet period” having a minute amount of heat accumulation, followed by a short and intermittent “active period” with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing largescale circulation across the cell and resets its rotational direction. The distribution function of the plume eruption amplitude is found to follow the generalized extreme value statistics with an upper bound, which changes with the physical properties of the convecting fluid. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in largescale flow pattern are often observed. 
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