Investigating Interfacial Effects on Surface Nanobubbles without Pinning Using Molecular Dynamics Simulation

Abstract

We investigated how the stability of aqueous argon surface nanobubbles on hydrophobic surfaces depends on gas adsorption, solid−gas interaction energy, and the bulk gas concentration using molecular dynamics simulation with the SPC/E water solvent. We observed stable surface nanobubbles without surface pinning sites for longer than 160 ns, contrary to previous findings using monoatomic Lennard-Jones solvent. In addition, the hydrophobicity of a substrate has an effect to reduce the requirement degree of oversaturation on water bulk. We found that the gas enrichment layer, gas adsorption monolayer on the hydrophobic substrate, and water hydrogen bonding near the interface are likely necessary conditions for nanobubble stability. We concluded that gas nanobubble stability does not necessarily require three-phase pinning sites.

Stability, Dynamics, and Tolerance to Undersaturation of Surface Nanobubbles

“…And what is the evidence that substrate hydrophobicity influences stability and dynamics? Molecular simulations have recently demonstrated that substrate hydrophobicity enhances nanobubbles' resilience to dissolution [25], while nucleation experiments with AFM show that the contact angles of nanobubbles increases with substrate hydrophobicity [26], as we predict [21].…”

“…But as the hydrophobicity increases, c à i =c sat → 0 asymptotically, at which limit gas molecules cannot be destabilized by any degree of degassing. Furthermore, the idea that tolerance develops from substrate hydrophobicity has recently been confirmed in molecular dynamics simulations [25]. Finally, the slow approach to c à i =c sat → 0 in the hydrophobic limit in Fig.…”

Quantifying interfacial tensions of surface nanobubbles: How far can Young's equation explain?

“…One is the overestimation of the gas density inside the nanobubbles in STXM observations caused by the existence of an ultradense gas molecule-adsorbed layer underneath them. The formation of the dense adsorbed layers of gas molecules has been reported not only by MD simulations 22,24,40 but also by AFM 40,50–52 and LPEM 53,54 experiments. Our analysis also found that even when the pressure P gas was reduced to 0.2 MPa corresponding to the nanobubbles with a footprint radius of 250 nm and a contact angle of 170°, the peak density inside the adsorbed layer was 37 and 241 kg m −3 for η = 1.0 and 1.5 (shown in ESI Fig.…”

“…The top graphene worked as a piston to attain the pressure in the liquid at the atmospheric pressure P 0 = 1 atm, as it has been successfully used in previous studies. 22,38,39 The Lennard-Jones (LJ) energy between the carbon atoms in the bottom graphite and N 2 molecules was changed by multiplying a coupling parameter η = 1.0 or 1.5 to investigate the influence of the solid–gas interaction. The details of the setup of these nanobubble simulations are described in the Methods section.…”

“…Chen et al carried out molecular dynamics (MD) simulations of steady argon surface nanobubbles without pinning sites and argued that the pinning was not necessarily needed for their stabilization. 22 Nag et al experimentally observed a free growth/shrinkage of surface nanobubbles for longer than 1000 seconds in a nano-confined water film using an in situ liquid phase electron microscopy (LPEM) technique, which indicated the existence of non-pinned surface nanobubbles. 23 Thus, it is still under debate whether the pinning is necessary to explain the unique properties of surface nanobubbles.…”