Underwater bubble collapse on a ridge-patterned structure

Abstract

This experimental study reports the collapse of an underwater bubble near a patterned structure with ridges and grooves. When a bubble is generated by a spark above a ridge, the entire bubble collapses toward the structure after its full expansion, or it is split into two smaller bubbles because of a radial jet induced by bubble contraction. These distinct collapse modes are dependent on the surface geometry of the structure and determined by the contracting speed of a bubble part inside the cross section of an adjacent groove. For a bubble that collapses in a groove, water flows induced from the tops of adjacent ridges collide with each other in the middle of the groove cross section, and this collision occurs if the effective width of the groove is small enough. For the bubble-splitting radial jet mode on the ridge and the collision mode in the groove, some energy of the bubble is lost during its contraction and, accordingly, the strength of the re-entrant jet toward the surface is weakened. Thus, these modes may be effective for reducing erosion on the structure surface, which is supported by our simple experiment for damage assessment.