In IC engines, deposition of fuel-spray on the engine walls is highly undesirable as it facilitates fuel-rich burning, incomplete combustion and soot formation. Examining a single drop behavior after impacting on a liquid film is useful in understanding and manipulating the impact outcome of bouncing or merging. During the impact, the merging of drop and film surfaces is rendered difficult by the trapped interfacial gas-layer, which as a result, plays an important role in deciding the outcome of the impact. However, the gas layer is not easily optically accessible due to its microscopic scale and the presence of curved interfaces. Here we report the spatial and temporal evolution of this critical gas layer thickness using high-speed color interferometry for drop impacting liquid film with various thicknesses. Compared to the impact on dry solid surfaces, we found that the presence of liquid film changes the gas layer dynamics drastically and non-monotonic behaviors were observed as the drop rebounds. The gas layer dynamics at different impact speeds and film thicknesses provide a guideline to promote bouncing thus preventing accumulation of fuel film on engine walls.
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