Ripples and Shear Bands in Plowed Granular Media

Monodisperse packings of dry, air-fluidized granular media typically exist between volume fractions from $\Phi$= 0.585 to 0.64. We demonstrate that the dynamics of granular drag are sensitive to volume fraction $\Phi$ and their exists a transition in the drag force and material deformation from smooth to oscillatory at a critical volume fraction $\Phi_{c}=0.605$. By dragging a submerged steel plate (3.81 cm width, 6.98 cm depth) through $300 \mu m$ glass beads prepared at volume fractions between 0.585 to 0.635 we find that below $\Phi_{c}$ the media deformation is smooth and non-localized while above $\Phi_{c}$ media fails along distinct shear bands. At high $\Phi$ 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 $l/w$ 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 $f$, peak acceleration $\Gamma$). Column collapse occurs in a characteristic time, $\tau$, which follows the relation $\tau = f^{-1} \exp(\Delta / \Gamma)$. $\Delta$ resembles an activation energy and is maximal at intermediate $l/w$. 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, $\phi$. 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

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