Laser-driven droplet deformation at low Weber numbers

We investigate droplet deformation following laser-pulse impact at low Weber numbers ( ${\textit{We}}\sim 0.1{-}100$ ). Droplet dynamics can be characterised by two key parameters: the impact Weber number and the width, W , of the distribution of the impact force over the droplet surface. By varying laser-pulse energy, our experiments traverse a phase space comprising (i) droplet oscillation, (ii) breakup or (iii) sheet formation. Numerical simulations complement the experiments by determining the pressure width and by allowing We and W to be varied independently, despite their correlation in the experiments. A single phase diagram, integrating observations from both experiments and simulations, demonstrates that all phenomena can be explained by a single parameter: the deformation Weber number ${\textit{We}}_{{d}}=f({\textit{We}},{W})$ that is based on the initial radial expansion speed of the droplet, following impact. The resulting phase diagram separates (i) droplet oscillation for ${\textit{We}}_{{d}}\lt 5$ , from (ii) breakup for $5\lt {\textit{We}}_{{d}}\lt 60$ and (iii) sheet formation for ${\textit{We}}_{{d}}\gt 60$ .