Response of a millimeter-sized opaque drop to tightly focused nanosecond laser pulse

Abstract

We experimentally study the blast of a millimeter-sized drop of water dyed red subjected to a tightly focused nanosecond 532 nm laser pulse. The red water drop is opaque to the green laser wavelength, but is transparent to the red illumination, which allows the detailed visualization of the phenomena inside the drop. The laser induced plasma or localized boiling at the drop front surface leads to a splash crown originating from the blast center. The various stages of this physical process are analyzed based on four characteristic time scales with different focal point positions and power densities. The initial blast on the drop front surface sends spherical shock waves into the drop. The spherical drop surface focuses the reflected rarefaction waves to the rear side of the drop, inducing cavitation bubbles as well as the ejections of liquid jets on the rear end. Various laser energy densities lead to different splash-crown-modes, which in turn affect the response of the opaque drop at the capillary time scale. The temporal evolution of the splash crown waist diameter follows a power law with respect to time, resembling characteristics of craters from mechanical impacts. Through the scaling law of the splash crown growth, we highlight the similarities among different splash processes that are triggered by various methods of point energy deposition.