Stroke frequency and size effects on metachronal paddling

Numerous aquatic organisms use a locomotion strategy known as metachronal paddling across a wide range of body sizes, from microscopic paramecia to lobsters nearly a meter in body length. This range of body size results in vastly different fluid dynamic effects on the swimming regime. Despite broad application of metachronal paddling in nature, the effect of paddle-based Reynolds number on the paddling wake and swimming performance is unknown. Using a paddling robot with fixed stroke amplitude and metachronal phase lag, we vary stroke frequency and fluid viscosity to investigate these effects. Varying stroke frequency allows us to examine the fluid dynamic effects of an organism stroking faster or slower, while varying fluid viscosity allows us to examine the different fluid dynamic effects acting on organisms of different sizes. We found that while viscosity has a stronger effect on the direction of the wake than frequency, the frequency does affect cycle to cycle interactions in the wake. Interestingly, the Strouhal number remained relatively unchanged (approximately St=0.25) for Reynolds numbers ranging from Re=50 to Re=50,000, indicating that metachronal paddling is robust across different sizes.