(79b) Advective Symmetry Breaking in Phoretic Migration of a Colloidal Particle

The phoretic migration of a colloidal particle is animated by an imposed gradient of a scalar field: e.g. neutral solute gradients cause diffusiophoresis, and temperature gradients drive thermophoresis. It is usually assumed that the scalar field evolves solely via diffusion, owing to its large diffusivity and the modest colloidal particle velocity. (That is, the Peclet number for scalar-field transport is assumed to be zero). This leads to Morrison's celebrated result that the translational phoretic velocity of a colloid is independent of its size, shape, or orientation with respect to the imposed gradient (moreover, the colloid does not rotate).  However, intuitively, as the colloid moves it sets up a fluid flow that advects the same scalar field that instigated its motion. In this talk, using asymptotic expansions, we explore the first effects of advection on the phoretic motion of colloidal particles (i.e. at the experimentally relevant conditions of small but finite Peclet number). In particular, we show that advection leads to symmetry breaking in the phoretic motion of fore-aft asymmetric particles, wherein the particle velocity depends on the direction of the imposed gradient. Moreover, we demonstrate that advection drives phoretic rotation of non-spherical colloids, which ultimately align with the imposed gradient.