The glass transition of soft colloids
We use a combination of light and X-ray scattering, rheology, and numerical simulations to provide a comprehensive picture of the glassy dynamics in soft colloids. We observe a rapid growth of the structural relaxation time with volume fraction at equilibrium, which is nearly independent of particle softness, in stark contrast with the commonly-accepted view that soft colloids form a distinct class of glass-formers. Softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this non-equilibrium regime, the relaxation time depends surprisingly weakly on packing fraction and time correlation functions change qualitatively. We attribute this novel regime to the `anomalous’ decrease of local order observed in dense packings of soft colloids. The generality of our results is confirmed by experiments performed on colloids of different nature, and by computer simulations covering a broad range of particle softnesses.