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

P5.32Wed 711:10-14:00Phase behavior of contact lens-like particles: purelyentropy driven competition between isotropic-nematicphase separation and cluster formationGiorgio Cinacchi 11 Department of Chemistry University of Pisa, Via Risorgimento 35, 56125, Pisa,ItalySuitable hard particle models are often sufficient to capture the basic features of the phase behaviorand properties of a variety of complex fluids. Onsager showed that a purely entropy drivenisotropic-nematic phase transition occurs in systems of hard rods, while later it was also demonstratedthat steric interactions suffice to give rise to smectic or columnar phases. In this communication,the novel phase behavior of hard partial spherical surfaces is reported, focusing onparticles close to the platelet limit, similar to contact lenses. They are basic models for bowlicmolecular and, especially, colloidal liquid crystals. Spherical caps of sufficiently large radius ofcurvature exhibit a transition from the isotropic to the nematic discotic phase. By reducing theradius further, however, the latter phase is destabilized. On increasing pressure, the nematic discoticis suppressed and a new phase emerges, characterized by the simultaneous aggregation ofthe centers of the parent spheres and the organization of the concave particles on the correspondingand interpenetrating spherical surfaces. Contact lens-like particles thus exhibit a competitionbetween a fluid-fluid phase transition and a clustering phenomenon, similar to what is observedin molecular systems forming micelles or colloidal suspensions forming cluster phases. For contactlens-like particles, however, this competition does not involve any energetic contribution: thephenomenology is remarkably purely entropy driven.32