A Molecular Dynamics Study On The Nucleation And Growth Of Gas

Crystal nucleation and growth have drawn considerable attention of scientists in fields of polymer chemistry, condensed matter physics, and materials science. Their studies mainly focused on the efficient controlling. Nucleation is a fast rare event, and even occurs at some extreme conditions. These then, to some extent, limit studies using experimental methods. Fortunately, these limitations do not bound computer simulations. In this paper, we investigated homogeneous and heterogeneous nucleation of noble gas using molecular dynamics.For polymorphic crystals, a defect-induced model has been introduced to elucidate their mechanism of formation. Cooling the systems, we found that the system almost always evolves into the polymorphic crystal with either fivefold-symmetric stacking faults or single-direction stacking faults. The detailed analysis reveals that such an evolution depends on the configuration of fcc/hcp concomitance in the nucleation stage. Through calculating the formation energies of the defective critical nuclei, we find that the polymorphic crystals seem to be determined by their defective critical nuclei, in which the relatively lower formation energy ensures the preponderance of the fivefold-symmetric cluster.We then investigate phase transitions at a smooth surface.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 a, i.e., the ratio between wall-particle and the particle-particle attraction strength. Three critical values of the ratio, namely,αp, αw, and αc, are used to define the qualitative nature of the phase behaviors at a smooth surface. Some interesting phenomena due to the increase of a 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 α=αp; second, the layering phase transition close to the wall antecedes that of the bulk when α=αw. Finally, the first-order phase transition in the first two layers is supplanted by a continuous phase transition when α=αc, which to some extent can be treated as a quasi-two-dimensional process. In addition, a commensurate substrate with triangular pattern is considered to investigate the phase behavior. For the moderately attractive substrates, an intermediate hexatic phase between liquid and crystal is detected in the first two layers where the hexatic-solid freezing process is continuous while, counterintuitively, the liquid-hexatic process is of first order. Moreover, we observe that liquid-hexatic and hexatic-solid transitions shift towards higher temperatures with the attraction strength increasing. By contrast, the liquid-hexatic transition shifts faster than the hexatic-solid process, significantly widening the temperature range of the hexatic phase.