Dynamics of microscale droplets impinging onto hot surfaces with oxide layers

首先，通过实验研究了基质电导率对吸湿性液体除湿液滴吸收水蒸气的影响。结果表明，衬底导热系数在非稳态换热过程中起着至关重要的作用。由于热效应，铜基板上的液滴比低导热系数聚四氟乙烯基板上的液滴表现出略高的蒸汽吸收速率。通过对水蒸气压力变化的分解进一步分析表明，与温度变化的影响相比，水滴表面水蒸气压力受水蒸气吸收过程中溶质浓度变化的影响较大，进而影响水蒸气的吸收速率。其次，建立了一个润滑型模型来模拟吸湿溶液离子水滴的蒸发和水蒸气吸收过程。这一研究使得充分阐明液滴正、负界面质量通量运动的调控机制成为可能。我们阐明，随着界面相变，毛细效应、热Marangoni效应和溶质Marangoni效应相互作用，导致吸湿溶液液滴的时变行为不同于蒸发挥发液滴。这些结论合理地解释了现有文献中伴随着水蒸气吸收的扩散和收缩现象，以及蒸发水溶液液滴外围的树枝状结构。

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.