Control of the acoustic waves generated by intense laser filamentation in water

Bubble dynamics and atomization of acoustically levitated diesel and biodiesel droplets using femtosecond laser pulses

This study focuses on the bubble dynamics and associated breakup of individual droplets of diesel and biodiesel under the influence of femtosecond laser pulses. The bubble dynamics were examined by suspending the droplets in the air through an acoustically levitated setup. The laser pulse energies ranged from 25 to 1050 µJ, and droplet diameters varied between 0.25 and 1.5 mm. High-speed shadowgraphy was employed to examine the influence of femtosecond laser intensity and multiple laser pulses on various spatial-temporal parameters. Four distinct sequences of regimes have been identified, depending on early and late times: bubble creation by individual laser pulses, coalescence, bubble rupture and expansion, and droplet fragmentation. At all laser intensities, early-time dynamics showed only bubble generation, while specifically at higher intensities, late-time dynamics revealed droplet breaking. The droplet breakup is further categorized into three mechanisms: steady sheet collapse, unstable sheet breakup, and catastrophic breakup, all following a well-known ligament and secondary breakup process. The study reveals that laser pulses with high repetition rates and moderate laser energy were the optimal choice for precise bubble control and cutting.

Characterization of nanosecond laser-induced underwater filamentation using acoustic measurements

This investigation aims to understand the temporal and spectral characteristics of underwater acoustic impulses due to ∼10n s laser-induced filamentation at various incident laser energies. In addition to the experimental study, finite element analysis (FEA) has been used in conjunction with experimentally acquired data to simulate and visualize acoustic impulse propagation and interaction across the water-rigid boundary. In the time domain, the increase in the underwater filament length is measured by the peak-to-peak (P _k-P _k) overpressures as a function of input laser energies. The P _k-P _k overpressures are maximum around the focal plane and are symmetrical on either side. This variation in the P _k-P _k overpressure is traced to characterize the spatial extent of the underwater filament. The arrival time of underwater acoustic impulse varies within the 8-9 µs for input laser energies. It was observed that with increasing laser beam power, the conversion efficiency of the acoustic impulse decreases, and the underwater filament length increases. In the frequency domain, with increasing input laser energy, the peak frequency of the underwater acoustic impulse shifts toward the lower frequencies. Using FEA, the short-time Fourier transform spectrogram simultaneously visualizes the arrival time and instantaneous frequency of the simulated acoustic impulse. The simulated acoustic impulse produced a single instantaneous peak frequency of around 76 kHz. Upon reflection from the water-rigid boundary, we observed that the reflected signal also has a single instantaneous peak frequency of around 76 kHz, while higher-frequency components are completely dissipated. Our results are promising for the development of remote laser-based acoustic generation and sensing applications.

Controlling bubble generation by femtosecond laser-induced filamentation

Femtosecond laser-induced optical breakdown in liquids results in filamentation, which involves the formation and collapse of bubbles. In the present work, we elucidate spatio-temporal evolution, interaction, and dynamics of the filamentation-induced bubbles in a liquid pool as a function of a broad spectrum of laser pulse energies (∼1 to 800 µJ), liquid media (water, ethanol, and glycerol), and the number of laser pulses. Filament attributes such as length and diameter have been demarcated and accurately measured by employing multiple laser pulses and were observed to have a logarithmic dependence on laser energy, irrespective of the medium. The size distribution of persisting microbubbles is controlled by varying the pulse energy and the number of pulses. Our experimental results reveal that introducing consecutive pulses leads to strong interaction and coalescence of the pulsating bubbles via Bjerknes force due to laser-induced acoustic field generation. The successive pulses also influence the population density and size distribution of the micro-bubbles. We also explore the size, shape, and agglomeration of bubbles near the focal region by controlling the laser energy for different liquids. The insights from this work on filamentation-induced bubble dynamics can be of importance in diverse applications such as surface cleaning, fluid mixing and emulsification, and biomedical engineering.