We study patchy models for colloidal systems to investigate the suppression of phase separation at low packing fraction Φ. We find that, on reducing the number of patches, the critical point moves towards zero packing fraction, providing the possibility to generate "empty liquids"[1]. Moreover, we find a new topology for the interplay of dynamical arrest with the phase separation region, that appears universal for various models of patchy systems[2]. Differently from spherically symmetric attraction where the glass line merges into the spinodal at large Φ allowing only for dynamic arrest as an arrested spinodal decomposition, for patchy interactions a whole region of optimal network formation opens up, where arrested states (gels) can be approached in equilibrium. We also discuss the deep analogy between patchy colloids and network-forming liquids (e.g. water, silica)[3].

[1] E. Bianchi et al., Phys. Rev. Lett. 97, 168301 (2006)

[2] E. Zaccarelli et al, J. Chem. Phys. 124, 124908 (2006)

[3] C. De Michele et al, J. Phys. Chem. B 110, 8064 (2006); J. Chem. Phys. 125, 204710 (2006).