Surface nanobubbles nucleated on rough hydrophilic steel exhibit topography-dependent shapes

On highly cleaned planar surfaces submerged in highly cleaned water, flat surface nanobubbles with an angle of attachment of ∼15^∘ are observed - never on engineering surfaces submerged in plain water, though here unidentified cavitation nuclei are always present and cause low tensile strength.

In the present study, surface nanobubbles are generated by standard experimental techniques on a polished steel surface, and we find that the shape and the angles of attachment of the bubbles are influenced by the local substrate topography. These observations align with the theory of non-adsorbed liquid zones, which explains a surface nanobubble as a bubble with a skin of contamination molecules, which bond along the bubble rim at a contact angle of ∼14.5^∘ to mirror hydrogen bonded water molecules on the clean solid surface. It couples the bubble shape with the solid surface structure. The curvature of the skin structure bends the mirror-hydrogen bonds, and it causes deviations from the solid liquid contact angle of 14.5^∘, dependent on the surface topography.

On engineering surfaces, contamination is widespread and has no rim, but gas molecules dissolved in the liquid support its detachment from concave solid surface structures. Therefore, solid surfaces appear more smooth in water than in air. The pattern of the detachments depends on the nature and topography of the solid surface, on the contamination, and on the gas saturation of the water, and it is decisive for the tensile strength of the liquid-solid system.