Formation of Surface Nanobubbles and the Universality of Their Contact Angles: A Molecular Dynamics Approach

Abstract

We study surface nanobubbles using molecular dynamics simulation of ternary (gas, liquid, solid) systems of Lennard-Jones fluids. They form for sufficiently low gas solubility in the liquid, i.e., for large relative gas concentration. For strong enough gas-solid attraction, the surface nanobubble is sitting on a gas layer, which forms in between the liquid and the solid. This gas layer is the reason for the universality of the contact angle, which we calculate from the microscopic parameters. Under the present equilibrium conditions the nanobubbles dissolve within less of a microsecond, consistent with the view that the experimentally found nanobubbles are stabilized by a nonequilibrium mechanism.When liquid comes into contact with a solid, nanoscopic gaseous bubbles can form at the interface: surface nanobubbles [1][2][3]. These bubbles were discovered about 15 years ago, after Parker et al. predicted their existence to explain the long-ranged attraction between hydrophobic surfaces in water [4]. Many Atomic Force Microscopy (AFM) and spectroscopy measurements have since then confirmed the existence of spherical cap-shaped, gaseous bubbles at the liquid-solid interface.Various open questions remain about surface nanobubbles, and in this Letter we will address three crucial ones: (i) How do surface nanobubbles form? This question is difficult to answer by experimental means, since the formation process is too fast to be observed by AFM.(ii) A second question regards the contact angle of surface nanobubbles which is found to disagree with Young's law: all recorded nanobubbles have a much lower gas-side contact angle than expected, and seem to be universal within 20 degrees. (iii) Finally, AFM showed that surface nanobubbles can be stable for hours or even days, whereas the pressure inside these bubbles due to their small radius of curvature (R c ∼ 100 nm) would be several atmospheres due to the Laplace pressure: ∆p = 2γ/R c , with γ the liquid-vapour surface tension. A simple calculation then shows that surface nanobubbles should dissolve within microseconds, which is 9 to 10 orders of magnitudes off with respect to the experimental data.In this paper, we use Molecular Dynamics simulations (MD) to study surface nanobubbles in simple fluids. Using MD, we are able to answer questions (i) and (ii), and provide important information with respect to question (iii). MD are well-suited for nanobubbles, because the temporal resolution is of order fs, and since all atom's motions are resolved, the spatial resolution is intrinsically high enough to resolve nanobubbles. This atomistic model allows to study microscopic details that are inaccessible by experimental means and standard continuum mechanics. Figure 1 shows how surface nanobubbles form in a typical simulation of a liquid containing gas. The gas will homogeneously nucleate to form a bubble, which subsequently attaches to the wall. We will analyze the nucleation process in detail and quantify how the contact angle of the bubble changes upon varying gas solubility. T...

Thermodynamics of Surface Nanobubbles

“…63 The gas enrichment layer alters the solid-fluid interfacial tensions. In the Koch-Amirfazli-Elliott framework, 63 a force resisting contact line motion is added, as shown in equations (25) and (26). This resistance force would empirically account for factors such as pinning that cause advancing and receding contact angles to be different from the equilibrium contact angle.…”

“…This explanation depends on the existence of the interfacial gas layer. Although there is some experimental support [40][41][42] and some simulation support 26,[43][44][45] for the existence of this gas enrichment layer, its existence is not universally accepted,…”

“…63 Next, we can obtain the value for ‫ܨ‬ ோ in the case of macroscopic contact angle measurement on the same hydrophobic surface as was used in the nanobubble experiments (but without the conditions of solvent exchange, water supersaturation and therefore a gas enrichment layer being present). For any smooth surface, an equation analogous to (26) can be subtracted from an equation analogous to (25) to yield: 63…”

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

“…They determined that bubble coalescence is associated with line tension in the pinned three-phase contact line. The accumulation of gas molecules on the structured substrates can be attributed to many factors, such as gas reduced diffusion perpendicular to the surface [22], wall-fluid interaction parameters [23], a depletion layer at the water-solid interface [22], contact line pinning [21] and hydrogenbond networks. In some cases, a higher concentration of gas molecules can cause gas bubbles to protrude into the region of flow [24].…”

Collapse of Surface Nanobubbles

“…2(e)]. Since the nanoscopic contact angle of the surface nanobubble θ (water side) is much larger than the equilibrium contact angle [15,35] θ eq , the interfacial tensions γ, γ LS , and γ SV at the liquid-solid-vapor CL of the nanobubble are not balanced [ Fig. 2(e)].…”