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Diffusiophoresis in a Near-critical Binary Fluid Mixture

We suppose that a rigid spherical particle is put into a binary fluid mixture with the critical composition in the homogeneous phase near the demixing critical point. A short-range interaction is assumed between each component and the particle surface, and one component is assumed to be attracted more than the other by the surface. The adsorption layer, where the preferred component is more concentrated, can be significantly thick owing to a large susceptibility. In this situation, an imposed composition gradient causes a particle motion, i.e., diffusiophoresis emerges from a mechanism not considered previously. We calculate how the mobility depends on the temperature and particle size.