Experimental study of bubble growth and detachment behavior under different constraint strengths in a von Kármán swirling flow

This work advances the systematic study of the mechanism by which a circular wettability dividing line (WDL) constrains the three-phase contact line of a growing bubble on a surface. This provides critical insights for improving the stability of air layers on superhydrophobic surfaces by discretization, which is essential for improving the application of superhydrophobic surfaces in drag reduction. The effects of two constraint parameters—the radius of the WDL and the size of the surface microstructures—on the growth and detachment behavior of bubbles in a von Kármán swirling flow were experimentally analyzed. The results show that as the WDL radius is increased from 2 to 8 mm, the additional constraint force it generates is weakened, leading to increases in the coefficient of variation of the bubble detachment volume, tilt angle, and asymmetry ratio, and a decrease in its maximum contact width. As the microstructure period is increased from 50 to 100 μm, the number of pinning points provided by the WDL for the three-phase contact line decreases, weakening the constraint force. Consequently, the coefficient of variation of the bubble detachment volume and the asymmetry ratio increase, while the tilt angle and maximum contact width decrease. Finally, as both the radius of the WDL and the size of the microstructures increase, the inertial force grows more rapidly than the buoyancy force at the moment of bubble detachment, resulting in a higher ratio of horizontal to vertical bubble displacement.