Molasses Tail in Dense Hard Core Fluids

The long slow decaying potential part of the shear-stress autocorrelation function has been called the ``molasses tail'' to differentiate it from the hydrodynamic origin of the long time tail in the velocity autocorrelation function and to emphasize its relation to the highly viscous glassy state [1]. Some twenty years ago, the molasses tail in dense liquids near the solid-fluid transition point was speculated to be due to transient crystal nuclei formation [2].This slow decaying process of the OACF and its decomposition (pair, triplet, and quadruplet) is a key factor in understanding the onset of the glass transition. With additional computer power, we are now investigating the origin of the molasses tail using a modern fast algorithm based on event-driven Molecular Dynamics (MD) simulation.We confirm the non-algebraic decay (stretched exponential) at intermediate times corresponding to the existence of various cluster sizes a solid cluster at high densities. The decay in dense systems thus consists of a three stage relaxation process, which are the kinetic regime, the molasses regime and the diffusional power regime[3]. [1] B. J. Alder, in Molecular Dynamics Simulation of Statistical-mechanical Systems, G. Ciccotti and W. G. Hoover, eds.(North-Holland, Amsterdam, 1986) 66. [2] A. J. C. Ladd, and B. J. Alder, J. Stat. Phys. 57, 473 (1989). [3] M. Isobe and B. J. Alder, Mol. Phys., 107, 609 (2009).