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

Wed 711:10-14:00P5.139Accurate simulation study of dipolar hard spheres. Noevidence of gas-liquid criticalityLorenzo Rovigatti, 1 John Russo, 1 and Francesco Sciortino 11 Universita’ di Roma La Sapienza, Piazzale Aldo Moro 2, 00138, Rome, ItalyThe existence of a ”gas-liquid” critical point in dipolar hard spheres (DHS) and the associatedvapour-liquid phase transition has been debated for several decades, following the pioneeringwork of De Gennes and Pincus [1]. Contradictory results have been reported, some suggestinga critical point around T = 0.15 − 0.16 [2,3] (in units of σ3 kT), other suggesting the absenceµ 2of any critical phenomenon [4], preempted by the formation of a network of long dipolar chainsinterconnected by network defects. We report [5] very accurate calculations of the distributionof density fluctuations, via grand-canonical successive umbrella sampling, incorporating efficientMonte Carlo moves, for temperatures well below the expected values of critical temperatures. Weshow that no bimodality is present in the density fluctuations, excluding the possibility of a phaseseparation between a dilute gas of chain ends and a high-density liquid of chain branching points[6], at least down to T = 0.125. Analysis of the equilibrium properties of the DHS fluid at lowT shows a huge increase in the number of rings and, consequently, a decrease in the number ofchain ends. At very low T the system percolates even at small reduced densities (ρ ∼ 0.01) andthe particles that are not in the percolating cluster mainly form rings. This strongly supports ascenario where no vapour-liquid phase separation is present [4].[1] P. G. De Gennes, P. A. Pincus, Phys. Kondens. Mater. , 11, 189 (1970).[2] Y. V. Kalyuzhnyi, I. A. Protsykevytch, and P. T. Cummings, EPL, 80, 56002 (2007).[3] P. J. Camp, J. C. Shelley, and G. N. Patey Phys. Rev. Lett. , 84, 115 (2000).[4] G. Ganzenmüller, G. N. Patey, and P. J. Camp Mol. Phys. 107, 403-413 (2009).[5] L. Rovigatti, J. Russo and F. Sciortino, in preparation.[6] T. Tlusty and S. A. Safran, Science 290, 1328 (2000).139