Observation of empty liquids and equilibrium gels in a colloidal clay

The relevance of anisotropic interactions in colloidal systems has recently emerged in the context of the rational design of new soft materials1. Patchy colloids of different shapes, patterns and functionalities2 are considered the new building blocks of a bottom-up approach toward the realization of selfassembled bulk materials with predefined properties3-7. The ability to tune the interaction anisotropy will make it possible to recreate molecular structures at the nano- and microscales (a case with tremendous technological applications), as well as to generate new unconventional phases, both ordered and disordered. Recent theoretical studies8 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 liquids8--liquid states with vanishing density--and equilibrium gels8-10-- arrested networks of bonded particles, which do not require an underlying phase separation to form11--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.