  In many technical applications, but also in natural processes like ice
nucleation in clouds, crystallization proceeds in the presence of stresses and
flows, hence the importance to understand the crystallization mechanism in
simple situations. We employ molecular dynamics simulations to study the
crystallization kinetics of a nearly hard sphere liquid that is weakly sheared.
We demonstrate that shear flow both enhances and suppresses the crystallization
kinetics of hard spheres. The effect of shear depends on the quiescent
mechanism: suppression in the activated regime and enhancement in the
diffusion-limited regime for small strain rates. At higher strain rates
crystallization again becomes an activated process even at densities close to
the glass transition.
