A new facility to study the hydrodynamic effects of surface deformation during ciliary propulsion

Ctenophores are the largest organisms which locomote using cilia. These cilia are bundled into paddle-like ctenes; ctene rows oscillate sequentially in a metachronal wave. Previous work on both ctenes (intermediate Re) and cilia (low Re) has assumed the propulsors are embedded in a rigid, flat substrate. However, in ctenophores, this substrate (mesoglea) is curved and deformable. Observation of swimming animals shows a wave traveling along the substrate at the same frequency as the metachronal wave on the ctenes above. It is not clear if this movement is active (e.g. to improve speed or propulsive efficiency), or passive (a consequence of the pressure field generated by the beating ctenes). We design a system that mimics the motion of a ctene together with the deforming substrate observed in ctenophores. A synchronized servomotor and linear actuator produce oscillation and surge in the paddle, and a flexible sheet imitates the mesoglea. We will use Particle Shadow Velocimetry (PSV) to evaluate the generated flow, exploring different combinations of substrate geometry (flat vs. curved) and substrate deformation (rigid vs. flexible) on the ctene-generated flows. This facility will enable the first study of the effects of surge on a rowing paddle: a common biological configuration that is often unaccounted for in numerical and laboratory experiments.