Formation, dissolution and properties of surface nanobubbles

Abstract

Surface nanobubbles are stable gaseous phases in liquids that form on solid substrates. While their existence has been confirmed, there are many open questions related to their formation and dissolution processes along with their structures and properties, which are difficult to investigate experimentally. To address these issues, we carried out molecular dynamics simulations based on atomistic force fields for systems comprised of water, air (N and O), and a Highly Oriented Pyrolytic Graphite (HOPG) substrate. Our results provide insights into the formation/dissolution mechanisms of nanobubbles and estimates for their density, contact angle, and surface tension. We found that the formation of nanobubbles is driven by an initial nucleation process of air molecules and the subsequent coalescence of the formed air clusters. The clusters form favorably on the substrate, which provides an enhanced stability to the clusters. In contrast, nanobubbles formed in the bulk either move randomly to the substrate and spread or move to the water-air surface and pop immediately. Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension smaller than that of an equivalent system under atmospheric conditions, and contact angles larger than those in the equivalent nanodroplet case. We anticipate that this study will provide useful insights into the physics of nanobubbles and will stimulate further research in the field by using all-atom simulations.

Surface nanobubbles: Theory, simulation, and experiment. A review

“…The density of nanobubbles has been estimated by means of all-atom MD simulation. The obtained value was 409 kg/m 3 [68], which is of the order of the density of liquids, in agreement with recent experiments [76]. Moreover, all-atom MD simulation has found that the surface tension in the case of NBs is about 50 mN/m, a value smaller than the water-air interface tension at atmospheric conditions [68].…”

“…Liu and Zhang proposed a mechanism for the three-phase contact-line pinning to obtain stable NBs, which result from the intrinsic nanoscale physical roughness or chemical heterogeneity of the substrate [67]. By using classical DFT, it has been shown that NBs are in thermodynamic metastable states [67], in agreement with MD simulation [68]. This theoretical assumption is consistent with nucleation theory and can predict relationships between the contact angle and size of the NBs, as well as the chemical potential [67].…”

“…On the contrary, molecular-level simulations have indicated that NBs have shorter lifetimes (i.e. 100 ns) due to the limited diffusion stemming from the limited system sizes, in contrast to the long lifetimes of experimental NBs [52,68]. While contact-line pinning is important for the stability of single surface NBs, it also plays an important role in suppressing the Ostwald ripening process between neighbouring NBs [156], which could result in the growing of small NBs (large curvature) and the shrinking of large NBs (small curvature).…”

“…Moreover, the contact angle in the case of nanodroplets is size and model dependent [78]. In addition to the contact-angle mismatch between nanodroplets and NBs, the all-atom MD simulation has found that the surface tension in NBs is smaller than the corresponding surface tension of a water-air interface under atmospheric conditions [68]. A similar model has recently focused on the mechanical stability of surface NBs by Dockar et al [79].…”

“…; (C) NB formation from N 2 and O 2 atoms on an HOPG substrate immersed in water from all-atom MD simulation. Reprinted with permission from Ref [68]…”