Nonmonotonic response of drop impacting on liquid film: Mechanism and scaling

Drop impacting on a liquid film with a finite thickness is omnipresent in nature and plays a critical role in numerous industrial processes. The impact can result in either bouncing or merging, which is mainly controlled by the impact inertia of the drop and film thickness. Although it is known that impact with inertia beyond a critical value on a thick film promotes merging through the breakage of the interfacial gas layer, here we demonstrate that for an impact inertia less than that critical value, increasing the film thickness leads to a nonmonotonic transition from merging to bouncing to merging and finally to bouncing again. For the first time, two different merging mechanisms are identified and the scaling laws of the nonmonotonic transitions are developed. These results provide important insights into the role of the film thickness in the impact dynamics, which is critical for optimizing operating conditions for spray or ink-jet systems among others.