Particle dynamics in colloidal suspensions above and below the glass-liquid re-entrance transition

We study colloidal particle dynamics of a model glass system using confocal and fluorescence microscopy as the sample evolves from a hard-sphere glass to a liquid with attractive interparticle interactions. The transition from hard-sphere glass to attractive liquid is induced by short-range depletion forces. The development of liquid-like structure is indicated by particle dynamics. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. As samples enter the attractive liquid region, particle speed during these motional events increases by about one order of magnitude, and the particles move more cooperatively. Interestingly, colloidal particles in the attractive liquid phase do not exhibit significantly larger displacements than particles in the hard-sphere glass. Introduction. – Theory, simulation, and experiment have demonstrated that a colloidal system can be driven from a hard-sphere glass to an attractive glass by increasing short-range attractions between colloidal particles [1–9]. In colloidal suspensions this effect is typically realized by adding nonadsorbing polymers to the colloidal suspension. Depletion forces [10–12], induced in this way, cause the particles to move closer to one another, and the system exhibits a transition from a hard-sphere glass to an attractive liquid [1–5]. Increasing the polymer concentration even further causes the system to enter an attractive glass phase [1–5]. Calculations and molecular dynamics simulations [6–9] suggest that reentrance to the glass phase is due to the existence of two qualitatively different glassy states. In hard-sphere colloidal suspensions the system enters a glass phase through a caging mechanism: as the volume fraction φ is increased, particles are increasingly trapped by their neighbors, until a critical volume fraction φg ∼ 0.58 is reached; then caging becomes effectively permanent, stopping long-range particle motion. In attractive glasses, the attractive part of the potential causes particles to move closer to one another and eventually binds them at contacts. In (a)E-mail: [email protected]