Self-diffusiophoresis of Janus particles in near-critical mixtures

We theoretically study the self-propulsion of a laser-heated Janus particle in a near critical waterlutidine mixture, and relate its velocity vp and squirmer parameter β to the wetting properties of its two hemispheres. For non-ionic surface forces, the particle moves the active cap ahead, except at strong driving, where a more complex behavior occurs if one of the hemispheres attracts water and the other lutidine. Both vp and β show non-monotonic dependencies on the heating power, and may even change sign. Including the drift of the counterions due to the composition gradient, we find that a charged cap with lutidine affinity drives the particle the cap ahead, and with water affinity in the opposite direction, in agreement with experimental findings. PACS numbers 05.70.Ln; 66.10.C-; 82.70.-Dd