Dynamics of Bubble Pairs in Water Induced by Focused Nanosecond Laser Pulse

Objective Focused laser-induced cavitation in liquid is crucial in numerous applications, e. g., targeted cell lysis, microfluidic operations (such as switching, pumping, and mixing), and perforation of cell membranes. Depending on the focusing conditions and laser pulse energy, single or multiple bubble formations may occur, which may be accompanied by bubble coalescence, high-speed jet formation, ring vortex generation, and multiple shock wave emission. Owing to its promising application prospect on microsurgery, micropumping, and tissue cutting, laser-induced multiple bubbles and their interactions have been studied extensively. It has been confirmed that the dynamics of multiple bubbles are strongly related to the relative bubble positions as well as the time and size difference between bubbles. For example, by adjusting these parameters, the strength and direction of the emerging liquid jets can be controlled. Shock wave and rebound bubbles generated after cavitation bubble collapse are susceptible to their asymmetrical collapse. Without a doubt, the mutual interaction of bubbles causes the asymmetrical oscillation process of bubbles. However, to the best of our knowledge, the influence of multiple bubble interactions on shock wave emission and rebound bubble process has not been studied yet. Therefore, in this study, two bubbles with similar sizes were generated using a single nanosecond laser pulse to investigate the influence of relative interval on multiple bubble dynamics, especially on collapse shock wave emission and rebound bubble generation. Methods A frequency-doubled Q-switched Nd: YAG laser was introduced to generate optical breakdown in water. The laser pulse was split into two parts using a variable beam splitter. Then, the split laser pulses were focused on water from different directions to generate bubble pairs. Three methods were introduced to measure the bubble pair dynamics: high-speed shadow-graph, optical scattering technique, and acoustic detection technique. It was easy to generate bubble pairs with variable relative interval by adjusting the incidence direction of laser pulses, focusing objective position, and pulse energy. First, the bubble pair dynamics with different relative intervals were discussed experimentally and compared with the Rayleigh-Plesset model. Second, the influences of the relative interval between bubble pairs on the collapse shock wave strength and rebound bubble oscillation period were investigated. In this part, a high-speed camera was replaced by an EMCCD to picture the plasmas generated during an optical breakdown, and the bubble size was calculated by its first oscillation period. Results and Discussion For a single bubble in free liquid, the maximum radius of the bubble linearly increases with the cube root of the pulse energy and its first oscillation period, respectively, (Fig. 3), which means that the bubble size can be calculated from the pulse energy or its first oscillation period. The oscillation process of bubble pairs is significantly influenced by its relative interval (gamma) (Fig. 4). For gamma = 1. 36, the bubble pairs oscillates spherically, without contacting each other before their first collapse, but both of their first oscillation periods significantly increases [Figs. 4(a) and (b)]. For gamma approximate to 0.49, the two bubbles begin to coalesce during the early stage of expansion, and their shapes deform. The evolution of the equivalent radius of the coalesced bubble fits well with the Rayleigh-Plesset simulation [Figs. 4(c) and (d)]. For gamma approximate to 0.18, the coalesced bubble oscillates spherically again during its first period [Figs. 4(e) and (f)], which is similar to the single bubble case. Then, we experimentally examine the influence of relative interval of bubble pairs on the collapse shock wave emission and rebound bubble generation. The results revealed that the first oscillation period was uninfluenced by the relative interval (gamma<0. 75) (Table 1). However, with the increase in relative interval, the collapse shock wave strength first reduced, then increased, and then reduced again; however, an opposite trend was observed in the rebound bubble oscillation period (Fig. 6), which meant that the relative interval affected the energy distribution between shock wave and rebound bubble. Owing to the multiple shock wave formation induced by the asymmetric collapse of the coalesced bubble, the evolution of shock wave strength and rebound bubble oscillation over relative interval were not synchronous. Conclusions In this study, we investigate the dynamics of laser-induced bubble pairs with variable relative interval. The bubble pair oscillation process significantly varies with relative interval. For a non-coalesced bubble pair, the oscillation is nearly spherically during the first period, but both of their oscillation processes are prolonged. For a coalesced bubble pair, the smaller the relative interval, the more spherical the bubble shape during its first period. The first oscillation period is longer than that of every single bubble and unaffected by the relative interval when it is less than 0. 75. Besides, the evolution of coalesced bubble could still be described by Rayleigh-Plesset model. However, the relative interval of bubble pairs significantly influences the collapse shock wave emission and rebound bubble generation after the collapse of the coalesced bubble. The findings of this study are expected to facilitate the applications of laser-induced bubbles in microfluidic operations, such as rapid mixing and cell sorting.