Centripetal focusing of gyrotactic phytoplankton in solid-body rotation

TL;DR

This study investigates how gyrotactic microalgae in a rotating vessel form an axial aggregate due to centripetal orientation, with experimental and mathematical analysis showing the balance between focusing and diffusion effects.

Contribution

The paper provides the first combined experimental and theoretical analysis of gyrotactic algae focusing in solid-body rotation, deriving a mathematical model that matches observed distributions.

Findings

Algae form an axial aggregate that intensifies with rotation speed.

The stationary distribution results from the balance between centripetal orientation and stochastic reorientation.

Numerical simulations agree quantitatively with experimental data.

Abstract

A suspension of gyrotactic microalgae Chlamydomonas augustae swimming in a cylindrical water vessel in solid-body rotation is studied. Our experiments show that swimming algae form an aggregate around the axis of rotation, whose intensity increases with the rotation speed. We explain this phenomenon by the centripetal orientation of the swimming direction towards the axis of rotation. This centripetal focusing is contrasted by diffusive fluxes due to stochastic reorientation of the cells. The competition of the two effects lead to a stationary distribution, which we analytically derive from a refined mathematical model of gyrotactic swimmers. The temporal evolution of the cell distribution, obtained via numerical simulations of the stochastic model, is in quantitative agreement with the experimental measurements in the range of parameters explored.