Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W14: The Role of Elasticity in the Dynamics of Thermal Glassy MaterialsInvited Live Streamed
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Sponsoring Units: DSOFT Chair: Rahul Chacko, UPenn; Rahul Chacko, UPenn Room: McCormick Place W-183B |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W14.00001: Dynamic Facilitation and Nucleation in Supercooled Liquids Below TMCT Invited Speaker: Rahul N Chacko Mean field theories predict a transition to a regime of elastic behaviour when a supercooled liquid is cooled below the mode-coupling theory temperature TMCT . According to Random First Order Transition theory, deep energy landscape minima are ubiquitous in this regime, but transitions between them are slow. As such, nucleation has been expected to play a key role in the sub-TMCT dynamics of these systems, with structural evolution proceeding via the size-limited nucleation of droplets of "aperiodic crystal" within domains of different, incompatible states. In this talk, we present direct evidence from molecular dynamics simulations supporting the prediction of elasticity below TMCT, but opposing that of nucleation. We find that dynamic facilitation, where local motion triggers further motion nearby, undergoes a crossover from short-ranged to elastic and long-ranged at TMCT. However, upon studying the equilibration dynamics of initially out-of-equilibrium systems, we see no evidence of nucleation on the time sale of structural relaxation. |
Thursday, March 17, 2022 3:36PM - 4:12PM |
W14.00002: Does mesoscopic elasticity control viscous slowing down in glassforming liquids? Invited Speaker: Edan Lerner The dramatic slowing down of relaxation dynamics of liquids approaching the glass transition remains a highly debated problem, where the crux of the puzzle resides in the elusive increase in the activation barrier ΔE(T) with decreasing temperature T. A class of theoretical frameworks—elastic models—attribute this temperature dependence to the variations of the liquid’s macroscopic elasticity, quantified by the high-frequency shear modulus G∞(T). While elastic models find some support in some experimental studies, these models do not take into account the spatial structures and length scales associated with structural relaxation in supercooled liquids. In my talk, I will propose and test the possibility that viscous slowing down is controlled by a mesoscopic elastic stiffness κ(T), defined as the characteristic stiffness of response fields to local dipole forces in the liquid’s underlying inherent structures. First, I will show that κ(T)—which is intimately related to the energy and length scales characterizing quasilocalized, nonphononic excitations in glasses—increases more strongly with decreasing T than the macroscopic inherent structure shear modulus G(T) in several computer liquids. Second, I will show that the simple relation ΔE(T) ∝ κ(T) holds remarkably well for some computer liquids, suggesting a direct connection between the liquid’s underlying mesoscopic elasticity and enthalpic energy barriers. On the other hand, I will show that for other computer liquids, the above relation fails. Finally, I will provide strong evidence that what distinguishes computer liquids in which the ΔE(T) ∝ κ(T) relation holds from those in which it does not is that the latter feature highly fragmented/granular potential energy landscapes, where many sub-basins separated by low activation barriers exist. Under such conditions, it appears that the sub-basins do not properly represent the landscape properties relevant for structural relaxation. |
Thursday, March 17, 2022 4:12PM - 4:48PM |
W14.00003: Structure and dynamics in supercooled liquids: A theory of localized excitations Invited Speaker: Kranthi K Mandadapu The microscopic motion of glass-forming liquids dramatically slows down with decreasing temperature. This slowdown is accompanied by dynamical heterogeneity where localized regions of particle mobility and extended immobile regions emerge from the liquid. There are two competing perspectives to explain these phenomena. One perspective proposes that structural/static properties of the liquid may be used to explain the slowdown. Another perspective, the dynamical facilitation (DF) theory, proposes that dynamics are driven by emergent excitations whose origins are believed to be independent of liquid structure, which then facilitate the creation and relaxation of nearby excitations. DF theory predicts many key properties of dynamics including, relaxation behaviors of single- and multi-component systems, temperature-dependence of heat capacities, breakdown of Stokes-Einstein diffusion, and competing dynamics of crystallization and vitrification. |
Thursday, March 17, 2022 4:48PM - 5:24PM |
W14.00004: Local shear transformations in hard-sphere colloidal glasses Invited Speaker: Katharine E Jensen Although defect-mediated plastic deformation in crystals has been well-understood for decades, understanding equivalent processes in glasses remains an area of active research. Theory and simulation predict the existence of "shear defects" or "shear transformation zones" (STZs) that mediate plastic deformation in glasses as dislocations do in crystals, but direct observations of these defects in amorphous structures remain elusive. An ideal experiment would catch the defects in action by observing the 3D, real-time motion of every microscopic constituent in a macroscopic glass sample. Colloidal glasses provides a unique experimental system in which we can realize such an ideal experiment and directly study structures, defects, and dynamics of amorphous materials across the complete range of relevant length and time scales. We analyze particle-level trajectories and local strain fields obtained from confocal microscopy experiments on ~1-μm-diameter, hard-sphere colloidal glasses under conditions of quiescence or uniform shear deformation. We find that shear transformation zones are active in both sheared and quiescent colloidal glasses, and we examine the evolution of the STZ population with time as well as increasing macroscopic strain. On strain reversal, we observe partial elastic recovery, followed by plastic deformation that compensates for irreversibly transformed regions. We further identify individual STZs directly by comparing their measured local strain fields to ideal Eshelby inclusions, fit for their positions and sizes, and investigate how their free volume, local density, coordination, and other structural predictors evolve leading up to and after the shear transformation. |
Thursday, March 17, 2022 5:24PM - 6:00PM |
W14.00005: Thermally activated flow in models of amorphous solids Invited Speaker: Marko Popovic Amorphous solids yield at a critical value Σc of the imposed stress Σ through a dynamical phase transition. While sharp in athermal systems, the presence of thermal fluctuations leads to the rounding of the transition and thermally activated flow even below Σc. Here, we study the steady state thermal flow of amorphous solids using a mesoscopic elasto-plastic model. In the Hébraud-Lequex (HL) model we provide an analytical solution of the thermally activated flow at low temperature. We then propose a general scaling law that also describes the transition rounding. Finally, we find that the scaling law holds in numerical simulations of the HL model, a 2D elasto-plastic model, and in previously published molecular dynamics simulations of 2D Lennard-Jones glass. |
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