Colloid thermophoresis as a non-proportional response

Thermophoresis (particle motion driven by a temperature gradient) is envisioned here as the ensemble-averaged motion of a Brownian particle in a bathing medium of nonuniform temperature. The theory of Brownian motion pertinent to colloids is reviewed and adapted to the case of a nonuniform temperature field. The thermophoretic velocity of the Brownian particle is not proportional to the force undergone by the particle, but contains a significant additive correction pushing the particle toward regions of lower friction. The correction is independent of the precise nature of the force. The physicochemical nature of the force is briefly discussed.