Critical Casimir interactions of colloids in micellar critical solutions.

We study the temperature-dependence of critical Casimir interactions in a critical micellar solution of the nonionic surfactant C12E5 dissolved in water. Experimentally, this is achieved with total internal reflection microscopy (TIRM) which measures the interaction between a single particle and a flat wall. For comparison, we also studied the pair interactions of a two dimensional layer of colloidal particles in the identical micellar system which yields good agreement with the TIRM results. Although, at the surfactant concentration considered here, the fluid forms a dynamical network of wormlike micelles whose structure is considerably more complex than that of simple critical molecular fluids, the temperature-dependence of the measured interactions is - for surface-to-surface distances above 160 nm - in excellent quantitative agreement with theory. Below 160 nm, deviations arise which we attribute to the adsorption of micelles to the interacting surfaces.