Singular jets and entrapments from compound drop impact

Compound drops impacting on a solid surface play an important role in industrial applications, such as combustion, the food industry, and drug encapsulation. An intriguing phenomenon associated with this process is the occurrence of singular jets that are up to dozens of times faster than the impact velocity. These jets break into microdroplets, which can produce aerosols and affect the quality of printing technologies. The collapsing dynamics directly determine the cavity geometry, jet velocity, and entrapments. Here, we investigate experimentally and numerically the jetting process after a coaxial water-in-oil compound drop impacts onto a glass substrate with different releasing heights and volumetric ratios. After impact, the water core spreads and retracts, giving rise to a vertical jet initially made of oil. For certain values of the impacting velocity, high-speed and very thin jets are observed, the so-called singular jets. Depending on the volumetric ratio, one or two velocity peaks can be observed when varying the impact velocity, triggered by the contraction dynamics of a deep and cylindrical cavity. The self-similar time-evolution of the collapse for the first singularity regime follows a 1/2 power law in time, which can be derived from bubble pinch-off. In contrast, the collapse at the second peak follows a 2/3 power law, which can be accounted for by a balance between inertial and capillary forces.