Optical investigation of the interaction of an automotive spray and thin films by utilization of a high-pressure spin coater

Abstract

Although the interaction of automotive sprays with thin films is of high technical relevance for IC engine applications, fundamental knowledge about underlying physical mechanisms is still limited. This work presents a systematic study of the influence of the film’s initial thickness—homogeneously spread over a flat wall before the initial spray impingement—on film surface structures and thickness after the interaction. For this purpose, interferometric film thickness measurements and complementary high-speed visualizations are used. By gradually increasing the initial film thickness on a micrometer scale, a shift from a regime of liquid deposition (increasing film thickness with respect to initial film thickness) to a regime of liquid removal (decreasing film thickness with respect to initial film thickness) is observed at the stagnation zone of the impinging spray. This transition is accompanied by the formation of radially propagating surface waves, transporting liquid away from the stagnation zone. Wavelengths and amplitudes of the surface waves are increased with increasing initial film thickness.