Abstract
In this work, we experimentally studied bubble formation on the superhydrophobic surface (SHS) under a constant gas flow rate and at quasi-static regime. SHS with a radius R-SHS ranging from 4.2 to 19.0 mm was used. We observed two bubbling modes A and B, depending on R-SHS. In mode A for small R-SHS, contact line fixed at the rim of SHS, and contact angle (theta) initially reduced, then maintained as a constant, and finally increased. In mode B for large R-SHS, contact line continuously expanded, and theta slowly reduced. For both modes, during necking, contact line retracts, and theta was close to the equilibrium contact angle. Moreover, the pinch-off of bubble at the early stage was similar to the pinch-off of bubble from a nozzle and followed a power-law relation R-neck similar to tau(0.54), where R-neck is the minimum neck radius and tau is the time to detaching. Furthermore, we calculated the forces acting on the bubble and found a balance between one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force). Last, we found a waiting time for a finite volume to be detected for large R-SHS. The detached volume was well predicted by Tate volume, which was derived based on balance between buoyancy and surface tension and was a function of bubble base radius.