Critical Casimir interactions of colloids in micellar Q1 critical solutions

Laurent Helden, Timo Knippenberg, Li Tian, Aubin Archambault, Felix Ginotb
and Clemens Bechinger

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.!divAbstract