Direct Simulations of Homogeneous Bubble Nucleation: Agreement with CNT and no Local Hot Spots

TL;DR

This study uses large-scale molecular dynamics simulations to measure homogeneous bubble nucleation rates, finding they generally agree with classical nucleation theory and do not support the hot spots hypothesis.

Contribution

The paper provides the first large-scale MD simulations of bubble nucleation that show rates close to CNT predictions and refute the hot spots hypothesis.

Findings

Nucleation rates are within two orders of magnitude of CNT predictions.

No evidence of local hot spots preceding bubble formation.

Critical bubble sizes match CNT predictions across temperatures.

Abstract

We present results from direct, large-scale molecular dynamics (MD) simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations contain half a billion Lennard-Jones (LJ) atoms and cover up to 56 million time-steps. The unprecedented size of the simulated volumes allows us to resolve the nucleation and growth of many bubbles per run in simple direct micro-canonical (NVE) simulations while the ambient pressure and temperature remain almost perfectly constant. We find bubble nucleation rates which are lower than in most of the previous, smaller simulations. It is widely believed that classical nucleation theory (CNT) generally underestimates bubble nucleation rates by very large factors. However, our measured rates are within two orders of magnitude of CNT predictions - only at very low temperatures does CNT underestimate the nucleation rate significantly. Introducing a small, positive Tolman length leads to very good agreement at all temperatures, as found in our recent vapor-to-liquid nucleation simulations. The critical bubbles sizes derived with the nucleation theorem agree well with the CNT predictions at all temperatures. Local hot spots reported in the literature are not seen: Regions where a bubble nucleation events will occur are not above the average temperature, and no correlation of temperature fluctuations with subsequent bubble formation is seen.