EAGER: Molecular and hybrid simulations of nanobubble stability

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

1256838 PI: Shell Surface nanobubbles remain among the most significant puzzles in interfacial science. These small, very flat bubbles form in water on hydrophobic surfaces, with widths 50-600 nm and heights 10-100 nm, and contain gases that were originally dissolved in the liquid. Classic predictions of bubble lifetimes suggest that they should last only for microseconds, owing largely to the significant predicted internal (Young-Laplace) pressure. Remarkably, however, nanobubbles are observed to 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 the unusual stability of nanobubbles remains lacking. The main goal of this project is to develop foundational simulation methodologies and models that can address fundamental, molecular aspects of nanobubble stability where experiments currently cannot. This work lies at the intersection of thermodynamics and continuum mechanics, and has two integrated aims. (1) The first is to quantify the relevance of thermodynamic contributions to stability that relate to the physics and unique properties of liquid water at hydrophobic interfaces. Advanced molecular simulation methods will be used to compute free energies of nanobubble formation, with a particular focus on the impact of dissolved gases. These calculations will be compared with bulk macroscopic arguments (e.g., based on bulk surface tensions and solubilities) to assess if they break down at the nanoscale. (2) The second aim is to understand how a dynamic, active-transport mechanism may underlie bubble stability. A recently-proposed mechanism suggests that there may be a recirculating flow of dissolved gases leaving the bubble apex and returning to it in the vicinity of contact line, and AFM experiments have now detected signatures of a jet of fluid above a nanobubble that strongly supports this picture. The ultimate goal of this work is to develop a detailed simulation picture of this recirculating gas transport mechanism consistent with experiments, critically assessing its viability as an explanation for stability. This part of the study will develop and adapt hybrid (molecular-continuum) simulation methods to the nanobubble problem that can span the large range of length scales expected for this mechanism. This study aims to addresses the important issue of nanobubble stability for the first time using detailed simulations and state-of-the-art molecular and hybrid techniques. It will provide new perspectives on fundamental interactions in water at hydrophobic interfaces. In turn, this will impact the understanding of longstanding issues in interfacial science such as flow slip at hydrophobic boundaries, attractive interactions between hydrophobic surfaces, and colloidal coagulation. Ultimately such knowledge will impact a range of exciting new and emerging technologies that rely on the ability of nanobubbles to radically modify the properties of solid surfaces, including anti-fouling and surface cleaning techniques, transport in microfluidic devices, delivery of medical therapeutics, and chemical processes involving biological or gas-liquid reactions, surface-induced crystallization, and catalysis. This study simultaneously aims to provide outstanding educational opportunities for students at multiple levels, including involvement of those from underrepresented groups. In particular, undergraduates will be actively incorporated in this work by involvement in outstanding professional development and research training programs at UCSB?s California Nanosystems Institute and Materials Research Lab (notably the RISE, SIMS, GRIP, and SABRE programs).