Minimal robophysical model for multi-flagellate propulsion

Microorganisms with appendages (e.g., flagella) rely on various strategies to generate locomotion in highly viscous environments. Despite possessing similar morphology, different quadriflagellate algae species consistently self-propel at different speeds. Here to test if these performance differences are a sole function of the diverse gaits (appendage coordination patterns) employed by different species, we developed a macroscopic robophysical model (four appendages, body length of 3.9 cm) with the capability to self-propel in a viscous fluid (mineral oil, 1,000 cSt). We tested swimming performance in three distinct gaits, the pronk, the trot, and the gallop, and tested the effects of appendage orientation relative to the cell body. With perpendicular appendages, the robot achieved a speed of 0.020-0.1 body lengths per second depending on the gait, with the trot displaying the greatest speed. Robot performance was comparable to the algae across gaits. With parallel appendages, swimming performance decreased significantly for all gaits. Our results show a minimal robophysical modeling can aid our understanding of control principles of low Reynolds swimming in biological systems and inspire design of autonomous microrobots.