Laser filament shattering of distilled water and seawater droplets

Laser filaments are produced by high-intensity ultrashort pulse lasers and consist of a low density (~10¹⁶ cm^-3) plasma channel and a clamped laser intensity (~10¹³ W/cm²) over tens of centimeters of propagation. Filament interaction with water droplets or aerosols is relevant to applications in machining, medicine, and as a tool for enabling free-space optical communication through fog and clouds. The existing body of work has extensively explored the interaction of a filament with a single aerosol and with a cloud of aerosols. Aerosol size is typically within 30 nm to 100 μm, while raindrops are much larger, with diameters on the scale of ~1 mm. Filaments produced by a near infrared femtosecond laser have high intensity cores comparable to the upper bound of these aerosol sizes (~150 μm full-width at half maximum), while still an order of magnitude smaller than a droplet. Therefore, it can be expected that the interaction of a filament with aerosols would be fundamentally different than a raindrop. In this work, we investigate laser filament interaction with millimeter scale distilled water and seawater droplets using high speed shadowgraphy techniques. Droplet evolution, including ejected mist and droplets, formation of cavitation bubbles, and droplet deformation, is analyzed as a function of laser energy and droplet composition. This work builds on the knowledge of filament-aerosol interactions to better inform how filamenting laser beams interact with large droplets like raindrops or seawater drops.