Nucleation is the formation of a tiny embryo of a new phase out of an existing metastable phase. This process is vitally important to many natural systems and industries, including fine chemicals, food, dyes, pharmaceuticals, nonlinear optics elements, and explosives. Despite its ubiquity and importance, the nucleation process is not well understood. Nucleation is extremely difficult to study experimentally because it is a stochastic process occurring on the nano-scale in both time and space. As such, theoretical investigations, as well as molecular simulation, are crucial to understanding nucleation at the molecular level.

Laser-induced nucleation Recent experiments have demonstrated that intense, nanosecond laser pulses can dramatically accelerate crystal nucleation from transparent solutions, a phenomenon termed nonphotochemical laser-induced nucleation (NPLIN). Previous work has proposed that this effect is due to the alignment of solute molecules in solution due to the electric field of the applied laser light, promoting crystalline order. We derive a new method of computing the diffusion coefficient along a reaction coordinate and apply this to previously proposed mechanisms for NPLIN. Our analysis shows that some current hypotheses about laser-induced nucleation mechanisms lead to a non-zero threshold laser pulse duration below which a laser pulse will not affect nucleation. We therefore propose experiments that might be used to test these hypotheses. We also note that the relationship that we derive has application not only to nucleation, but also to any system exhibiting diffusive crossing of a free energy barrier.

We have also used simulations of NPLIN to examine how an orientational bias in solution (such as that proposed by the "optical Kerr effect") affects nucleation with Monte Carlo simulations of a Potts lattice gas model. We examine this effect within both a classical, one-step nucleation framework as well as in the context of two-step nucleation. Our results indicate that an orientational bias can reduce the free energy barrier to nucleation within the one-step picture, as well as promoting the crystallization of precritical nuclei (the rate-determining step in the two-step picture). However, these effects are only present with field strengths that are much greater than those used in experiments. We conclude that the optical Kerr effect is far too weak to explain the acceleration of crystal nucleation seen in NPLIN experiments.

Experimentally, we demonstrate that laser pulses similar to those used in NPLIN experiments induce the nucleation of carbon dioxide bubbles from carbonated water. Enhanced alignment or attraction of CO2 molecules would not help facilitate bubble formation; thus previously proposed mechanisms for NPLIN cannot be at work. Additionally, in water that is co-supersaturated with argon and glycine, argon bubbles escaping from the water can induce crystal nucleation without a laser. The combined evidence suggests a possible unified theory for the mechanism of all laser-induced nucleation experiments: extremely small and transient bubbles may form in NPLIN experiments with unfocused lasers and catalyze crystal nucleation without being observed.

Methane hydrates nucleation Clathrate hydrates are ice-like inclusion compounds of hydrophobic guest molecules. Clathrate hydrates of natural gases ("gas hydrates") constitute the most abundant fossil fuel on the planet, and are thus an enormous potential resource of fossil energy. However, they also hinder the oil and natural gas industry by blocking pipelines and exacerbating oil well blowouts. They are also important to global climate change, gas transportation, and gas storage. The molecular mechanism by which these compounds form at conditions relevant to industry and nature remains a mystery. To understand the mechanism of methane hydrate nucleation from supersaturated aqueous solutions, we perform molecular dynamics (MD) simulations at controlled and realistic supersaturation. Utilizing direct MD results and classical nucleation theory (CNT), we find that critical nuclei are extremely large and that homogeneous nucleation rates are extremely slow. Our findings suggest that nucleation of methane hydrates at these realistic conditions cannot occur by a homogeneous mechanism.