  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
selfassembled bulk materials with predefined properties. 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 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.
