The relevance of anisotropic interactions in colloidal systems has recently emerged in the context of the rational design of new soft materials. Patchy colloids of different shapes, patterns and functionalities are considered the new building blocks of a bottom-up approach toward the realization of self-assembled bulk materials with predefined properties. The ability to tune the interaction anisotropy will make it possible to recreate molecular structures at the nano- and micro-scales (a case with tremendous technological applications), as well as to generate new unconventional phases, both ordered and disordered. Recent theoretical studies suggest that the phase diagram of patchy colloids can be significantly altered by limiting the particle coordination number (that is, valence). New concepts such as empty liquids—liquid states with vanishing density—and equilibrium gels—arrested networks of bonded particles, which do not require an underlying phase separation to form—have been formulated. Yet no experimental evidence of these predictions has been provided. Here we report the first observation of empty liquids and equilibrium gels in a complex colloidal clay, and support the experimental findings with numerical simulations.

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 (C(w)=0.003-0.031). At variance with previous determination, indicating a stable liquid phase for C(w)

The phase diagram of a charged colloidal system (Laponite) has been investigated by dynamic light scattering in a previously unexplored range of salt and clay concentrations. Specifically the clay weight and salt molar concentrations have been varied in the ranges C w = 0.004 ÷ 0.025, C s = (1 × 10 −3 ÷ 5 × 10 −3 ) M respectively. As in the case of free salt water samples (C s ≃ 1 × 10 −4 M) an aging dynamics towards two different arrested phases is found in the whole examined C w and C s range. Moreover a transition between these two different regimes is found for each investigated salt concentration. It is clear from these measurements that a revision of the phase diagram is necessary and a new "transition" line between two different arrested states is drawn.

Using experiments, theory and simulations, we show that the arrested state observed in a colloidal clay at intermediate concentrations is stabilized by the screened Coulomb repulsion (Wigner glass). Dilution experiments allow us to distinguish this high-concentration disconnected state, which melts upon addition of water, from a low-concentration gel state, which does not melt. Theoretical modelling and simulations reproduce the measured Small Angle X-Ray Scattering static structure factors and confirm the long-range electrostatic nature of the arrested structure. These findings are attributed to the different timescales controlling the competing attractive and repulsive interactions.PACS numbers: 82.70. Dd, 64.70.kj, 64.70.pv Dynamical arrest in soft colloidal systems has recently become the subject of an intense research activity. The fine tuning of control parameters opens the possibility to tailor the macroscopic properties of the resulting nonergodic states. Several mechanisms of dynamical arrest have been identified. Building on the knowledge on the hard spheres glass [1], it has become recently clear that when both attractive and repulsive terms are present in the interaction potential, a re-entrant liquid-glass line, surrounded by two distinct glasses, has been predicted and experimentally observed in short-ranged attractive colloids at high concentrations [2]. A rich phenomenology also takes place at low concentrations: here gelation occurs, which may result from different routes [3]. Interesting scenarios arise when, in addition to a short-ranged attraction, particles have a residual electrostatic charge which builds up a long-range repulsion in the effective potential. In this case, particles can form equilibrium clusters [4], which provide the building blocks of arrest [5]. Recent works have shown that both Wigner glasses [6], intended as arrested states formed by disconnected particles or clusters and stabilized by the electrostatic repulsion, and equilibrium gels, which occur at larger packing fractions when the clusters branch into a percolating network, can form under these conditions [7,8].To investigate the formation of multiple arrested states, colloidal clays [9, 10] have emerged as suitable candidates. The anisotropy of the particles, combined with the presence of attractive and repulsive terms in the interactions, makes the phase diagram of such colloidal systems very complex. Among these, Laponite suspensions have been widely studied not only for their appealing industrial applications [11] but also for their interesting experimental/theoretical properties [12][13][14][15][16][17][18][19][20]. In particular, Laponite displays a non-trivial aging dynamics [15] and (at least) two final arrested states, which are obtained by a simple increase of Laponite volume fraction from low (C w < 2.0%) to moderate (C w ≥ 2.0%) values, at fixed salt concentration C s = 10 −4 M [15, 17].More recently, the static properties of these two states have been investigated in detail [18], showing that they are...

At variance with previous determinations, recent investigations on water suspension of a synthetic clay (Laponite) have shown the presence of an arrested phase also at very low clay concentrations (down to Cw = 0.003). This surprising behaviour has been studied in a wide clay concentration range. As a result the existence of two different routes towards the arrested phase, applying to low and high Laponite concentrations, has been found. We can speculate that at high clay concentration the system would form a Wigner glass whose elementary units are single Laponite platelets, as already indicated in previous works. At low clay concentrations, in contrast, the Wigner glass is supposed to be composed of clusters of Laponite platelets; in this case the clusters would be stabilized by the competition of long-range electrostatic repulsion and short-range attractive interactions. A similar behaviour has been recently found in a simulation work (Sciortino et al 2003 Preprint cond-mat/0312161).