Routes to gelation in a clay suspension

The gelation of water suspension of a synthetic clay (Laponite) has been studied by dynamic light scattering in a wide range of clay weight concentration (Cw = 0.003-0.031). At variance with previous determination, indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the gelation takes actually place in the whole examined Cw range. More importantly, we find that Cw* marks the transition between two different routes to gelation. We hypothesize that at low concentration Laponite suspension behaves as an attractive colloid and that the slowing down of the dynamics is attained by the formation of larger and larger clusters while at high concentration the basic units of the gel could be the Debye Huckel spheres associated to single Laponite plates.