Drop deformation by laser-pulse impact

TL;DR

This paper investigates how laser pulses impact liquid drops, causing deformation and fragmentation, by developing a two-stage model that links pulse shape to the resulting drop dynamics and shape.

Contribution

It introduces a combined analytical and numerical model that predicts drop deformation based on laser pulse shape and properties, linking energy partition to deformation behavior.

Findings

Tightly focused pulses produce thin, curved sheets with maximum expansion.

Uniform illumination results in smaller, flat, symmetric sheets.

Model accurately matches experimental observations.

Abstract

A free-falling absorbing liquid drop hit by a nanosecond laser-pulse experiences a strong recoil-pressure kick. As a consequence, the drop propels forward and deforms into a thin sheet which eventually fragments. We study how the drop deformation depends on the pulse shape and drop properties. We first derive the velocity field inside the drop on the timescale of the pressure pulse, when the drop is still spherical. This yields the kinetic-energy partition inside the drop, which precisely measures the deformation rate with respect to the propulsion rate, before surface tension comes into play. On the timescale where surface tension is important the drop has evolved into a thin sheet. Its expansion dynamics is described with a slender-slope model, which uses the impulsive energy-partition as an initial condition. Completed with boundary integral simulations, this two-stage model explains the entire drop dynamics and its dependance on the pulse shape: for a given propulsion, a tightly focused pulse results in a thin curved sheet which maximizes the lateral expansion, while a uniform illumination yields a smaller expansion but a flat symmetric sheet, in good agreement with experimental observations.