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.

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...

Colloidal suspensions are characterized by a variety of microscopic interactions, which generate unconventional phase diagrams encompassing fluid, gel and glassy states and offer the possibility to study new phase and/or state transitions. Among these, glass-glass transitions are rare to be found, especially at ambient conditions. Here, through a combination of dilution experiments, X-ray photon correlation spectroscopy, small angle X-ray scattering, rheological measurements and Monte Carlo simulations, we provide evidence of a spontaneous glass-glass transition in a colloidal clay. Two different glassy states are distinguished with evolving waiting time: a first one, dominated by long-range screened Coulombic repulsion (Wigner glass) and a second one, stabilized by orientational attractions (Disconnected House of Cards glass), occurring after a much longer time. These findings may have implications for heterogeneously charged systems out-of-equilibrium and for applications where a fine control of the local order and/or long term stability of the amorphous materials are required.

The aging of a charged colloidal system has been studied by small-angle x-ray scattering, in the exchanged momentum range Q=0.03-5 nm(-1) , and by dynamic light scattering, at different clay concentrations (C(w)=0.6-2.8%) . The static structure factor S(Q) has been determined as a function of both aging time and concentration. This is the direct experimental evidence of the existence and evolution with aging time of two different arrested states in a single system simply obtained only by changing its volume fraction: an inhomogeneous state is reached at low concentrations, while a homogeneous one is found at high concentrations.

An unexpected dichotomic long time aging behaviour is observed in a glassy colloidal clay suspension investigated by X-ray photon correlation spectroscopy and dynamic light scattering. In the long time aging regime the intensity autocorrelations are non-exponential, following the Kohlrausch-Williams-Watts functional form with an exponent beta(Q). We show that for spontaneously aged samples a stretched behaviour (beta(Q) < 1) is always found. Surprisingly a compressed exponent (beta(Q) > 1) appears only when the system is rejuvenated by application of a shear field. In both cases the relaxation times scale as Q(-1). These observations shed light on the origin of compressed exponential behaviour and help in classifying previous results in the literature on anomalous dynamics