Bubble formation via multidrop impacts

Gas bubbles are often generated when droplets impact a liquid-air interface. For the impact of single droplets, a critical impact velocity must be exceeded for air to be entrained in the form of bubbles. Here we establish that bubbles can be generated at much lower velocities provided that two or more drops impact the liquid-air interface within a sufficiently short time interval. Using high-speed imaging, we show that bubbles are entrained when a drop lands in the impact crater of a previous drop. We quantify the critical crater depth formed upon impact and the necessary time interval between drop impacts for bubble entrainment to occur. For 1 mm diameter water drops falling at 1 m/s, the critical separation time is approximately 5 ms. This critical time is consistent with a scaling analysis of the time required for an impact crater to close by capillarity.