Abstract
Gaseous domains formed on solid–liquid interface have attracted scientists’ attentions in recent 2 decades, and the existence of interfacial nanobubble (INB) has been basically confirmed. However, an overall understanding on INB is still lacking. This research studied the influence of surface chemical heterogeneity on the morphology of INB by molecular dynamics simulations technique. The results showed that the gaseous domains could not nucleate on the hydrophilic substrate, while only dense gas layer (DGL) could be observed from the time-averaged density map for homogeneously hydrophobic substrate due to the random moving of INB. If there was a hydrophobic patch on the hydrophilic surface, INB could form on the hydrophobic patch with contact line being pinned at the boundary of the patch. In this case, the contact angle (gas-side) increased with the gas oversaturation degree and decreased with surface hydrophobicity of the patch. For the case that there existed a more hydrophobic patch/site on the hydrophobic surface, the INB could have moved randomly along the hydrophobic surface, but its receding contact line was pinned by the more hydrophobic patch/site. Hence, the INB could only move in the vicinity of this pinning patch/site, so that an INB profile instead of a DGL formed due to the pinning effect, and the apparent contact angle of the INB is significantly lower than the actual one. Throughout this study, the apparent INB we observed from experiments may be different from its instantaneous state and is significantly affected by surface heterogeneity.
Highlights
Interfacial nanobubble (INB) was first proposed by Parker et al (1994) in 1994 due to the steps and the long-range attractive force in the force curve between two approaching hydrophobic surfaces
When we inspected the trajectory of simulation for SM surface, we found that the interfacial nanobubble (INB) moved along the solid–liquid interface randomly all the time
The following conclusions were obtained: (1) For smoothly homogeneous ideal-surface, the gaseous domain could not nucleate on the hydrophilic surface, and only dense gas layer (DGL) forms on strong hydrophobic surface
Summary
Interfacial nanobubble (INB) was first proposed by Parker et al (1994) in 1994 due to the steps and the long-range attractive force in the force curve between two approaching hydrophobic surfaces. Until 2000, the first image of solid-water interfacial nanobubble was independently obtained through tapping mode of the atomic force microscopy by Lou et al (2000) and Ishida et al (2000). Research on INBs have been extensively focused on due to their potential applications in many fields including froth flotation, protein adsorption, drag reduction, catalysis, and electrolysis (Lohse and Zhang, 2015). INBs are usually referred to as gas aggregates that are confined in a spherical cap with height less than 100 nm and contact line diameter less than 1 μm (Alheshibri et al, 2016). According to Young–Laplace equation, the internal pressure of INB could be described as Surface Heterogeneity on Nanobubble Morphology.
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