Motion of an active particle in a linear concentration gradient

Abstract

Janus particles self-propel by generating local tangential concentration gradients along their surface. These gradients are present in a layer whose thickness is small compared to the particle size. Chemical asymmetry along the surface is a prerequisite to generate tangential chemical gradients, which gives rise to diffusio-osmotic flows in a thin region around the particle. This results in an effective slip on the particle surface. This slip results in the observed “swimming” motion of a freely suspended particle even in the absence of externally imposed concentration gradients. Motivated by the chemotactic behavior of their biological counterparts (such as sperm cells, neutrophils, macrophages, bacteria, etc.), which sense and respond to external chemical gradients, the current work aims at developing a theoretical framework to study the motion of a Janus particle in an externally imposed linear concentration gradient. The external gradient along with the self-generated concentration gradient determines the swimming velocity and orientation of the particle. The dominance of each of these effects is characterized by a non-dimensional activity number A (ratio of applied gradient to self-generated gradient). The surface of Janus particle is modeled as having a different activity and mobility coefficient on the two halves. Using the Lorentz reciprocal theorem, an analytical expression for the rotational and translational velocity is obtained. The analytical framework helps us divide the parameter space of surface activity and mobility into four regions where the particle exhibits different trajectories.