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We present a refractive-index-matched colloidal system that allows direct observation of critical Casimir induced aggregation with a confocal microscope. We show that in this system, in which van der Waals forces are negligible, a simple competition between repulsive screened Coulomb and attractive critical Casimir forces can account quantitatively for the reversible aggregation. Above the temperature T(a), the critical Casimir force drives aggregation of the particles into fractal clusters, while below T(a), the electrostatic repulsion between the particles breaks up the clusters, and the particles resuspend by thermal diffusion. The aggregation is observed in a remarkably wide temperature range of as much as 15 degrees. We derive a simple expression for the particle pair potential that accounts quantitatively for the temperature-dependent aggregation and aggregate breakup.

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We report fluid-fluid and fluid-solid phase transitions of charge-stabilized polystyrene particles suspended in a binary liquid mixture of 3-methylpyridine and water. These thermally reversible phase transitions occur in the homogeneous phase of the binary liquid mixture below the coexistence temperature of the two liquids. Close density matching of the particles and the solvent allows us to follow the phase behavior until complete coexistence of macroscopic phases with temperature as the control parameter. We use small angle x-ray scattering to characterize these phases as colloidal gas, liquid, fcc crystal, and glass.

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