Drop Impact on Hot Surfaces

液滴冲击热表面在技术中经常遇到，例如电子设备的喷雾冷却，金属的喷雾淬火，内燃机或使用洒水器的消防。在足够热的表面上，液滴漂浮在一层蒸汽上（莱顿弗罗斯特效应）。这极大地减少了从基材到液滴液体的热量传递，从而降低了蒸发速率。液滴对等温基底的冲击是流体动力学的一个经典领域，对轻沉积液滴的莱顿弗罗斯特效应进行了频繁的研究。然而，很少的物理背后滴冲击热基板尚未揭示。液滴对热表面的冲击涉及流体力学、传热和相变效应的复杂相互作用。它包括液体、周围气体和固体的性质，并结合了大范围的时间和长度尺度。最近，特温特大学流体物理小组利用全内反射成像（TIR）技术取得了突破性进展，揭示了撞击液滴下的沸腾状态和复杂的润湿动力学。Leidenfrost或部分Leidenfrost体系的独特条件也代表了理想超疏水表面的模型系统，因此可以对纯粹的液滴撞击相关问题（如液滴扩散、飞溅和其他分解机制）有重要的见解。该项目将主要使用光学高速成像技术以及伴随的分析和可能的数值模拟来探索液滴撞击热表面的基本物理现象。我们的目标是阐明不同沸腾状态的发生取决于基材、液体和环境气体的性质（包括传热系数、潜热、粘度、表面张力、基材润湿性和潜在的图案）。另一个核心方面将是了解湿润动力学的起源和水滴撞击热光滑基底的模式。此外，我们将研究衬底条件的影响。我们将从总体上解决水滴扩散和飞溅的物理原因。项目的最后几个月将专门研究斜滴冲击，这是应用中的常见情况，但研究和了解的情况比永久情况要少得多。该项目将在Detlef Lohse教授领导的流体物理小组进行。Lohse教授在研究和指导初级研究人员方面具有出色的专业知识。特温特大学和马克斯普朗克复杂流体动力学中心为这项研究和富有成效的（国际）合作提供了理想的环境。总的来说，这个博士后研究奖学金有望为申请人提供最好的资格，以继续她在德国的科学事业，例如连续领导一个初级研究小组。

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.