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.