Dynamics of microscale droplets impinging onto hot surfaces with oxide layers

首先，实验研究了基材电导率对吸湿性液体干燥剂液滴吸收水蒸气的影响。结果表明，衬底导热性在瞬态传热过程中起着至关重要的作用。由于热效应，铜基上的液滴比低热传导率的聚四氟乙烯基上的液滴表现出略高的蒸汽吸收率。进一步分解水蒸气压的变化分析表明，相对于温度变化的影响，液滴表面水蒸气压受水蒸气吸收过程中溶质浓度变化的影响较大，因此水蒸气吸收率也受到影响。其次，建立了润滑型模型，模拟了吸湿离子溶液液滴的蒸发和蒸汽吸收过程。该研究使充分阐明具有正负界面质量通量的液滴运动的控制机制成为可能。研究表明，随着界面相变，毛细效应、热马兰戈尼效应和溶质马兰戈尼效应相互作用，导致吸湿性溶液液滴的时变行为不同于蒸发性挥发性液滴。这些结论为现有文献中水蒸气吸收过程中的扩张和收缩现象以及蒸发水溶液液滴周围的枝晶结构提供了合理的解释。

First,the effect of substrate conductivity on the vapor absorption into hygroscopic liquid desiccant droplets is investigated experimentally. Results indicate that substrate conductivity plays a crucial role in the transient heat transfer process. As a result of the thermal effect, droplets on copper substrates show a slightly higher rate of vapor absorption than those on low-thermal conductivity PTFE substrates. Further analyses by decomposing the variation of water vapor pressure indicate that, compared to the influence of temperature change, the water vapor pressure at the droplet surface is greatly affected by the change of solute concentration during vapor absorption, and therefore the rate of vapor absorption.Second, a lubrication-type model is developed to simulate the evaporation and vapor absorption process of hygroscopic ionic solution droplet. The investigation makes it possible to fully elucidate the governing mechanisms of droplet motions with both positive and negative interfacial mass fluxes. We elucidate that, along with interfacial phase change, the capillary effect, the thermal Marangoni effect and the solutal Marangoni effect interact with each other, and lead to the time-varying behaviors of hygroscopic solution droplets that differ from evaporating volatile droplets. The conclusions provide reasonable explanations for the spreading and contracting phenomena along with vapor absorption, and for the dendritic structures at the periphery of evaporating aqueous solution droplets in the existing literature.