Palmetshofer Patrick, Geppert Anne K, Steigerwald Jonas, Arcos Marz Tim, Weigand Bernhard
Institute of Aerospace Thermodynamics, University of Stuttgart, 70569, Stuttgart, Germany.
Sci Rep. 2024 Jan 11;14(1):1102. doi: 10.1038/s41598-024-51382-3.
We experimentally observe a new phenomenon, the formation of a toroidal region of lower film thickness in the center of the lamella formed during high Weber number water droplet impacts onto smooth heated walls. This region forms around the air bubble, which is entrapped during the initial impact phase at the impact center. Our study encompasses a variation of the droplet size, impact velocity, surface wettability and temperature. We show how this phenomenon can be explained considering a two-step process involving thermocapillary convection in two separate regions: The temperature gradient along the surface of the entrapped air bubble caused by heat conduction induces flow that pumps warmer liquid to the lamella-ambient interface due to the Marangoni effect. The non-uniform temperature distribution along it then causes fluid acceleration in the radial direction, depleting the fluid volume around the bubble in a self-amplifying manner. We use direct numerical simulations of a stagnant liquid film with an enclosed bubble at the wall to confirm this theory.
我们通过实验观察到一种新现象，即在高韦伯数水滴冲击光滑加热壁面过程中形成的薄片中心出现了一个薄膜厚度较低的环形区域。该区域围绕着气泡形成，气泡在初始冲击阶段于冲击中心处被截留。我们的研究涵盖了液滴尺寸、冲击速度、表面润湿性和温度的变化。我们展示了如何通过一个两步过程来解释这一现象，该过程涉及两个独立区域的热毛细对流：由热传导引起的截留气泡表面的温度梯度会引发流动，由于马兰戈尼效应，这种流动会将较热的液体泵送到薄片与周围环境的界面。沿其不均匀的温度分布随后会导致流体在径向方向上加速，以自增强的方式耗尽气泡周围的流体体积。我们使用壁面处带有封闭气泡的静止液膜的直接数值模拟来证实这一理论。
