Dynamics of laser-induced bubble pairs

The interaction of two laser-induced bubbles in bulk water is investigated. The strength and direction of the emerging liquid jets can be controlled by adjusting the relative bubble positions, the time difference between bubble generation, and the laser pulse energies determining the bubble sizes. Experimental and numerical studies are performed for millimetre-sized bubble pairs. Taking bubbles of equal energy, a maximum jet velocity is found for close anti-phase bubbles, i.e. when the second bubble is produced at the maximum volume of the first one and the bubble walls are almost touching and not merging. Under these conditions, one bubble produces a fast jet with a peak velocity of about 150 m s^-1 that reaches a distance into the surrounding liquid of at least three times the maximum bubble radius. Collapse of the other bubble results in a slow jet of large mass that rapidly converts into a ring vortex. Correspondingly, the interaction with adjacent structures is dominated either by localized jet impact or by shear stresses extending over a larger area. Furthermore, interactions between micrometre-sized bubble pairs are investigated numerically to understand and predict how the effects of the physical parameters on bubble dynamics would change when the bubbles become smaller. The results are discussed with respect to micropumping and opto-injection.