Drop impact on nonwetting nanoporous surfaces: formation of a novel air film and its influencing factors

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Drop impact on solid surfaces is a ubiquitous phenomenon in nature and technological processes. It is generally accepted that a thin air film is entrapped underneath impinging drops. The dynamics of such air film is crucial for the outcome of drop impact and affects the heat transfer efficiency and drag in technological processes. For smooth substrates, the dynamics of the thin air films underneath impinging drops have been studied in detail. However, on rough or nanoporous surfaces, a fundamental understanding of the stability and dynamics of the air film is still missing as a function of drop and substrate properties. In preliminary experiments on nonwetting nanoporous alumina surfaces, we observed a novel kind of air film underneath impinging drops under ambient conditions. We suppose that the air in the closed pores couples to the entrapped air between impinging drops and the substrate. This crosstalk contributes to the formation of the novel air film. The surface structure, the liquid properties and the surrounding conditions affect the formation and dynamics of the air film during the impact. This project focuses on the formation mechanism and dynamics of this novel air film on nanoporous surfaces, elucidating the effects of the influencing factors involved and resolving the contribution of the novel air film on the drop impact dynamics. We intend to use diverse nanoporous surfaces (with either open or closed pores) and vary systematically the pore diameter and pore length. We apply scanning electronic microscopy and atomic force microscopy to characterize the surface structure. Side-view and top-view imaging using high-speed cameras and bottom-view high-speed confocal imaging reveal the dynamics of the impact process. Drop impact experiments are performed under well-controlled conditions, varying systematically the ambient air pressure, surface temperature, surface inclination, liquid surface tension and viscosity. These parameter variations enable us to unravel the origin and dynamics of the air film, giving quantitative data on the maximum drop radius, the radius of the water-surface contact area, and the radius and thickness of the air film, the dynamics and lifetime of the air film, as well as the critical impact velocities for drop bouncing and splashing. Heated substrates lead to an enhanced evaporation of the drop and add a vapor source between the drop and the substrate. Lower ambient pressures reduce the amount of gas between the drop and the substrate. Also, surfaces with open pores reduces the air film due to air flow through the pores. To summarize, we aim for a quantitative understanding of the stability of the air film in drop impact on nanoporous surfaces and its contribution on the drop spreading, bouncing and splashing dynamics. Our results contribute to the rational design of functional surfaces to control the dynamics of the air film, to achieve special nonwetting properties and to control heat transfer and fluid drag.