(398b) Phoretic Motion in Active Matter

A new continuum perspective for phoretic motion is developed that is applicable for any shaped particle in 'microstructured' fluids such as a suspension of solute or bath particles. Using the reciprocal theorem for Stokes flow it is shown that the local osmotic pressure of the solute adjacent to the phoretic particle generates a thrust force (via a 'slip' velocity) which is balanced by the hydrodynamic drag such that there is no net force on the body. For a suspension of passive Brownian bath particles this perspective recovers the classical result for the phoretic velocity owing to an imposed concentration gradient. In a bath of active particles which self-propel with characteristic speed U₀ for a time Ï„ and then change direction randomly, taking a step of size l = U₀Ï„, at high activity the phoretic velocity is U^{act ∼} - U₀ l ∇φ_b , where φ_b is a measure of the 'volume' fraction of the active bath particles. The phoretic velocity is independent of the size of the phoretic particle and of the viscosity of the suspending fluid.