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Search Results: 1 - 10 of 652 matches for " Gregg Lois "
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Force networks and the dynamic approach to jamming in sheared granular media
Gregg Lois,Jean M. Carlson
Physics , 2006, DOI: 10.1209/0295-5075/80/58001
Abstract: Diverging correlation lengths on either side of the jamming transition are used to formulate a rheological model of granular shear flow, based on the propagation of stress through force chain networks. The model predicts three distinct flow regimes, characterized by the shear rate dependence of the stress tensor, that have been observed in both simulations and experiments. The boundaries separating the flow regimes are quantitatively determined and testable. In the limit of jammed granular solids, the model predicts the observed anomalous scaling of the shear modulus and a new relation for the shear strain at yield.
The Breakdown of Kinetic Theory in Granular Shear Flows
Gregg Lois,Anael Lemaitre,Jean M. Carlson
Physics , 2005, DOI: 10.1209/epl/i2005-10605-1
Abstract: We examine two basic assumptions of kinetic theory-- binary collisions and molecular chaos-- using numerical simulations of sheared granular materials. We investigate a wide range of densities and restitution coefficients and demonstrate that kinetic theory breaks down at large density and small restitution coefficients. In the regimes where kinetic theory fails, there is an associated emergence of clusters of spatially correlated grains.
Long Range Correlations in Granular Shear Flow I: Numerical Evidence
Gregg Lois,Anael Lemaitre,Jean M. Carlson
Physics , 2006, DOI: 10.1103/PhysRevE.76.021302
Abstract: We investigate the emergence of long-range correlations in granular shear flow. By increasing the density of a simulated granular flow we observe a spontaneous transition from a dilute regime, where interactions are dominated by binary collisions, to a dense regime characterized by large force networks and collective motions. With increasing density, interacting grains tend to form networks of simultaneous contacts due to the dissipative nature of collisions. We quantify the size of these networks by measuring correlations between grain forces and find that there are dramatic changes in the statistics of contact forces as the size of the networks increases.
Long Range Correlation in Granular Shear Flow II: Theoretical Implications
Gregg Lois,Anael Lemaitre,Jean M. Carlson
Physics , 2006, DOI: 10.1103/PhysRevE.76.021303
Abstract: Numerical simulations are used to test the kinetic theory constitutive relations of inertial granular shear flow. These predictions are shown to be accurate in the dilute regime, where only binary collisions are relevant, but underestimate the measured value in the dense regime, where force networks of size $\xi$ are present. The discrepancy in the dense regime is due to non-collisional forces that we measure directly in our simulations and arise from elastic deformations of the force networks. We model the non-collisional stress by summing over all paths that elastic waves travel through force networks. This results in an analytical theory that successfully predicts the stress tensor over the entire inertial regime without any adjustable parameters.
Momentum Transport in Granular Flows
Gregg Lois,Anael Lemaitre,Jean M. Carlson
Physics , 2006,
Abstract: We investigate the error induced by only considering binary collisions in the momentum transport of hard-sphere granular materials, as is done in kinetic theories. In this process, we first present a general microscopic derivation of the momentum transport equation and compare it to the kinetic theory derivation, which relies on the binary collision assumption. These two derivations yield different microscopic expressions for the stress tensor, which we compare using simulations. This provides a quantitative bound on the regime where binary collisions dominate momentum transport and reveals that most realistic granular flows occur in the region of phase space where the binary collision assumption does not apply.
A percolation model for slow dynamics in glass-forming materials
Gregg Lois,Jerzy Blawzdziewicz,Corey S. O'Hern
Physics , 2008, DOI: 10.1103/PhysRevLett.102.015702
Abstract: We identify a link between the glass transition and percolation of mobile regions in configuration space. We find that many hallmarks of glassy dynamics, for example stretched-exponential response functions and a diverging structural relaxation time, are consequences of the critical properties of mean-field percolation. Specific predictions of the percolation model include the range of possible stretching exponents $1/3 \leq \beta \leq 1$ and the functional dependence of the structural relaxation time $\tau_\alpha$ and exponent $\beta$ on temperature, density, and wave number.
Protein folding on rugged energy landscapes: Conformational diffusion on fractal networks
Gregg Lois,J. Blawzdziewicz,Corey S. O'Hern
Physics , 2009, DOI: 10.1103/PhysRevE.81.051907
Abstract: We employ simulations of model proteins to study folding on rugged energy landscapes. We construct ``first-passage'' networks as the system transitions from unfolded to native states. The nodes and bonds in these networks correspond to basins and transitions between them in the energy landscape. We find power-laws between the folding time and number of nodes and bonds. We show that these scalings are determined by the fractal properties of first-passage networks. Reliable folding is possible in systems with rugged energy landscapes because first passage networks have small fractal dimension.
The jamming transition and new percolation universality classes in particulate systems with attraction
Gregg Lois,Jerzy Blawzdziewicz,Corey S. O'Hern
Physics , 2007, DOI: 10.1103/PhysRevLett.100.028001
Abstract: We numerically study the jamming transition in particulate systems with attraction by investigating their mechanical response at zero temperature. We find three regimes of mechanical behavior separated by two critical transitions--connectivity and rigidity percolation. The transitions belong to different universality classes than their lattice counterparts, due to force balance constraints. We also find that these transitions are unchanged at low temperatures and resemble gelation transitions in experiments on colloidal and silica gels.
Numerical Tests of Constitutive Laws for Dense Granular Flows
Gregg Lois,Ana?l Lema?tre,Jean M. Carlson
Physics , 2005, DOI: 10.1103/PhysRevE.72.051303
Abstract: We numerically and theoretically study macroscopic properties of dense, sheared granular materials. In this process we first introduce an invariance in Newton's equations, explain how it leads to Bagnold's scaling, and discuss how it relates to the dynamics of granular temperature. Next we implement numerical simulations of granular materials in two different geometries-- simple shear and flow down an incline-- and show that measurements can be extrapolated from one geometry to the other. Then we observe non-affine rearrangements of clusters of grains in response to shear strain and show that fundamental observations, which served as a basis for the Shear Transformation Zone (STZ) theory of amorphous solids, can be reproduced in granular materials. Finally we present constitutive equations for granular materials, based on the dynamics of granular temperature and STZ theory, and show that they match remarkably well with our numerical data from both geometries.
Reliable protein folding on non-funneled energy landscapes: the free energy reaction path
Gregg Lois,Jerzy Blawzdziewicz,Corey S. O'Hern
Quantitative Biology , 2008, DOI: 10.1529/biophysj.108.133132
Abstract: A theoretical framework is developed to study the dynamics of protein folding. The key insight is that the search for the native protein conformation is influenced by the rate r at which external parameters, such as temperature, chemical denaturant or pH, are adjusted to induce folding. A theory based on this insight predicts that (1) proteins with non-funneled energy landscapes can fold reliably to their native state, (2) reliable folding can occur as an equilibrium or out-of-equilibrium process, and (3) reliable folding only occurs when the rate r is below a limiting value, which can be calculated from measurements of the free energy. We test these predictions against numerical simulations of model proteins with a single energy scale.
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