Nanobubble-Assisted Nanopatterning Reveals the Existence of Liquid Quasi-Two-Dimensional Foams Pinned to a...

This paper reports on our observation of a quasi-two-dimensional (quasi-2D) liquid nanofoam spontaneously appearing on a submersed solid surface. Unlike common liquid foams existing on top of the liquid, the quasi-2D liquid nanofoam is pinned to a water-immersed solid surface. The foam imaging was performed by a nanobubble imprint technique, which allows recording the positions of the surface nanobubbles by their imprints in a polystyrene film, as described in our previous papers [Tarábková et al. Langmuir 2014, 30, 14522; Tarábková et al., Langmuir 2016, 32, 11221]. Nanobubble imprints are then examined by ex situ atomic force microscopy. Besides randomly distributed nanoprotrusions corresponding to solitary nanobubbles, quasi-periodic arrangements of a tight cellular structure and more spaced round-shaped patterns, corresponding to "dry" and "wet" quasi-2D micro- and nanofoams, respectively, are identified. Although randomly spread solitary nanobubbles can occupy up to 30% of an immersed solid surface, their self-organization in a quasi-2D nanofoam leads to surface gas coverage reaching up to 80%, which implies significantly lowered surface wetting. Existence of a submersed quasi-2D nanofoam thus opens the novel question on the impact of dense surface nanobubble assemblies on heterogeneous processes at the solid-liquid interface.