8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.4Wed 711:10-14:00Phase separation and equilibrium gels in a colloidal clayRoberta Angelini 11 CNR-IPCF and Physics Department of University ”La Sapienza”, Piazzale aldo Moro 2,00185, Rome, ItalyThe relevance of anisotropic interactions in colloidal systems has recently emerged in the contextof the rational design of novel soft materials. Patchy colloids of different shapes, patterns andfunctionalities are considered the novel building blocks of a bottom-up approach toward therealization of self-assembled bulk materials with pre-defined properties. New concepts suchas empty liquids and equilibrium gels have been formulated. Yet no experimental evidenceof these predictions has been provided. Here we report the first observation of empty liquidsand equilibrium gels in a complex colloidal clay, and support the experimental findings withnumerical simulations. We investigate dilute suspensions of Laponite, a synthetic clay made ofnanometer-sized discotic platelets with inhomogeneous charge distribution and directional interactionsextending the observation time for low concentration samples to time-scales significantlylonger than those previously studied. We discover that, despite samples appear to be arrested onthe second timescale, a significant evolution takes place on the year timescale. Samples undergoan extremely slow, but clear phase separation process which terminates at a finite but very lowclay concentration, above which the samples remain in a homogeneous arrested state. Moreover,the slow aging dynamics peculiar of Laponite suspensions drive an arrest transition through avery slow rearrangement, so that equilibrium gels are formed. The observed features are similarto those predicted in simple models of patchy particles, suggesting that Laponite forms an emptyliquid at very low concentrations. These new phenomenologies have been observed by directvisual inspection and by Small Angle X-ray measurements performed for more than one year andthe experimental results have been confirmed by extensive numerical simulations with a primitivemodel of patchy Laponite discs [1].[1] B. Ruzicka et al. Nature Materials 10, 56 (2011)4