Why and when merging surface nanobubbles jump

Abstract

Gas bubble accumulation on substrates reduces the efficiency of many physicochemical processes, such as water electrolysis. For microbubbles, where buoyancy is negligible, coalescence-induced jumping driven by the release of surface energy provides an efficient pathway for their early detachment. At the nanoscale, however, gas compressibility breaks volume conservation during coalescence, suppressing surface energy release and seemingly disabling this detachment route. Using molecular dynamics simulations, continuum numerical simulations, and theoretical analysis, we show that surface nanobubbles with sufficiently large contact angles can nevertheless detach after coalescence. In this regime, detachment is powered by the release of pressure energy associated with nanobubble volume expansion. This finding thus establishes a unified driving mechanism for coalescence-induced bubble detachment across all length scales.

Figures (4)

• Figure 1: (a) Sketch of the coalescence-induced nanobubble detachment. The nanobubbles (dashed cap) before coalescence are identical for simplicity and have a gas-side contact angle $\theta$ and a radius of curvature $R_1$, and the thermodynamic state $(P_1,V_1, N_1,T_1)$. The merged nanobubble (solid sphere) after coalescence has a radius of $R_2$ and the thermodynamic state $(P_2,V_2,N_2,T_2)$. (b) A perspective view of the initial configuration of nanobubble coalescence on the hydrophilic substrate in MD simulations. The system's condition is maintained at $T=300$ K and $P_{\infty}=1$ atm.
• Figure 2: (a) Coalescence process of surface nanobubbles (NBs) in MD simulations, characterized by five typical states: I, initial contact; II, top liquid bridge flattening after expansion; III, maximum NB volume; IV, NB departure from the substrate; V, equilibrated NB. (b) Coalescence process of free NBs in MD simulations, characterized by four typical states (no departure): I, initial contact; II, liquid bridges flattening after expansion; III, maximum NB volume; IV, equilibrated NB. Notably, the flattening of liquid bridges corresponds to the minimum of surface area. (c) Surface NB's volume and area changes, normalized by their initial values. (d) Free NB's normalized volume and area changes. (e) Instant surface NB's center-of-mass velocity $u_{com}$ measured from MD. (f) Energy release (nondimensional) during surface NBs' coalescence in MD. $E_p$ is the pressure energy and $E_s$ is the surface energy.
• Figure 3: Energy release as a function of bubble radii, nondimensionalized by the total surface energy $8\pi\gamma R^2$. Solid lines are for free NBs or surface NBs with $\theta=180^{\circ}$. Dashed lines are for surface NBs with $\theta=150^{\circ}$.
• Figure 4: (a) Pressure energy $E_p$ as a function of contact angles with a fixed radius of curvature and the effects of viscous dissipation energy $E_{d}$. The MD snapshots given for three different initial contact angles $\theta$ show whether NBs can jump after coalescence. (b) Jumping distance $H$ (nondimensionalized by NB radii) as a function of initial NB radii, obtained by direct numerical simulations (DNS) of compressible NB coalescence. The inset pictures are simulation snapshots for 100 nm NBs with $\theta=180^{\circ}$, and $t^*=t/\sqrt{\rho_lR^3/\gamma}$.