Wall-induced phase transition controlled by layering freezing.

Molecular dynamics simulations of the Lennard-Jones model are used to study phase transitions at a smooth surface. Our motivation is the observation that the existence of an attractive wall facilitates crystallization. To investigate how this wall influences phase transitions, the strength of wall-particle interaction is varied in our studies. We find that the phase behavior depends on the strength parameter alpha, i.e., the ratio between wall-particle and the particle-particle attraction strength. Three critical values of the ratio, namely, alpha(p), alpha(w), and alpha(c), are used to define the qualitative nature of the phase behaviors at a smooth surface. Some interesting phenomena due to the increase of alpha are observed. First, a set of close-packed planes, i.e., {111} planes in fcc structures or {0001} planes in hcp structures, are "rotated" from intersecting to parallel to the wall when alpha = alpha(p); second, the layering phase transition close to the wall antecedes that of the bulk when alpha = alpha(w). Finally, the first-order phase transition in the first two layers is supplanted by a continuous phase transition when alpha = alpha(c), which to some extent can be treated as a quasi-two-dimensional process. We find that bulk freezing always discontinuously occurs through a first-order phase transition, and seems to be isolated from the freezing process occurring close to the attractive surfaces. Moreover, during the heating process, we observe minimal dependence at a strongly attractive surface.