40 research outputs found
Ripples and Shear Bands in Plowed Granular Media
Monodisperse packings of dry, air-fluidized granular media typically exist
between volume fractions from = 0.585 to 0.64. We demonstrate that the
dynamics of granular drag are sensitive to volume fraction and their
exists a transition in the drag force and material deformation from smooth to
oscillatory at a critical volume fraction . By dragging a
submerged steel plate (3.81 cm width, 6.98 cm depth) through glass
beads prepared at volume fractions between 0.585 to 0.635 we find that below
the media deformation is smooth and non-localized while above
media fails along distinct shear bands. At high the
generation of these shear bands is periodic resulting in the ripples on the
surface. Work funded by The Burroughs Wellcome Fund and the Army Research Lab
MAST CT
Entangled granular media
We study the geometrically induced cohesion of ensembles of granular
"u-particles" which mechanically entangle through particle interpenetration. We
vary the length-to-width ratio of the u-particles and form them into
free-standing vertical columns. In laboratory experiment we monitor the
response of the columns to sinusoidal vibration (frequency , peak
acceleration ). Column collapse occurs in a characteristic time,
, which follows the relation .
resembles an activation energy and is maximal at intermediate .
Simulation reveals that optimal strength results from competition between
packing and entanglement.Comment: 4 pages, 5 figure
Lateral contact yields longitudinal cohesion in active undulatory systems
Many animals and robots move using undulatory motion of their bodies. When in
close proximity undulatory motion can lead to novel collective behaviors such
as gait synchronization, spatial reconfiguration, and clustering. Here we study
the role of contact interactions between model undulatory swimmers: three-link
robots in experiment and multi-link robots in simulation. The undulatory gait
of each swimmer is generated through a time-dependent sinusoidal-like waveform
which has a fixed phase offset, . By varying the phase relationship
between neighboring swimmers we seek to study how contact forces and spatial
configurations are governed by the phase difference between neighboring
swimmers. We find that undulatory actuation in close proximity drives
neighboring swimmers into spatial equilibrium configurations that depend on the
actuation phase difference. We propose a model for spatial equilibrium of
nearest neighbor undulatory swimmers which we call the gait compatibility
condition, which is the set of spatial and gait configurations in which no
collisions occur. Robotic experiments with two, three, and four swimmers
exhibit good agreement with the compatibility model. To probe the interaction
potential between undulatory swimmers we perturb the each longitudinally from
their equilibrium configurations and we measure their steady-state
displacement. These studies reveal that undulatory swimmers in close proximity
exhibit cohesive longitudinal interaction forces that drive the swimmers from
incompatible to compatible configurations. This system of undulatory swimmers
provides new insight into active-matter systems which move through body
undulation. In addition to the importance of velocity and orientation coherence
in active-matter swarms, we demonstrate that undulatory phase coherence is also
important for generating stable, cohesive group configurations.Comment: 13 pages, 10 figure
Utilization of granular solidification during terrestrial locomotion of hatchling sea turtles
Biological terrestrial locomotion occurs on substrate materials with a range of rheological behaviour, which can affect limb-ground interaction, locomotor mode and performance. Surfaces like sand, a granular medium, can display solid or fluid-like behaviour in response to stress. Based on our previous experiments and models of a robot moving on granular media, we hypothesize that solidification properties of granular media allow organisms to achieve performance on sand comparable to that on hard ground. We test this hypothesis by performing a field study examining locomotor performance (average speed) of an animal that can both swim aquatically and move on land, the hatchling Loggerhead sea turtle (Caretta caretta). Hatchlings were challenged to traverse a trackway with two surface treatments: hard ground (sandpaper) and loosely packed sand. On hard ground, the claw use enables no-slip locomotion. Comparable performance on sand was achieved by creation of a solid region behind the flipper that prevents slipping. Yielding forces measured in laboratory drag experiments were sufficient to support the inertial forces at each step, consistent with our solidification hypothesis
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Sidewinding with minimal slip: Snake and robot ascent of sandy slopes
Limbless organisms like snakes can navigate nearly all terrain. In particular, desert-dwelling sidewinder rattlesnakes (C. cerastes) operate effectively on inclined granular media (like sand dunes) that induce failure in field-tested limbless robots through slipping and pitching. Our laboratory experiments reveal that as granular incline angle increases, sidewinder rattlesnakes increase the length of their body in contact with the sand. Implementing this strategy in a physical robot model of the snake enables the device to ascend sandy slopes close to the angle of maximum slope stability. Plate drag experiments demonstrate that granular yield stresses decrease with increasing incline angle. Together these three approaches demonstrate how sidewinding with contact-length control mitigates failure on granular media.This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in the journal Science on 10 October 2014, Volume 346 number 6206, DOI: 10.1126/science.1255718. The published article is copyrighted by the American Association for the Advancement of Science and can be found at: http://www.sciencemag.org/journals
Synchronized swimming: adaptive gait synchronization through mechanical interactions instead of communication
The 9.5th international symposium on Adaptive Motion of Animals and Machines. Ottawa,Canada (Virtual Platform). 2021-06-22/25. Adaptive Motion of Animals and Machines Organizing Committee