Meeting Abstract
We observed that Atlantic razor clams burrow out of the substrate very rapidly using a simple strategy: cyclic extension and contraction of a soft, hollow muscular foot. This is notably different from its burrowing-in strategy where closing/opening of the shell and dilation of the foot are also involved. Inspired by the burrowing-out strategy, we designed a minimum self-burrowing pneumatic soft robot, which consists of a silicone tube reinforced with a symmetrical, double-helix wrapping of inextensible thread. The threads limit the actuator to axial extension/contraction motion under inflation/deflation. The cyclic extension/contraction of the robot naturally drive it out of the sand, mimicking razor clams. The burrowing-out behavior of the robot was studied by varying the inflation-deflation periods and the relative density of the sands. Each burrowing cycle features an initial advancement due to inflation, followed by a slip due to deflation, resulting in a net stride. When the robot burrows out, the stride first increases due to the decrease in overburden pressure, and then decreases after the top of the actuator moves out of the soil, due to the reduction in the effective length of the actuator. The results also indicate that the average burrowing-out speed decreases with increasing soil relative density. A simplified model based on soil mechanics is developed to model the burrowing-out behaviors and its prediction matches very well with the experiment results. From this model, the burrowing-out behavior is readily explained by the asymmetric nature of the resistant force on the two ends of the actuator. An insight is that in order to burrow-in, additional symmetry breaking features such as asymmetric geometry, friction or external load are needed to increase the resistant force (anchorage) in the upward direction and to reduce the resistant force in the downward direction.
