Self-diffusiophoresis of Janus particles in near-critical mixtures
Résumé
We theoretically study the self-propulsion of a laser-heated Janus particle in a near critical water-lutidine 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 at the front, whereas a charged hydrophilic cap leads to backward motion, in agreement with experiment. Both vp and β show non-monotonic dependencies on the heating power, and may even change sign. The variation of β is expected to strongly affect the collective behavior of dense squirmer systems. PACS numbers 05.70.Ln; 66.10.C-; 82.70.-Dd PACS numbers: In recent years, artificial microswimmers have been realized by Janus particles which move along the concentration or temperature gradients generated by their own chemical or thermal activity [1–5]. Oriented autonomous motion has been achieved throug dynamical feedback [6] or rectification in a periodically structured channel [7], opening applications such as targeted transport and pumping of passive particles. In dense systems, active Janus particles aggregate in dynamical clusters [8, 9]. This observation was related to short-range hydro-dynamic effects [10–12], in terms of the squirmer model originally developed for the motility of bacteria. Self-propulsion mechanisms have generally a strong diffusio-phoretic component [13, 14]; in the case of ionic molecular solutes, self-generated electric fields and ion effects may contribute to the motion [15, 16]. Diffusiophoresis was first rationalized by Derjaguin et al. [17], when observing that wax particles dispersed in a non-uniform glucose solution, migrate toward lower sugar concentration. Because of its unfavorable interaction with wax (u > 0), sugar is depleted in the boundary layer, the adsorption parameter Γ = ∞ 0
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