Exploring noisy Jeffery orbits: A combined Fokker-Planck and Langevin analysis in two and three dimensions.

Abstract

The behavior of nonspherical particles in a shear flow is of significant practical and theoretical interest. These systems have been the object of numerous investigations since the pioneering work of Jeffery a century ago. His eponymous orbits describe the deterministic motion of an isolated, rodlike particle in a shear flow. Subsequently, the effect of adding noise was investigated. The theory has been applied to colloidal particles, macromolecules, anisometric granular particles, and most recently to microswimmers, for example, bacteria. We study the Jeffery orbits of elongated (uniaxial, prolate) particles subject to noise using Langevin simulations and a Fokker-Planck equation. We extend the analytical solution for infinitely thin needles (β=1) obtained by Doi and Edwards to particles with arbitrary shape factor (0≤β≤1) and validate the theory by comparing it with simulations. We examine the rotation of the particle around the vorticity axis and study the orientational order matrix. We use the latter to obtain scalar order parameters s and r describing nematic ordering and biaxiality from the orientational distribution function. The value of s (nematic ordering) increases monotonically with increasing Péclet number, while r (measure of biaxiality) displays a maximum value. From perturbation theory, we obtain simple expressions that provide accurate descriptions at low noise (or large Péclet numbers). We also examine the orientational distribution in the v-grad v plane and in the perpendicular direction. Finally, we present the solution of the Fokker-Planck equation for a strictly two-dimensional (2D) system. For the same noise amplitude, the average rotation speed of the particle in 3D is larger than in 2D.