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.