Effects of nozzle size and surface hydrophobicity on microbubble generation

Microbubble dynamics in distilled water were experimentally investigated during three stages: formation, expansion, and detachment, emphasizing the combined effects of nozzle size and surface hydrophobicity. To achieve superhydrophobicity, nozzles with radii of 5, 15, 25, and 50 μm were fabricated using laser precision machining technology and treated with trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane. High-speed, high-spatial-resolution imaging was used to precisely measure the contact angle, instantaneous volume, radii of curvatures and detachment time of the microbubbles. The experimentally measured data were used to calculate the forces acting on bubbles, including the Young–Laplace pressure, surface tension, buoyancy, momentum, and added mass force. The results show that superhydrophobic nozzles generate larger, more elongated bubbles with extended detachment time, whereas non-hydrophobic nozzles form smaller, spherical bubbles that detach faster. Furthermore, it was observed that increasing the nozzle size decreases the size of bubbles generated by superhydrophobic nozzles, while it increases the bubble size for non-hydrophobic nozzles. A force balance analysis confirmed that in the expansion stage, the pressure force on bubbles from non-hydrophobic nozzles drops, while the surface tension and momentum force remain constant, leading to faster bubble growth and detachment. In contrast, for bubbles from superhydrophobic nozzles, the pressure force decreases gradually, while the surface tension changes continuously, resulting in a prolonged attachment and the formation of larger bubbles.