A large variety of engaging phenomena, ranging from crystallization in protein solutions to the formation of colloidal gels and glasses via depletion forces, stems from the occurrence of very short-ranged attractive forces. From depolarized light scattering measurements of equilibrium sedimentation profiles, we obtain an accurate description of the equation of state and of the phase diagram of colloids where depletion forces are tuned by the addition of a surfactant. For weak depletion, a colloidal fluid fully described by Baxter’s “sticky” hard sphere model coexists with ultradense colloidal crystals. For stronger attractive interactions, kinetically arrested looser gels form, showing an elastic modulus that scales as a power law of the local particle concentration.

• Received 17 May 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.098301