Equation of motion for density distribution of many circling particles with an overdamped circle center

Abstract

We first established the dynamic equations to describe the noisy circling motion of a single particle and the corresponding probability conservation equation in both two dimensions and three dimensions, and then developed the evolution equation of density distribution of many circling particles with overdamped circle center. For many circling particle system without any external force, the density gradient in one direction can induce a flow perpendicular to this direction. While for single circling particle, similar phenomena occurs only for non-zero external force. We performed numerical evolution of the density distribution of many circling particles, the density distribution behaves as a decaying Gaussian distribution propagating along the channel. We computed the particle flow field and the effective force field. Vortex shows up in the high density region. The force field drive particles to the transverse direction perpendicular to the density gradient. We applied this non-equilibrium evolution equation to understand the diffusion phenomena of many sperms(J. Exp. Biol. 210, 3805-3820). Numerical evolution gave us similar density distribution as experimental measurement. The transverse flow we predicted provide a theoretical understanding to the bias concentration of many sperms(J. Exp. Biol. 210. 3805-3820).