Phase separation in two-dimensional binary fluids: A molecular dynamics study

We present results of molecular dynamics computer simulation experiments of phase separation in a two-dimensional model binary fluid. For critical quenches the tubularlike domain structure grows at late times according to a power law with an exponent 1/2 which later crosses over to 2/3. This latter result has recently been questioned by Ossadnik et al. [Phys. Rev. Lett. 72, 2498 (1994)], and we present further evidence that the fast exponent is indeed present in the dynamical growth. That these exponents are hydrodynamic in origin is demonstrated by performing runs where the velocity field is disturbed at regularly spaced intervals of time. The perturbation consists of resetting the velocities of the particles to those characteristic of a Boltzmann distribution, which destroys the hydrodynamic tail of the velocity autocorrelation function. The resulting exponent changes to that predicted by experiments using stochastic techniques (Monte Carlo), i.e., 1/3. Also, we present results of experiments for off-critical quenches. The late-stage growth exponent for not too off-critical concentrations is close to 1/3, but we observe a continuous transition from this exponent to that characteristic of a critical quench.