Eva Noya and Francesco Sciortino
Dipartimento di Fisica, Università di Roma La Sapienza, Roma, Italia
We revisit the assembly of colloidal tetrahedral patchy particles, a toy model for water. Previous studies have shown that crystallization depends more critically on patch width than on interaction range: particles with patches narrower than 40 degree crystallize readily and those with wide patches form disordered glass states. Here we find that the crystalline structure also depends on the patch width. Whereas particles with intermediate patches assemble into diamond (random stacking of cubic and hexagonal diamond layers), particles with narrow patches (with angular width about 20 degrees) crystallize quite frequently into clathrates. Clathrates formed form the melt exhibit sII structure, which is explained because sII has a lower free energy than sI due to lower deviations from the perfect tetrahedral arrangement. Curiously, particles with narrow patches show also a rather strong tendency to assemble into 100-particles clusters with icosahedral symmetry. The structure of this unit can be described as a central dodecahedral cage surrounded by twenty edge-sharing 15-particles cages in which two pentagonal rings are linked by five 6-member boat rings. These clusters are often joined by interpenetration forming disordered aggregates. Cluster free energy calculations suggest that the similarity between the tetrahedral angle on the particles and the internal angle of a regular pentagon is behind the preferential formation of pentamer rings in highly directional tetrahedral particles.