Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R59: Athermal and Statistical MechanicsInvited

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Sponsoring Units: GSNP Chair: Mark Shattuck, City College of New York CUNY Room: LACC Petree Hall D 
Thursday, March 8, 2018 8:00AM  8:36AM 
R59.00001: Forcebased ensembles in granular materials Invited Speaker: Karen Daniels It has long been known that athermal materials, including real granular materials with friction, exhibit highly heterogeneous patterns of force transmission. I will describe two routes to understanding this phenomenon, both arising from the consideration of statistical mechanical ensembles in a quasi twodimensional experiment of disks floating on a gentle cushion of air. First, force network ensembles examine the configurationspace of allowed interparticle forces, given a fixed particle configuration. We perform experiments tabulating up to 1000 realizations of the force network in the same particle packing. Using the tools of network science, we evaluate the causes of individual particles being on the strong network, as well as whether or not particles are reliably on the strong network. Second, the Edwardslike forcemoment ensemble predicts the existence of a temperaturelike variable which would provide an equation of state. To test the efficacy of this approach, we perform experiments under three different loading conditions. From measurements of the interparticle forces, we find that the distributions of the normal component of the coarsegrained forcemoment tensor are exponentialtailed, while the deviatoric component is Gaussiandistributed. This implies that the correct ensemble will need to consider both forcemoment tensor (responsible for the linear term in the exponent) and the MaxwellCremona forcetiling area (the quadratic term). These two terms lead to two candidate variables of state, correspondingly the tensorial angoricity and a new temperaturelike quantity associated with the forcetile area which we name keramicity. We observe an equation of state in which each of these is inversely proportional to the global confining pressure; however only keramicity exhibits the protocolindependence expected of a state variable, while angoricity behaves as a variable of process. 
Thursday, March 8, 2018 8:36AM  9:12AM 
R59.00002: Stress anisotropy in quasistatically sheared granular packings Invited Speaker: Corey O'Hern We seek to understand the development of shear and normal stress 
Thursday, March 8, 2018 9:12AM  9:48AM 
R59.00003: Plastic flow in amorphous solids: from particlescale to mesoscale. Invited Speaker: Craig Maloney Amorphous solids flow plastically when driven at sufficiently large stress. Examples include: metallic and polymeric glasses; soft glassy materials such as dense suspensions, pastes, emulsions, and foams; granular materials, etc. During flow, particles rearrange in socalled shear transformation zones (STZs) to accommodate the deformation and relieve the imposed stress. Models can describe the kinematics and dynamics of these rearrangements either explicitly at the particlescale or, more schematically, at a coarsegrained scale where particlelevel details are abstracted away. We will discuss how the results from coarsegrained simulations agree with or differ from particlebased simulations and how this depends on: the way disorder is introduced, the precise way the STZs are coupled to each other elastically, the way finite rate driving is modeled, etc. In particular, we will discuss: the spectrum of dissipative stress relaxation events; the localstrain and particle displacement statistics; emergent spatial structures; and the dependence of these on driving rate or stress. 
Thursday, March 8, 2018 9:48AM  10:24AM 
R59.00004: Universal avalanche dynamics: From nanocrystals, to bulk metallic glasses, to earthquakes and stars? Invited Speaker: Karin Dahmen Slowlycompressed nanocrystals, bulk metallic glasses, rocks, granular materials, and the earth all deform via intermittent slips or “quakes”. We find that although these systems span 12 decades in length scale, they all show the same scaling behavior for their slip size distributions and other statistical properties. Remarkably, they also appear to show similar slip dynamics. A simple mean field model for avalanches of slipping weak spots explains the agreement across scales. It predicts the observed slipsize distributions, and the temporal slip profiles. The analysis draws on tools from statistical physics and the renormalization group. The results enable extrapolations from one scale to another, and from one force to another, across different materials and structures, from nanocrystals to earthquakes. Connections to neuron avalanches in the brain and recent observations on stars will also be discussed, extending the range of scales to 16 decades in length. 
Thursday, March 8, 2018 10:24AM  11:00AM 
R59.00005: Granular Statistical Mechanics: Bridging Between Statics and Dynamics Invited Speaker: Raphael Blumenfeld A firstprinciples statistical mechanics is formulated for slow dense granular fluids, where prolonged intergranular contacts vitiate collision theory. The contacts are regarded as virtual particles, exchanged during the dynamics between different structure types. 
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