Abstract :
Molecular dynamics simulations are performed for a binary liquid mixture (α + β) to measure, among others, the collective velocity auto-correlation function Zm(t) and the time correlation function of the concentration fluctuations Fm(t). Both α-α and β-β pairs of molecules interact through the same Lenard-Jones potential, but α-β pairs interact only through the repulsive part of the potential. From the results for the relaxation time of Fm(t), we estimate the mutual diffusion coefficients. All measurements probing the individual molecular motions indicate that the molecules execute randomly diffusing motions both at T> TC and T < TC. The measurements probing their collective motions indicate otherwise. At T> TC, the results are as expected from those of the individual motions. At T < TC, the results indicate cooperative behavior which is not evident in the individual motions. Most importantly, we observe an oscillatory mode of collective motions which is analogous to the optical phonons in two-component crystals.
