Shape and displacement fluctuations in soft vesicles filled by active particles

Active granular particles can harness unbiased mechanical vibrations in the environment to generate directed motion. We provide a theoretical framework that connects the geometrical shape of a three dimensional object to its self-propulsion characteristics over a vertically vibrated plate. We find that a maximally efficient propulsion is achieved for structures having tilted flexible legs forming a characteristic angle with the vertical. Our predictions are verified by experimental observations on a class of 3D printed structures with smoothly varying geometrical features.