Drop Impact on Hot Surfaces

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Liquid drops impacting on hot surfaces are frequently encountered in technology, e.g. in spray cooling of electronics, in spreay quenching of metals, in combustion engines or in firefighting using sprinklers. On sufficiently hot surfaces, liquid drops float on a layer of vapour(Leidenfrost effect). This immensely reduces the heat transfer from the substrate to the drop liquid, and thus the evaporation rate. Drop impact on isothermal substrates is a classic field of fluid dynamics, and Leidenfrost effects on gently deposited drops have been frequently investigated. However, little of the physics behind drops impacting hot substrates has yet been revealed. Drop impact on hot surfaces involves a complex interplay of hydrodynamic, heat transfer and phase transition effects. It includes the properties of the liquid, the surrounding gas and the solid, and it combines a large range of time and length scales. Recent breakthroughs revealing the boiling regimes and complex wetting dynamics underneath impacting drops have been achieved using total internal reflection imaging (TIR) in the Physics of Fluids Group at the University of Twente. The unique conditions of the Leidenfrost, or partial Leidenfrost regimes, also represent model system for ideal superhydrophobic surfaces, and thus allows important insights into purely drop-impact related problems such as drop spreading, splashing and other decomposition mechanisms. This project will explore the fundamental physics of drop impact on hot surfaces using mainly optical high-speed imaging techniques and accompanying analytical and where possible also numerical modelling. We aim to shed light on the occurrence of different boiling regimes in dependence of substrate, liquid and ambient gas properties (including e.g. heat transfer coefficients, latent heat, viscosity, surface tension, substrate wettability and potentially also patterning). Another central aspect will be understanding the origin of the wetting dynamics and patterns underneath droplets impacting on hot, smooth substrates. Besides, we will study the influence of substrate conditions. We will address drop spreading and the physical reason of splashing in general. The last months of the project will be dedicated to oblique drop impact, which is the usual case in application but much less studied and understood than the perpedicular case. The project will be carried out in the Physics of Fluids Group led by Prof. Detlef Lohse. Prof. Lohse has excellent expertise in both research and supervision of junior researchers. The University of Twente and the Max Planck Center for Complex Fluid Dynamics provide an ideal environment this research and for fruitful (international) collaborations. Altogether, this postdoctoral research fellowship is expected to equip the applicant with the best possible qualifications to continue her scientific carreer in Germany by e.g. consecutively leading a junior research group.