Deniz Ertas

Granular flow down an inclined plane: Bagnold scaling and rheology

We have performed a systematic, large-scale simulation study of granular media in two and three dimensions, investigating the rheology of cohesionless granular particles in inclined plane geometries, i.e., chute flows. We find that over a wide range of parameter space of interaction coefficients and inclination angles, a steady-state flow regime exists in which the energy input from gravity balances that dissipated from friction and inelastic collisions. In this regime, the bulk packing fraction (away from the top free surface and the bottom plate boundary) remains constant as a function of depth z, of the pile. The velocity profile in the direction of flow vx(z) scales with height of the pile H, according to vx(z) proportional to H(alpha), with alpha=1.52+/-0.05. However, the behavior of the normal stresses indicates that existing simple theories of granular flow do not capture all of the features evidenced in the simulations.

Geometry of frictionless and frictional sphere packings

We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from z=6 as the friction coefficient mu between two particles is increased.

Irreversibility, mechanical entanglement and thermal melting in superconducting vortex crystals with point impurities

Abstract We discuss the onset of irreversibility and entanglement of vortex lines in high T c superconductors due to point disorder and thermal fluctuations using a simplified cage model. A combination of Flory arguments, known results from directed polymers in random media, and a Lindemann criterion are used to estimate the field and temperature dependence of irreversibility, mechanical entanglement and thermal melting. The qualitative features of this dependence, including its nonmonotonicity when disorder is sufficiently strong, are in good agreement with recent experiments.

Lateral Separation of Macromolecules and Polyelectrolytes in Microlithographic Arrays

A new approach to separation of a variety of microscopic and mesoscopic objects in dilute solution is presented. The approach takes advantage of unique properties of a specially designed separation device (sieve), which can be readily built using already developed microlithographic techniques. Due to the broken reflection symmetry in its design, the direction of motion of an object in the sieve varies as a function of its self-diffusion constant, causing separation transverse to its direction of motion. This gives the device some significant and unique advantages over existing fractionation methods based on centrifugation and electrophoresis.

Self-driven mode switching of earthquake activity on a fault system

Theoretical results based on two different modeling approaches indicate that the seismic response of a fault system to steady tectonic loading can exhibit persisting fluctuations in the form of self-driven switching of the response back and forth between two distinct modes of activity. The first mode is associated with clusters of intense seismic activity including the largest possible earthquakes in the system and frequency‐size event statistics compatible with the characteristic earthquake distribution. The second mode is characterized by relatively low moment release consisting only of small and intermediate size earthquakes and frequency‐size event statistics following a truncated power law. The average duration of each activity mode scales with the time interval of a large earthquake cycle in the system. The results are compatible with various long geologic, paleoseismic, and historical records. The mode switching phenomenon may also exist in responses of other systems with many degrees of freedom and nonlinear dynamics.

QUASISTATIC CRACK PROPAGATION IN HETEROGENEOUS MEDIA

The dynamics of a single crack moving through a heterogeneous medium is studied in the quasistatic approximation, which may be directly applicable to cracks grown under fatigue. In a model scalar system and for mode III (tearing) loading, the crack surface is found to be self-affine with a roughness exponent of

Critical dynamics of contact line depinning.

\ensuremath{\zeta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2

Granular gravitational collapse and chute flow

. Mode I (tensile) loading, however, leads to a crack surface that is only logarithmically rough, quite unlike those seen in most experiments. Residual stresses are found, potentially, to lead to increased crack surface roughness. But elastic wave propagation effects may be needed to explain the very rough crack surfaces observed experimentally.

Velocity correlations in dense gravity-driven granular chute flow

The depinning of a contact line is studied as a dynamical critical phenomenon by a functional renormalization group technique. In

Gravity-driven dense granular flows

D=2-\epsilon