26 research outputs found
Velocity correlations in dense granular flows observed with internal imaging
We show that the velocity correlations in uniform dense granular flows inside
a silo are similar to the hydrodynamic response of an elastic hard-sphere
liquid. The measurements are made using a fluorescent refractive index matched
interstitial fluid in a regime where the flow is dominated by grains in
enduring contact and fluctuations scale with the distance traveled, independent
of flow rate. The velocity autocorrelation function of the grains in the bulk
shows a negative correlation at short time and slow oscillatory decay to zero
similar to simple liquids. Weak spatial velocity correlations are observed over
several grain diameters. The mean square displacements show an inflection point
indicative of caging dynamics. The observed correlations are qualitatively
different at the boundaries.Comment: 11 pages, 4 figure
Friction of a slider on a granular layer: Non-monotonic thickness dependence and effect of boundary conditions
We investigate the effective friction encountered by a mass sliding on a
granular layer as a function of bed thickness and boundary roughness
conditions. The observed friction has minima for a small number of layers
before it increases and saturates to a value which depends on the roughness of
the sliding surface. We use an index-matched interstitial liquid to probe the
internal motion of the grains with fluorescence imaging in a regime where the
liquid has no significant effect on the measured friction. The shear profiles
obtained as a function of depth show decrease in slip near the sliding surface
as the layer thickness is increased. We propose that the friction depends on
the degree of grain confinement relative to the sliding surfaces.Comment: 4 pages, 6 figure
Fast decay of the velocity autocorrelation function in dense shear flow of inelastic hard spheres
We find in complementary experiments and event driven simulations of sheared
inelastic hard spheres that the velocity autocorrelation function
decays much faster than obtained for a fluid of elastic spheres at
equilibrium. Particle displacements are measured in experiments inside a
gravity driven flow sheared by a rough wall. The average packing fraction
obtained in the experiments is 0.59, and the packing fraction in the
simulations is varied between 0.5 and 0.59. The motion is observed to be
diffusive over long times except in experiments where there is layering of
particles parallel to boundaries, and diffusion is inhibited between layers.
Regardless, a rapid decay of is observed, indicating that this is a
feature of the sheared dissipative fluid, and is independent of the details of
the relative particle arrangements. An important implication of our study is
that the non-analytic contribution to the shear stress may not be present in a
sheared inelastic fluid, leading to a wider range of applicability of kinetic
theory approaches to dense granular matter.Comment: 6 pages, 4 figure
Solid-fluid transition in a granular shear flow
The rheology of a granular shear flow is studied in a quasi-2d rotating
cylinder. Measurements are carried out near the midpoint along the length of
the surface flowing layer where the flow is steady and non-accelerating.
Streakline photography and image analysis are used to obtain particle
velocities and positions. Different particle sizes and rotational speeds are
considered. We find a sharp transition in the apparent viscosity ()
variation with rms velocity (). In the fluid-like region above the depth
corresponding to the transition point (higher rms velocities) there is a rapid
increase in viscosity with decreasing rms velocity. Below the transition depth
we find for all the different cases studied and the
material approaches an amorphous solid-like state deep in the layer. The
velocity distribution is Maxwellian above the transition point and a Poisson
velocity distribution is obtained deep in the layer. The observed transition
appears to be analogous to a glass transition.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Lubrication effects on the flow of wet granular materials
We investigate the dynamics of a partially saturated grain-liquid mixture
with a rotating drum apparatus. The drum is partially filled with the mixture
and then rotated about its horizontal axis. We focus on the continous
avalanching regime and measure the impact of volume fraction and viscosity of
the liquid on the dynamic surface angle. The inclination angle of the surface
is observed to increase sharply to a peak and then decrease as a function of
liquid volume fraction. The height of the peak is observed to increase with
rotation rate. For higher liquid volume fractions, the inclination angle of the
surface can decrease with viscosity before increasing. The viscosity where the
minima occurs decreases with the rotation rate of the drum. Limited
measurements of the flow depth were made, and these were observed to show only
fractional changes with volume fraction and rotation speeds. We show that the
qualitative features of our observations can be understood by analyzing the
effect of lubrication forces on the timescale over which particles come in
contact.Comment: 7 pages, 8 figure