  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.
