Salt dependence

Critical Casimir forces can be conveniently applied for tuning colloidal interactions. Both, the temperature of the solvent and adsorption preference of the confining geometries allow the control of strength and the sign of this type of interaction. When salt is added to a critical mixture, we observe that for symmetric boundary conditions attractive forces already arise several Kelvin below the critical temperature. This can be attributed to smaller particle wall distances, i.e. to larger critical Casimir forces. More interestingly, for antisymmetric boundary conditions a temperature dependent crossover from attractive towards repulsive interactions is observed. This unexpected behavior demonstrates the rich properties of colloidal interactions in critical mixtures.