Meeting Abstract
Impulsive movements in deformable media are generated during locomotion on water or sand. When basilisk lizards run on water, reaction forces arise resulting from combinations of hydrostatic pressure, drag forces with quadratic velocity dependence and virtual mass forces associated with accelerating fluid. We are interested in understanding how and if such forces contribute to impulsive interactions of limbs with granular media (GM), which can act like fluids and solids. To gain insight into these interactions, we study the jump height of a one-dimensional locomotor, a self-actuated spring mass robot with a ~7 cm flat circular foot in a deep bed of 1 mm poppy seeds, varying the compaction of the GM. We also vary parameters associated with the self-deformation of the robot, including a 1-cycle sine wave “single jump” starting with a low center of mass and a counter-movement induced “stutter jump” consisting of a preliminary hop. While both jumps perform well on compact GM (and on hard ground), they perform poorly in loose GM, reaching only ~40% and ~50% of the compact GM jump height. Introducing a delay time between the pull-up phase and the push-off phase of the stutter jump (which we call a “delayed stutter jump”) results in improved jump heights at low compaction, achieving ~80% of the compact GM height. A simulation of the robot in conjunction with particle image velocimetry monitoring of sidewall grain flow reveals that, like in fluids, a combination of hydrostatic-like pressure, quadratic drag and a granular virtual mass are required to reproduce experimental results. Virtual mass effects are important in loose GM, especially for the stutter jump.
