Observation of empty liquids and equilibrium gels in a colloidal clay

The observation of empty liquids and equilibrium gels in a complex colloidal clay has been obtained through dilution experiments, small angle x ray scattering measurements and extensive numerical simulations. We investigate the behaviour of dilute suspensions of a synthetic clay, Laponite, for low concentrations, Cw <= 1%, to waiting times significantly longer than those previously studied. A significant evolution takes place on the year timescale and samples undergo a clear phase separation process which terminates at a finite clay concentration Cw = 1%, above which they remain in a homogeneous arrested state. Moreover, the slow aging dynamics peculiar of Laponite suspensions drive an arrest transition through a very slow rearrangement, so that equilibrium gels are formed. The observed features are similar to those predicted in simple models of patchy particles, suggesting that Laponite forms an (arrested) empty liquid at very low concentrations.