Study of Transient Nuclei near Freezing

The long slow decaying potential part of the shear-stress autocorrelation function (SACF) 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]. The molecular dynamics (MD) simulation on the long time tail of SACF investigated in the early 80s, which show that the amplitude of SACF was found to be orders of magnitude greater than predicted by MCT. Twenty years ago, Ladd and Alder have speculated that the long time tail of SACF is due to transient crystal nuclei formation [2]. They found that the potential part of the SACF and the angular orientational auto-correlation function (OACF) are identical in the long time limit and show non-algebraic decay in time. Since the evidence for nonalgebraic decay is slow structural relaxation around the peak of the structure factor rather than hydrodynamic flow, an attempt was made to understand by decomposing the OACFs into two-, three-, and four-body correlations, however, these correlation functions have not been obtained accurately due to the computer limitation at that time. Recently, we revisited this problem in a twoand threedimensional system consisting of elastic hard spheres near the solid-fluid transition point placed in a square box with periodic boundary conditions, using a modern fast algorithm based on eventdriven MD simulation [3]. We confirmed that there exist three regimes in the relaxation of the pair orientational autocorrelation function, namely the kinetic, molasses (stretched exponential), and diffusional power decay [4]. The cause of the stretched exponential relaxation in the molasses regime is considered to be due to the distribution of different life times of transient clusters of nuclei in dense fluid systems. To confirm the above speculation, the bond orientational order as an alternative to two-body correlation is calculated. Furthermore, the density dependence of both the molasses and diffusional power regimes are evaluated and the latter compares with theoretical predictions in three dimensions. The largest cluster at the freezing density of only a few sphere diameters in size persist for only about 30 picoseconds in the real liquids [5].