Direct observation of critical adsorption on colloidal particles.

Despite the fundamental role adsorbed solvent layers play in generating critical Casimir forces between colloidal particles, the structure of these layers has yet to be directly determined. Using small-angle neutron scattering, we have measured critical adsorption on the surface of small spherical silica particles suspended in a binary mixture of lutidine and water. The surface concentration profile and excess adsorption Γ were studied as functions of temperature at the critical concentration and three off-critical concentrations. We are able to differentiate three distinct contributions to the excess adsorption including the intrinsic shape of the concentration profile. The adsorption associated with the profile shape is found to increase monotonically with increasing 2,6-lutidine concentration and to decrease with increasing temperature, this later observation is consistent with expectations for curvature induced corrections to planar adsorption and leads to Γ∼(T-Tc)/Tc (-0.52), where Tc is the critical temperature. This scaling relation corresponds to a stronger divergence than found in the planar case, but a substantially weaker divergence than found previously.