SBOR: a minimalistic soft self-burrowing-out robot inspired by razor clams.

We observe that the Atlantic razor clam (Ensis directus) burrows out of sand rapidly by simply extending and contracting its muscular foot. This is notably different from its well-known downward burrowing strategy or the dual-anchor mechanism, where closing/opening of the shell and dilation of the foot are also involved. Inspired by this burrowing-out strategy, we design a simple self-burrowing-out robot (SBOR) consisting of a single segment of fiber-reinforced silicone tube actuator and an external control board. The reinforcing fibers limit the motion of the actuator to axial extension/contraction under inflation/deflation. For an actuator that is vertically buried in the sand, cyclic inflation and deflation naturally drives it out of the sand, mimicking the motion of a razor clam. We characterize the burrowing-out behavior of the actuator by varying the actuation period and the relative density (packing) of the sand. Each burrowing cycle features an initial upward advancement during inflation, followed by a downward slip during deflation, resulting in a net upward stride. During the burrowing-out process, the stride length first increases due to a drop in the overburden pressure, the end pull-out resistance, and the side frictional resistance; the stride length then decreases after the top of the actuator moves out of the sand layer, due to a reduction in the effective length of the actuator. The results also indicate that the average burrowing-out speed decreases with the relative density of the sand and changes with the actuation pressure. We developed a simplified model based on soil mechanics to predict the burrowing-out processes in relatively loose dry sands, and the modeling results match well with the experiment results. From this model, the burrowing-out behavior is readily explained by the asymmetric nature of the resistant forces on the two ends of the actuator and the flowing nature of sand upon yielding. Our findings imply that razor clams leverage the natural stress gradient of sand deposits to burrow upward. Another insight is that in order to burrow downward into the sand, additional symmetry-breaking features such as asymmetric geometry, friction, stress state or external load are needed to increase the resistant force (anchorage) in the upward direction and to reduce the resistant force (drag) in the downward direction.