Influence of wall elasticity on growth and collapse of bubbles near a wall

The growth and collapse of a laser-induced bubble near an agarose gel were observed with a high-speed video camera in which 0.7% (Young's modulus E = 13 kPa), 1.5% (E = 55 kPa), 3.0% (E = 200 kPa), and 5.0% (E 570 kPa) agarose gels were used as tissue-mimicking phantoms. The effects of Young's modulus of agarose and the dimensionless bubble-boundary distance gamma on the dynamics of laser-induced bubbles, i.e., bubble oscillation time, bubble migration, bubble shape, jetting, and penetration of the liquid jet into the elastic wall were investigated. It was shown that as Young's modulus of the wall increased, the amount of migration of the bubble centroid toward the wall increased. The results also showed that as gamma decreased, the bubble shape in the late collapse stage changed from a spherical shape, a cone shape, and to a mushroom shape when E = 13, 55, and 200 kPa, while, when E = 570 kPa, it changed from a cone shape, a mushroom shape, and to a volcano shape: the threshold values of gamma, where the bubble shape changed from a cone shape to a mushroom shape and from mushroom shape to a volcanic shape, increased as E increased. These bubble shapes in the late collapse stage were determined from the bubble shapes at the maximum expansion. In the case of mushroom-shaped bubbles, the jet velocity was higher than volcano-shaped bubbles. The value of gamma at the onset of the penetration of the liquid jet into the elastic wall increased with an increase in E. The penetration area of bubbles into the elastic wall took the maximum value when the bubble shape in the late collapse stage changed from a mushroom shape to a volcano shape. The difference between the bubble behavior near the agarose wall and that near the PAA wall was also discussed.