  In this work, we present experiments and simulations on the nucleation and
successive dynamics of laser-induced bubbles inside liquid droplets in
free-fall motion, i.e. a case with a free boundary in all directions. The
droplets of a millimetric size have a nearly spherical shape by the moment the
bubble is nucleated. We have investigated the nucleation of secondary bubbles
induced by the rarefaction wave that is produced when the shock wave emitted by
the laser-induced plasma reflects at the drop surface. Interestingly,
three-dimensional clusters of cavitation bubbles are observed. Their shape is
compared with the negative pressure distribution computed with a CFD model and
allows us to estimate a cavitation threshold value. High-speed recordings of
the drop/bubble dynamics are complemented by the velocity and pressure fields
simulated for the same initial conditions. The effect of the proximity of a
curved free surface on the jetting dynamics of the bubbles was qualitatively
assessed by classifying the cavitation events using a non-dimensional stand-off
parameter which depends on the drop size, the bubble maximum radius and the
relative position of the bubble inside the drop. Additionally, we studied the
role of the drop's curvature by implementing a structural similarity algorithm
to compare cases with bubbles produced near a flat surface to the bubbles
inside the drop. This quantitative comparison method indicated the existence of
equivalent stand-off distances at which bubbles influenced by different
boundaries behave in a very similar way. The oscillation of the laser-induced
bubbles promote the onset of Rayleigh-Taylor and Rayleigh-Plateau
instabilities, observed on the drop's surface. This phenomenon was studied by
varying the ratio of the maximum radii of the bubble and the drop. The specific
mechanisms leading to the destabilisation of the droplet surface were
identified.
