61 research outputs found
Maximum and Minimum Stable Random Packings of Platonic Solids
Motivated by the relation between particle shape and packing, we measure the
volume fraction occupied by the Platonic solids which are a class of
polyhedron with congruent sides, vertices and dihedral angles. Tetrahedron,
cube, octahedron, dodecahedron, and icosahedron shaped plastic dice were
fluidized or mechanically vibrated to find stable random loose packing
and densest packing , respectively with standard deviation . We find that obtained by all protocols peak at the cube, which is
the only Platonic solid that can tessellate space, and then monotonically
decrease with number of sides. This overall trend is similar but systematically
lower than the maximum reported for frictionless Platonic solids, and
below of spheres for the loose packings. Experiments with ceramic
tetrahedron were also conducted, and higher friction was observed to lead to
lower
Spatial distribution functions of random packed granular spheres obtained by direct particle imaging
We measure the two-point density correlations and Voronoi cell distributions
of cyclically sheared granular spheres obtained with a fluorescence technique
and compare them with random packing of frictionless spheres. We find that the
radial distribution function is captured by the Percus-Yevick equation
for initial volume fraction . However, small but systematic
deviations are observed because of the splitting of the second peak as
is increased towards random close packing. The distribution of the Voronoi free
volumes deviates from postulated distributions, and the orientational
order metric shows disorder compared to numerical results reported for
frictionless spheres. Overall, these measures show significant similarity of
random packing of granular and frictionless spheres, but some systematic
differences as well.Comment: 4 pages, 4 figure
Dynamic wrinkling and strengthening of a filament in a viscous fluid
We investigate the wrinkling dynamics of an elastic filament immersed in a
viscous fluid submitted to compression at a finite rate with experiments and by
combining geometric nonlinearities, elasticity, and slender body theory. The
drag induces a dynamic lateral reinforcement of the filament leading to growth
of wrinkles that coarsen over time. We discover a new dynamical regime
characterized by a timescale with a non-trivial dependence on the loading rate,
where the growth of the instability is super-exponential and the wavenumber is
an increasing function of the loading rate. We find that this timescale can be
interpreted as the characteristic time over which the filament transitions from
the extensible to the inextensible regime. In contrast with our analysis with
moving boundary conditions, Biot's analysis in the limit of infinitely fast
loading leads to rate independent exponential growth and wavelength
Escape dynamics of confined undulating worms
We investigate the escape dynamics of oligochaeta Lumbriculus variegatus by confining them to a quasi-2D circular chamber with a narrow exit passage. The worms move by performing undulatory and peristaltic strokes and use their head to actively probe their surroundings. We show that the worms follow the chamber boundary with occasional reversals in direction and with velocities determined by the orientation angle of the body with respect to the boundary. The average time needed to reach the passage decreases with its width before approaching a constant, consistent with a boundary-following search strategy. We model the search dynamics as a persistent random walk along the boundary and demonstrate that the head increasingly skips over the passage entrance for smaller passage widths due to body undulations. The simulations capture the observed exponential time-distributions taken to reach the exit and their mean as a function of width when starting from random locations. Even after the head penetrates the passage entrance, we find that the worm does not always escape because the head withdraws rhythmically back into the chamber over distances set by the dual stroke amplitudes. Our study highlights the importance of boundary following and body strokes in determining how active matter escapes from enclosed spaces. © 2023 The Royal Society of Chemistry
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
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