  We report an experimental and numerical investigation of the fragmentation
mechanisms of micrometer-sized metal droplet irradiated by ultrashort laser
pulses. The results of the experiment show that the fast one-side heating of
such a droplet may lead to either symmetric or asymmetric expansion followed by
different fragmentation scenarios. To unveil the underlying processes leading
to fragmentation we perform simulation of liquid-tin droplet expansion produced
by the initial conditions similar to those in experiment using the smoothed
particle hydrodynamics (SPH) method. Simulation demonstrates that a thin heated
surface layer generates a ultrashort shock wave propagating from the frontal
side to rear side of the droplet. Convergence of such shock wave followed by a
rarefaction tale to the droplet center results in the cavitation of material
inside the central region by the strong tensile stress. Reflection of the shock
wave from the rear side of droplet produces another region of highly stretched
material where the spallation may occur producing a thin spallation layer
moving with a velocity higher than expansion of the central shell after
cavitation. It is shown both experimentally and numerically that the threshold
laser intensity necessary for the spallation is higher than the threshold
required to induce cavitation in the central region of droplet. Thus, the
regime of asymmetrical expansion is realized if the laser intensity exceeds the
spallation threshold. The transverse and longitudinal expansion velocities
obtained in SPH simulations of different regimes of expansion are agreed well
with our experimental data.
