(658d) The Puzzling Stability of Nanobubbles: Theoretical and Simulation Insights (Invited Talk)

Surface nanobubbles remain among the most significant puzzles in interfacial science, and represent a basic challenge to understanding interactions at hydrophobic interfaces.  These small, flat bubbles form in water on hydrophobic surfaces, with widths 50-600 nm and heights 10-100 nm, and primarily contain gases that were originally dissolved in the liquid.  Classic predictions of bubble lifetimes based on diffusion arguments suggest that they should last only for microseconds, owing to the significant predicted internal pressure.  Remarkably, nanobubbles persist for at least days, some nine orders of magnitude longer than expected.  There have been intense efforts to understand this glaring discrepancy, and experiments have recently succeeded in producing detailed characterizations of nanobubble geometries and size distributions, and in delineating the response of the bubbles to changing conditions (dissolved gas concentration, temperature, salt, pH, etc.)  Despite these impressive achievements, a definitive theory that explains nanobubbles’ unusual stability remains lacking. 

We describe a variety of theoretical and molecular simulation studies aimed at understanding two distinct contributions to nanobubble stability.  The first is a thermodynamic contribution due to the behavior of liquid water at hydrophobic interfaces.  We use molecular simulations to compute free energies of nanobubble formation as a function of bubble size, shape, and composition.  When compared with bulk macroscopic arguments (e.g., using bulk surface tensions and solubilities), we find evidence of distinct and unexpected nanoscale contributions to stability.  The second focus is the contribution of transport processes within and around the bubble.  A dynamic equilibrium model suggests that an influx of gas in the vicinity of the bubble contact line balances the outflux of gas from the bubble apex. We find that this model captures many nontrivial observed behaviors, specifically, that stable nanobubbles exist in narrow temperature and dissolved gas concentration ranges, that there is a maximum and minimum possible bubble size, and that nanobubble radii decrease with temperature.