Impact dynamics of a viscous drop containing a particle

When a particle is attached under a liquid drop by surface tension, it forms a solid–liquid compound drop. We investigate experimentally the impact dynamics of this compound drop onto a solid surface. After impact, the particle rebounds from the solid surface and rises through the drop. The particle can either remain in the deposited liquid at low impact velocities or separate from the drop above a critical impact velocity. We demonstrate here that as the liquid viscosity increases, this separation threshold transitions from a capillary threshold, characterized by a critical particle Weber number, to a viscous threshold, captured by a critical particle Stokes number. However, the particle can still separate from the drop below this viscous limit if the particle is shifted away from the axis of symmetry before the impact of the compound drop. This shifting is observed experimentally at large falling heights, where the particle is destabilized by the air drag. In addition, we show that the shifting of the particle can also induce an inclination in the vertical liquid jetting, with a tangent of its angle proportional to the shifting distance of the particle. Finally, we confirm the focusing mechanism responsible for this liquid jetting by combining the observations of two synchronized cameras, from side view and bottom view.