Session S3: Aging, Shear and Rejuvenation: Mechanics of Glasses, Colloids and Granular Matter

Jamming: Relating Shear and Effective Temperature

In an equilibrium system, temperature not only influences the average properties of a system, such as its pressure or density, but also controls the fluctuations around those averages. In systems driven far from equilibrium, however, temperature is no longer well-defined, and fluctuations can be non-thermal in origin. I will discuss a class of such systems, namely steadily-sheared glasses, for which there is a considerable body of evidence that the idea of an effective temperature is useful, at least in certain regimes. Using nonequilibrium molecular dynamics simulations, we have now calculated seven different definitions that yield a consistent value for the effective temperature, which can be many orders of magnitude higher than the bath temperature. However, if we want to understand the behavior of a material, measuring its temperature is only a start. I will discuss recent results that show that when shear-induced fluctuations dominate over thermal fluctuations, the effective temperature controls materials properties such as the rheology and the extreme slowing down of the dynamics as the system jams.   

Simulations of shear banding in metallic glasses

Metallic glasses represent a promising high strength material, but their use is limited by the onset of a shear banding instability when their material strength is exceeded. Recent simulation studies of the initiation and development of localized deformation in molecular dynamics simulations of a number of amorphous systems reveal the structural changes that accompany plastic deformation and localization involve a decrease in the local short range ordering. We have simulated both two-dimensional and three-dimensional systems in nanoindentation [1,2], uniaxial tension [3] and compression [4] in plane strain. The degree of strain localization depends sensitively on the quench rate during sample preparation, with localization only arising in more gradually quenched samples. A systematic analysis of simulated systems in simple shear geometries [5] reveals that a Boltzmann-like relationship between strain rate and structure holds over large variations in both the applied strain rate and the initial structural state of the glass. Scaling is observed over eight orders of magnitude in strain rate. The consequences of this scaling for constitutive models of glass plasticity will be discussed. \\ $[1]$ Y. Shi and M.L. Falk, ``Structural transformation and localization during simulated nanoindentation of a non-crystalline metal film,'' Applied Physics Letters, Vol. 86, pp. 011914 (2005). \\ $[2]$ Y. Shi and M.L. Falk, ``The structural origin of shear band formation in metallic glass studied via simulated nanoindentation,'' Acta Materialia , Vol. 55, pp. 4317 (2007). \\ $[3]$ Y. Shi and M.L. Falk, ``Strain localization and percolation of stable structure in amorphous solids,'' Physical Review Letters, Vol. 95, pp. 095502 (2005). \\ $[4]$ Y. Shi and M.L. Falk, ``Atomic-scale simulations of strain localization in three-dimensional model amorphous solids,'' Physical Review B, Vol. 73, pp. 214201 (2006). \\ $[5]$ Y. Shi, M.B. Katz, H. Li and M.L. Falk, ``Evaluation of the `disorder temperature' and `free volume' formalisms via simulations of shear banding in amorphous solids,'' Physical Review Letters, Vol. 98, 185505 (2007).   

Single particle dynamics of aging in colloidal systems

When a liquid is quenched to form a glass it becomes trapped in a non-equilibrium state, and many of the system's properties depend on the time elapsed since the quench. This phenomenon is known as aging. We study concentrated colloidal suspensions, a model system which has a glass transition when the particle concentration is increased. We use an optical confocal microscope to view the motion of these colloidal particles in three dimensions. Aging is most commonly detected by measuring the evolution of variables (such as the mean squared displacement) averaged over the entire system, but these quantities cannot yield information about the detailed, structural changes that occur during aging. In contrast, confocal microscopy lets us study the the relationship between local structure and the motion of the colloidal particles, in both monodisperse and binary colloidal glasses. We find that particle motion occurs in cooperative groups, and that this motion is facilitated by the relatively poor packing of the particles in these regions. Work done with G. C. Cianci, J. M. Lynch, and R. E. Courtland.   

Aging dynamics and the mechanical behavior of glassy solids

Although aging is observed in most glassy materials, a molecular level understanding of the underlying microscopic processes is incomplete. The intrinsic slow relaxation dynamics in glasses can cause intriguing phenomena: aging changes the mechanical properties of the material, but deformation and yielding also modify aging. This talk will present a series of molecular simulations that provide new insight into the complex, microscopic origins of the elastoplastic behavior of amorphous matter and its relationship to the macroscopic material response. In particular, we investigate the interplay between aging and plastic deformation in coarse-grained models for polymeric and metallic glasses. Molecular dynamics simulations are used to determine the macroscopic shear yield stress as well as the compliance of the model glasses for different loading conditions, temperatures, strain rates and aging times, as well as reveal their relationship to the underlying microscopic distribution of relaxation times. As in experiments on polymer glasses, we find that large stresses can decrease relaxation times and cause mechanical rejuvenation. Furthermore, we find new behavior when the aging glass undergoes more involved thermal protocols such as a temperature step. Phenomenological models will be developed that describe the data over a wide range of temperature, stress and strain rates. We also discuss how continuum models such as energy landscape pictures and the recently formulated shear transformation zone (STZ) theory of amorphous plasticity can account for the aging effects observed in the molecular simulations.   

Faults \& Earthquakes as Granular Phenomena: Controls on Stick-Slip Dynamics

Granular and continuous materials fail in fundamentally different ways, yet inherently discontinuous natural fault materials have often been modeled as continuum processes. Within a sheared or compressed granular material, the internal stresses take the form of a network of force chains. This network of strong connections among the particles is observed to be highly heterogeneous, and the magnitude of the stress varies widely over short distances. I will present the results of laboratory experiments which highlight the granular conrols on earthquake fault behavior. We perform experiments in a quasi-two-dimensional shear zone containing several thousand 5 mm circular and elliptical photoelastic plastic disks, allowing us to monitor the spatiotemporal evolution of both internal stress and strain. While the time, length, and strength scales are vastly different from the natural case, the frictional behavior is found to be in agreement. Therefore, the experiments allow us to isolate the effects of granular interactions and choice of boundary conditions on the fault behavior, through the observation of large populations of stick-slip and creep events.