Control of the shape of bubble growth on underwater substrates with different sizes of superhydrophobic circles

Abstract

Inspired by the everyday experience of changing the shape of a blown-up balloon by imposing a constraint, a method to control the shape of underwater bubbles is proposed by tangential constraint forces generated by the wettability difference (WD), and two bubble growth modes are distinguished based on the tangential constraint force strength and the minimum apparent contact angle (CA) of the bubble after the WD constraint. First, the critical growth shape of the bubble with a combined shape of a vertical cylinder and hemispherical top is identified, and its corresponding critical contact radius RCritical = 2.7 mm is solved by the Young–Laplace equation. Then, the effects of the radii of the superhydrophobic circle (SBC) on the bubble growth shapes are studied experimentally. The result shows that as the SBC radius decreases, the minimum apparent CA of the bubble decreases, and the minimum tangential constraint forces increase. Therefore, the bubble growth mode changes from the bell mode (with a minimum apparent CA greater than 90°) with a weaker constraint to the Ω mode (with a minimum apparent CA less than 90°) with a stronger constraint, and the bubble growth shape tends toward spherical from a flattened sphere. The maximum bubble trapping rate, Laplace pressure difference at the apex and bottom of the bubble, the aspect ratio, and the bubble filling ratio also increase as the SBC radius deceases. Furthermore, our results suggest that the proper WD-patterned arrays on underwater substrates can enhance their application efficiency, and the size of SBCS # R4 is probably the best choice in all cases.