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An analysis of phoretic propulsion of Janus particles: Effects of surface activity and mobility distributions

Sourav Ganguly,Kiran Raj M

Published 2026 in The Physics of Fluids

ABSTRACT

The mapping from surface properties to phoretic slip is well established for autophoretic particles, yet the influence of realistic, graded surface profiles on propulsion and transport remains underexplored. Building on the phoretic–squirmer framework, we present a detailed parametric study of spherical Janus particles, examining six families of activity shapes together with three different mobility laws across surface coverage fractions from 10% to 90%. Our numerical analysis, benchmarked against closed-form solutions for step–cap activity with constant mobility, quantifies how these functional forms distinctly shape the solute concentration field, tangential slip velocity, squirmer coefficients B1 and B2, and the resulting swimming speed. By integrating these deterministic results into a stochastic active Brownian particle model, we further calculate the mean-squared displacement, effective diffusivity, and persistence length, thereby directly connecting microscopic surface design to macroscopic transport metrics. We demonstrate that exponential activity, combined with linear mobility, can enhance the propulsion speed at optimal coverage compared to the classical step-Janus configuration and that mobility gradients can shift the optimal coverage in a monotonically increasing manner. Apart from these, we also showed that linear activity, along with linear mobility, provides the longest ballistic regime, compared to constant and quadratic mobility, which specifies the largest diffusivities. These findings provide concrete design rules for synthetic micromotors aimed at targeted cargo transport and enhanced mixing in microfluidic devices, demonstrating how the interplay between activity gradients and mobility heterogeneity offers a powerful two-parameter strategy for tuning colloidal microswimmer performance beyond the standard symmetric Janus architecture.

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