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 water-lutidine mixture, and we relate its velocity v(p) and squirmer parameter beta to the wetting properties of its two hemispheres. For nonionic surface forces, the particle moves the active cap at the front, whereas a charged hydrophilic cap leads to backward motion, in agreement with the experiment. Both v(p) and beta show nonmonotonic dependencies on the heating power, and they may even change sign. The variation of beta is expected to strongly affect the collective behavior of dense squirmer systems.