100 research outputs found
The mechanism of long-term coarsening of granular mixtures in rotating drums
Three fundamental segregation and pattern formation processes are known in
granular mixtures in a rotating cylindrical drum: radial segregation, axial
banding, and coarsening of the band pattern. While the mechanism for the first
effect is well understood and for the second effect, several models have been
proposed, the long-term coarsening mechanism remained unexplained so far. We
demonstrate that the unidirectional flow between the bands in an axially
segregated pattern is driven by small differences in size of the small beads at
the band edges. Due to a process of microsegregation inside each band of small
particles, which was so far unrecognized, this difference in diameter will be
effective in all experiments with polydisperse beads. In consequence the
stability of individual bands can be easily controlled by minor alterations of
their composition. Our results make evident that a new mechanism as the driving
force behind the axial particle flow has to be sought. We suggest possible
hypotheses for such a mechanism
On-off Intermittency in Stochastically Driven Electrohydrodynamic Convection in Nematics
We report on-off intermittency in electroconvection of nematic liquid crystals driven by a dichotomous stochastic electric voltage. With increasing voltage amplitude we observe laminar phases of undistorted director state interrupted by shorter bursts of spatially regular stripes. Near a critical value of the amplitude the distribution of the duration of laminar phases is governed over several decades by a power law with exponent -3/2. The experimental findings agree with simulations of the linearized electrohydrodynamic equations near the sample stability threshold
Cooling of a granular gas mixture in microgravity
Granular gases are fascinating non-equilibrium systems with interesting
features such as spontaneous clustering and non-Gaussian velocity
distributions. Mixtures of different components represent a much more natural
composition than monodisperse ensembles, but attracted comparably little
attention so far. We present the first experimental observation and
characterization of a mixture of rod-like particles with different sizes and
masses in microgravity. Kinetic energy decay rates during granular cooling and
collision rates were determined and Haff's law for homogeneous granular cooling
was confirmed. Thereby, energy equipartition between the mixture components and
between individual degrees of freedom is violated. Heavier particles keep a
slightly higher average kinetic energy than lighter ones. Experimental results
are supported by numerical simulations.Comment: 16 Pages, 7 figures; corrected typo in author's nam
Effects of grain shape on packing and dilatancy of sheared granular materials
Granular material exposed to shear shows a variety of unique phenomena:
Reynolds dilatancy, positional order and orientational order effects may
compete in the shear zone. We study granular packings consisting of macroscopic
prolate, oblate and spherical grains and compare their behaviour. X-ray
tomography is used to determine the particle positions and orientations in a
cylindrical split bottom shear cell. Packing densities and the arrangements of
individual particles in the shear zone are evaluated. For anisometric
particles, we observe the competition of two opposite effects. One the one
hand, the sheared granulate is dilated, but on the other hand the particles
reorient and align with respect to the streamlines. Even though aligned
cylinders in principle may achieve higher packing densities, this alignment
compensates for the effect of dilatancy only partially. The complex
rearrangements lead to a depression of the surface above the well oriented
region while neigbouring parts still show the effect of dilation in the form of
heaps. For grains with isotropic shapes, the surface remains rather flat.
Perfect monodisperse spheres crystallize in the shear zone, whereby positional
order partially overcompensates dilatancy effects. However, already slight
deviations from the ideal monodisperse sphere shape inhibit crystallization.Comment: 12 pages, 13 figures, accepted in Soft Matte
- …