Dynamics of Laser-Induced Bubbles Inside and Outside Water-Agarose Gel Boundaries

The growth and collapse of a laser-induced bubble inside and outside water-agarose boundaries were observed with a high-speed camera. Agarose gels with Young's modulus E = 2.3 kPa, 7.1 kPa, and 13 kPa were used as tissue-mimicking phantoms. The effects of Young's modulus of agarose and the dimensionless stand-off distance γ (negative γ means that the bubble is produced inside the agarose gel) on the dynamics of laser-induced bubbles (i.e., bubble oscillation time and bubble migration) were investigated. It was shown that the oscillation time of a bubble became longer with an increase in γ. When a bubble was produced in the agarose gel, the oscillation time became shorter with an increase in Young's modulus. It was also shown that the bubble migrated toward the water region and penetrated the water-agarose gel boundary when it was produced in the gel and close to the boundary; the larger Young's modulus of agarose became, the larger the bubble migration became. When a bubble grew and collapsed across the water-agarose gel boundary, the bubble part occupied in the agarose collapsed earlier than in the water because the pressure gradient around the former part differed from that around the latter. This pressure gradient around the bubble resulted in the acceleration of the bubble migration toward the water region.