Emergent coordination and propulsion of a model spherical ciliate

Abstract

A longstanding challenge in biofluid dynamics research is a mechanistic understanding of the coordinated movement of motile cilia and its resulting ability to facilitate fluid transport. In this study, we develop numerical techniques to simultaneously compute the emergent coordination of and propulsion by filamentous model cilia covering the surface of a sphere. To accomplish this, we develop what we refer to as the filament oscillator model, in which each cilium has two dynamic degrees of freedom: a phase variable that maps to a specific shape in a prescribed sequence, and an angle that describes the overall orientation of the sequence. By varying a parameter related to cilium stiffness, we show that there is bistability between symplectic-like and diaplectic metachronal waves, provided that the stiffness is sufficiently low. Above the critical stiffness, only diaplectic waves emerge. Further, we analyse the propulsive capabilities and flow fields of the two emergent states, showing that diaplectic waves provide more efficient propulsion due to their shorter wavelengths. In addition, we examine how introducing beat-plane tilt leads to ciliate rotation while maintaining nearly identical emergent states and comparable swimming speeds.