Enhancement of crystallization of hard spheres by gravity: Monte Carlo simulation

We present the effects of gravity on the growth of a hard-sphere (HS) crystal as determined by Monte Carlo simulations. HSs were confined between hard walls at the top (z = L-z) and the bottom (z = 0) of the system with a periodic boundary condition in the horizontal direction. After preparing a melt state as an initial state, the gravity was suddenly switched on. The values of the gravity were mg sigma/k(B)T(equivalent to g*) = 0.1,0.2,...,2.0, where m was the mass of a HS, g the acceleration of gravity, sigma the HS diameter, and k(B)T the temperature multiplied by Boltzmann's constant. We observed the enhancement of crystallization due to gravity up to g* = 0.7 and that the crystals of largest size were formed at g* = 0.7-0.9. On the other hand, for g* >= 1.0, the top position of the HS crystal that was grown from the bottom became lower with an increase in the gravity, where crystals with various axis directions coexisted. The polycrystallization is speculated to occur due to polynucleation. These results are qualitatively consistent with those of an experimental study of the centrifugal sedimentation of colloidal crystallization.