Vapor bubble formation in liquids is important for many processes including boiling, removal of CO2 from the anode of methanol fuel cells, and various microfluidic applications. Phase change and two phase systems are difficult to understand and control for several reasons, including the multiple time and length scales involved and the difficulty in dealing with heterogeneous systems.
This work was concerned with heterogeneous bubble nucleation on nanometer length scales and nanosecond time scales. The focus was on the effects of surface chemistry and nanometer scale geometric defects. The main tool for these studies was non-equilibrium molecular dynamics simulations under constant temperature and constant heat flux conditions. Additionally, a mean field thermodynamic model based on the Redlich-Kwong equation of state was used to explain and extend the molecular dynamics results.
The molecular dynamics results at constant temperature showed that heterogeneous nucleation on an atomically smooth surface was always more favorable than homogeneous nucleation and that weaker surface-fluid interactions increased the nucleation rate compared with stronger interactions. Increasing the strength of surface-fluid interactions induced more ordering in the fluid near the solid surface causing nucleation to occur at the solid surface for weak interactions and above the surface for stronger interactions. In the cases where nucleation occurred above the surface, the nucleation rate did not decrease much with increasing interaction strength due to the similar nucleation environment.
For nucleation on indented surfaces, constant temperature molecular dynamics showed that the nucleation rate increased by two orders of magnitude compared to a flat surface for an indentation that was large compared to the critical size. At constant heat flux, nucleation was even more favored in the large indentation due to concentrated heating of the fluid, especially with weak solid-fluid interactions.
The mean field model explained the location of nucleation on flat surfaces and why small indentations have no effect on nucleation. Weak surface interactions created a region of tension near the surface and stronger surface interactions created a region of high pressure. The minimum pressure for weak interactions was associated with the indentation only for large enough indentations.