Influence of the wettability dividing line on bubble growth and detachment behavior in a von Kármán swirling flow field

The contact-angle (CA) hysteresis generated by the difference between the advancing and receding CAs of a droplet on an inclined surface generates an adhesion force that can hold the droplet on the surface. Similarly, the CA hysteresis generated by a wettability dividing line (WDL) between the superhydrophobic and superhydrophilic regions can improve the anti-shear stability of bubbles in water, with a maximum range of more than 150° for the difference between the advancing and receding CAs. Based on this mechanism, the influence of a WDL on the growth and detachment behavior of bubbles at different Reynolds numbers (Re) in a von Kármán swirling flow field was experimentally studied. The results show that the WDL significantly improves the anti-shear stability of bubbles when compared to uniform superhydrophobic and hydrophilic surfaces. Furthermore, the detachment volume and maximum contact width decrease with increasing Re, while the tilt angle and asymmetry ratio increase significantly. The fluctuation amplitudes of these parameters gradually increase with rising Re. Based on variations in these parameters, bubbles' growth behavior can be classified into three modes: stabilization, transition, and oscillation. Additionally, the constraint mechanism of a WDL on a bubble was analyzed based on variations in the CA hysteresis generated by the WDL and the maximum contact width. Finally, the detachment dynamics of bubbles in the three modes were analyzed based on a force–balance model.