Laser-driven droplet deformation at low Weber numbers

We investigate droplet deformation following laser-pulse impact at low Weber numbers (We ~ 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 We_d = f(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 We_d < 5, from (ii) breakup for 5 < We_d < 60 and (iii) sheet formation for We_d > 60.