Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B14: GSNP Student and Postdoctoral Speaker Awards SessionPrize/Award

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Sponsoring Units: GSNP Chair: Christian Santangelo, University of Massachusetts, Amherst Room: 273 
Monday, March 13, 2017 11:15AM  11:27AM 
B14.00001: Mapping current fluctuations of stochastic pumps to nonequilibrium steady states. Grant Rotskoff We show that current fluctuations in stochastic pumps can be robustly mapped to fluctuations in a corresponding timeindependent nonequilibrium steady state. We thus refine a recently proposed mapping so that it ensures equivalence of not only the averages, but also the optimal representation of fluctuations in currents and density. Our mapping leads to a natural decomposition of the entropy production in stochastic pumps, similar to the ``housekeeping'' heat. As a consequence of the decomposition of entropy production, the current fluctuations in weakly perturbed stochastic pumps satisfy a universal bound determined by the steady state entropy production. [Preview Abstract] 
Monday, March 13, 2017 11:27AM  11:39AM 
B14.00002: A change in stripes for cholesteric shells via modulated anchoring Lisa Tran, Maxim Lavrentovich, Guillaume Durey, Alexandre Darmon, Martin Haase, Ningwei Li, Daeyeon Lee, Kathleen Stebe, Randall Kamien, Teresa LopezLeon Many of the patterns found in biological systems are also found to selfassemble into cholesteric liquid crystal (CLC) systems. In this work, we probe the effect of varying the perpendicular anchoring strength of a CLC that is confined to a spherical shell. The shell geometry gives the confinement and curvature conditions for the formation of a rich array of metastable states, revealing an unexplored region between degenerate parallel anchoring and strong perpendicular anchoring. We modulate the anchoring strength in experiments with two methods: by adjusting the surfactant concentration or, interestingly, by varying the temperature. We find two states not previously reported for CLC shells: a Bouligand arches state, where larger, lateral stripes on the shell can be filled with smaller, longitudinal substripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. We use a Landaude Gennes model of the CLC to simulate the director configurations of these states. This work identifies the Bouligand arches state in CLC shells and builds upon the existing knowledge of cholesteric FCDs, structures that not only have potential for use as intricate, selfassembly blueprints but are pervasive in biological systems. [Preview Abstract] 
Monday, March 13, 2017 11:39AM  11:51AM 
B14.00003: Using particle rearrangement statistics to quantify ductility in amorphous solids Meng Fan, Minglei Wang, Yanhui Liu, Jan Schroers, Mark Shattuck, Corey O'Hern The response of amorphous solids to applied shear has several distinct regimes: quasielastic, yielding, and plastic flow regimes in the absence of fracture. Both nonaffine particle motion and particle rearrangement events give rise to the strong nonlinear behavior of the stress versus strain curve. Here, we focus on computational studies of the mechanical behavior of binary LennardJones glasses in three spatial dimensions that are prepared over a wide range of cooling rates. We apply athermal quasistatic pure shear to the glasses and uniquely identify each particle rearrangement event. We then determine the frequency of rearrangements and the energy drop after each event. We also quantify ductility by measuring the critical strain at which the material fractures during tensile tests. We find that more rapidly cooled glasses undergo more frequent particle rearrangements with larger energy drops on average. In contrast, rearrangements are much less frequent and dissipate less energy in more slowly cooled glasses, and thus are more susceptible to fracture than rapidly cooled glasses. In fact, we can predict the ductility of amorphous solids by measuring the total energy loss per strain in the putative linear stress versus strain regime before fracture occurs. [Preview Abstract] 
Monday, March 13, 2017 11:51AM  12:03PM 
B14.00004: From microscopic rules to macroscopic dynamics with active colloidal snakes. Jie Zhang, Jing Yan, Steve Granick Seeking to learn about selfassembly far from equilibrium, these imaging experiments inspect selfpropelled colloidal particles whose heads and tails attract other particles reversibly as they swim. We observe processes akin to polymerization (short times) and chain scission and recombination (long times). The steadystate of dilute systems consists of discrete rings rotating in place with largely quenched dynamics, but when concentration is high, the system dynamics share features with turbulence. The dynamical rules of this model system appear to be scaleindependent and hence potentially relevant more generally. [Preview Abstract] 
Monday, March 13, 2017 12:03PM  12:15PM 
B14.00005: Entropy Driven Solid—Solid Transitions in Colloids Chrisy Xiyu Du, Greg van Anders, Richmond Newman, Sharon Glotzer In classical, equilibrium statistical mechanics, entropydriven order remains one of the most enigmatic phenomena. Although there is considerable work on entropydriven fluidsolid transitions, the multiplicity of crystals that form in systems of hard, anisotropically shaped colloids suggests the possibility of studying entropydriven solidsolid phase transitions. Here, we introduce a family of minimal model systems that exhibit solid—solid phase transitions that are driven by changes in the shape of colloidal particles. We carry out a detailed investigation of the thermodynamics of a series of isochoric, diffusionless solid—solid phase transitions within a single shape family, and find transitions that require thermal activation, or are “discontinuous”, and transitions that occur without thermal activation, or are “continuous”. Our results have direct implications for designing reconfiguration in soft materials, and our approach opens new avenues for the detailed study of the basic physics of solidsolid transitions, with potential applications in other areas of physics. [Preview Abstract] 
Monday, March 13, 2017 12:15PM  12:27PM 
B14.00006: Break

Monday, March 13, 2017 12:27PM  12:39PM 
B14.00007: Jamming transition in granular systems of regular polygons Cecey Stevens Bester, Yiqiu Zhao, Jonathan Bares, Yuanyuan Xu, Meredith Cox, Robert Behringer The study of the onset of mechanical stability, known as the jamming transition, of granular systems provides key insights into properties of amorphous materials. A fundamental challenge to understanding this transition is to determine the influence of particle properties. Here, we investigate how nontrivial particle shapes affect the jamming transition as controlled by the packing fraction. Our experiments are performed by compression of twodimensional arrangements of photoelastic particles, allowing us to visualize force information. To explore the role of particle shape, we systematically change the number of sides of polygonal particles used in the experiments and compare the force chain network, contact number and pressure evolution of compressed systems of polygons to the wellstudied systems of disks. We also explore the influence of geometric features, such as faceface contacts and ordering within packings, in connection with the jamming transition. [Preview Abstract] 
Monday, March 13, 2017 12:39PM  12:51PM 
B14.00008: A rigidity transition and glassy dynamics in a model for confluent 3D tissues Matthias Merkel, M. Lisa Manning The origin of rigidity in disordered materials is an outstanding open problem in statistical physics. Recently, a new type of rigidity transition was discovered in a family of models for 2D biological tissues, but the mechanisms responsible for rigidity remain unclear. This is not just a statistical physics problem, but also relevant for embryonic development, cancer growth, and wound healing. To gain insight into this rigidity transition and make new predictions about biological bulk tissues, we have developed a fully 3D selfpropelled Voronoi (SPV) model. The model takes into account shape, elasticity, and selfpropelled motion of the individual cells. We find that in the absence of selfpropulsion, this model exhibits a rigidity transition that is controlled by a dimensionless model parameter describing the preferred cell shape, with an accompanying structural order parameter. In the presence of selfpropulsion, the rigidity transition appears as a glasslike transition featuring caging and aging effects. Given the similarities between this transition and jamming in particulate solids, it is natural to ask if the two transitions are related. By comparing statistics of Voronoi geometries, we show the transitions are surprisingly close but demonstrably distinct. Furthermore, an index theorem used to identify topologically protected mechanical modes in jammed systems can be extended to these vertextype models. In our model, residual stresses govern the transition and enter the index theorem in a different way compared to jammed particles, suggesting the origin of rigidity may be different between the two. [Preview Abstract] 
Monday, March 13, 2017 12:51PM  1:03PM 
B14.00009: Improving SelfAssembly by Varying the Temperature Periodically with Time Oren Raz, Christopher Jarzynski Selfassembly (SA) is the process by which basic components organize into a larger structure without external guidance. These processes are common in Nature, and also have technological applications, e.g. growing a crystal with a specific structure. So far, artificial SA processes have been designed mostly using diffusive building blocks with high specificity and directionality. The formation of the selfassembled structures is then driven by freeenergy minimization into a thermodynamically stable state. In an alternative approach to SA, macroscopic parameters such as temperature, pressure, pH, magnetic field etc., are varied periodically with time. In this case, the SA structures are the stable periodic states of the driven system. Currently there are no design principles for periodically driven SA, other than in the limits of fast or weak driving. We present guiding ideas for selfassembly under periodic driving. As an example, we show a particular case in which selfassembly errors can be dramatically reduced by varying a system's temperature periodically with time. [Preview Abstract] 
Monday, March 13, 2017 1:03PM  1:15PM 
B14.00010: Stochastic thermodynamics and fluctuation theorems of active Brownian dynamics Dibyendu Mandal, Katherine Klymko Active biological systems reside far from equilibrium, dissipating heat even in their steady state, and thus requiring an extension of the conventional equilibrium thermodynamics and statistical mechanics. In this study, we have extended the emerging framework of stochastic thermodynamics to active Brownian particles. In particular, for the active OrnsteinUhlenbeck model, we have provided consistent definitions of thermodynamic quantities like work, energy, and entropy at the level of single, stochastic trajectories and derived all the major integral fluctuation relations, for total entropy production, excess entropy production, and housekeeping heat. We have developed the equivalent of the Clausius inequality and it reflects the underlying nonHamiltonian nature of the dynamics. For this active, overdamped model, we have also discovered some subtleties in the detailed fluctuation theorems for the excess and the housekeeping heat that are absent in passive overdamped dynamics. We have illustrated our results with explicit numerical studies. These studies will ultimately reflect on the thermodynamic efficiency of active, biological processes. [Preview Abstract] 
Monday, March 13, 2017 1:15PM  1:27PM 
B14.00011: Mechanical response and buckling of a polymer simulation model of the cell nucleus Edward Banigan, Andrew Stephens, John Marko The cell nucleus must robustly resist extra and intracellular forces to maintain genome architecture. Micromanipulation experiments measuring nuclear mechanical response reveal that the nucleus has two force response regimes: a linear shortextension response due to the chromatin interior and a stiffer longextension response from lamin A, comprising the intermediate filament protein shell. To explain these results, we developed a quantitative simulation model with realistic parameters for chromatin and the lamina. Our model predicts that crosslinking between chromatin and the lamina is essential for responding to small strains and that changes to the interior topological organization can alter the mechanical response of the whole nucleus. Thus, chromatin polymer elasticity, not osmotic pressure, is the dominant regulator of this force response. Our model reveals a novel buckling transition for polymer shells: as force increases, the shell buckles transverse to the applied force. This transition, which arises from topological constrains in the lamina, can be mitigated by tuning the properties of the chromatin interior. Thus, we find that the genome is a resistive mechanical element that can be tuned by its organization and connectivity to the lamina. [Preview Abstract] 
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