Dynamics of microscale droplets impinging onto hot surfaces with oxide layers

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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.