Molecular theory of Langevin dynamics for active self-diffusiophoretic colloids

TL;DR

This paper derives microscopic Langevin equations for small active colloids propelled by self-diffusiophoresis, providing detailed expressions for forces, torques, and reaction rates that are valid at microscopic scales.

Contribution

It offers a fully microscopic derivation of Langevin equations for self-diffusiophoretic colloids, including explicit formulas for friction tensors and reaction rates.

Findings

Provides microscopic expressions for translational and rotational friction tensors.

Derives nonequilibrium averages of fluid fields for active motion.

Extends theoretical understanding to small-scale active colloids.

Abstract

Active colloidal particles that are propelled by a self-diffusiophoretic mechanism are often described by Langevin equations that are either postulated on physical grounds or derived using the methods of fluctuating hydrodynamics. While these descriptions are appropriate for colloids of micrometric and larger size, they will break down for very small active particles. A fully microscopic derivation of Langevin equations for self-diffusiophoretic particles powered by chemical reactions catalyzed asymmetrically by the colloid is given in this paper. The derivation provides microscopic expressions for the translational and rotational friction tensors, as well as reaction rate coefficients appearing in the Langevin equations. The diffusiophoretic force and torque are expressed in terms of nonequilibrium averages of fluid fields that satisfy generalized transport equations. The results provide a description of active motion on small scales where descriptions in terms of coarse grained continuum fluid equations combined with boundary conditions that account for the presence of the colloid may not be appropriate.