Lattice Boltzmann Simulation of the Effects of Nanobubble Morphology and Surface Nanofeatures on Superheat-Driven Nucleation and Bubble Growth

Bubble nucleation associated with nucleate boiling at superheated surfaces has typically focused on surfaces with features on the order of microns and bubble embryos with comparable interface radii of curvature. For such surfaces the vapor embryo growth or collapse behavior is consistent with surface tension and wetting forces being confined to the contact line region at the surface. In a pure fluid saturating a superheated nanopororus layer, random density fluctuations can lead to the formation of nanoscale bubble embryos. Said fluctuations increase as the liquid is superheated and can lead to macroscopic nucleation. Entrapment of gas when nanostructured surfaces are flooded with liquid can also result in nanoscale bubble embryos in or near the porous layer. For highly wetted nanostructured surfaces, the fluid-to-surface attractive forces are strong over much of a nanobubble embryo, and the critical bubble size that results in spontaneous bubble growth is affected more strongly by surface forces. A Lattice Boltzmann model (LBM) is used to simulate the time evolution behavior of bubble embryos, with radii ranging from 5 to 15 nanometers, close to or within nanoscale interstitial spaces of a nanostructured surface. Single vapor nanobubbles are seeded in surrounding fluid with varying degrees of contact with solid surfaces to simulate smooth or nanostructured surfaces. The effects of varying adsorption coefficient (which dictates contact angle), varying bubble surface radius of curvature, mean distance of wall nanostructures from the embryo, and varying degrees of enclosure of the embryo by surrounding wall structures are explored. The simulation results indicate that the critical radius is largely impacted by the proximity of nanostructures, demonstrating how the fluid-surface forces affect the stability of a vapor embryo. The results suggest that the hydrophilic nature of the surfaces contributes to the suppression in the onset of nucleate boiling which is often seen in hydrophilic nanoporous layers. The implications of these results on convective and nucleate boiling at and within nanostructured surfaces are also discussed.