A Self-propelled Cruiser in 2D Granular Media under Gravity

We propose an extremely simple but efficient self-propelled cruiser, able to travel freely in 2D granular materials made of bidisperse dissipative particles under gravity. The cruiser has a round shape and a square indentation on its edge. To move into a given direction, perpendicular or parallel to gravity, we orient the indentation in line with the desired direction, and the cruiser shifts the granular particles entering its indent-region to its rear-half by a prescribed distance and then ejects them backward to gain thrust for moving forward. Using frictionless molecular dynamics (MD) method, we identify three universal phases of the cruiser during one particle-ejection process: 1) acceleration due to the ejection-thrust, 2) deceleration by the compressed particles ahead and 3) relaxation with the decompressed particles. We also confirm that the cruiser can travel continuously within the granular medium by successive particle-ejection, and the cruising performance increases with the ejection-strength and decreases by the interference from gravity. We believe our study demonstrates a novel approach to engineer self-propelled machines in granular materials.