Influence of Surface Wettability on Transport Mechanisms Governing Water Droplet Evaporation

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• 作者：Zhenhai Pan ; Justin A. Weibel ; Suresh V. Garimella
• 刊名：Langmuir
• 出版年：2014
• 出版时间：August 19, 2014
• 年：2014
• 卷：30
• 期：32
• 页码：9726-9730
• 全文大小：405K
• ISSN：1520-5827

文摘

Prediction and manipulation of the evaporation of small droplets is a fundamental problem with importance in a variety of microfluidic, microfabrication, and biomedical applications. A vapor-diffusion-based model has been widely employed to predict the interfacial evaporation rate; however, its scope of applicability is limited due to incorporation of a number of simplifying assumptions of the physical behavior. Two key transport mechanisms besides vapor diffusion鈥攅vaporative cooling and natural convection in the surrounding gas鈥攁re investigated here as a function of the substrate wettability using an augmented droplet evaporation model. Three regimes are distinguished by the instantaneous contact angle (CA). In Regime I (CA 60掳), the flat droplet shape results in a small thermal resistance between the liquid鈥搗apor interface and substrate, which mitigates the effect of evaporative cooling; upward gas-phase natural convection enhances evaporation. In Regime II (60 CA 90掳), evaporative cooling at the interface suppresses evaporation with increasing contact angle and counterbalances the gas-phase convection enhancement. Because effects of the evaporative cooling and gas-phase convection mechanisms largely neutralize each other, the vapor-diffusion-based model can predict the overall evaporation rates in this regime. In Regime III (CA 90掳), evaporative cooling suppresses the evaporation rate significantly and reverses entirely the direction of natural convection induced by vapor concentration gradients in the gas phase. Delineation of these counteracting mechanisms reconciles previous debate (founded on single-surface experiments or models that consider only a subset of the governing transport mechanisms) regarding the applicability of the classic vapor-diffusion model. The vapor diffusion-based model cannot predict the local evaporation flux along the interface for high contact angle (CA 鈮?90掳) when evaporative cooling is strong and the temperature gradient along the interface determines the peak local evaporation flux.