Droplet evaporation has been intensively investigated in past decades owing to its emerging applications in diverse fields of science and technology. Yet the role of transport mechanisms has been the subject of a heated debate, especially the presence of Marangoni flow in water droplets. This work aims to draw a clear picture of the switching transport mechanisms inside a drying pinned sessile water droplet in both the presence and absence of thermocapillarity by developing a comprehensive model that accounts for all pertinent physics in both phases as well as interfacial phenomena at the interface. The model reveals a hitherto unexplored mixed radial and buoyant flow by shedding light on the transition from buoyancy induced Rayleigh flow to the radial flow causing the coffee ring effect. Predictions of the model excellently match previous experimental results across varying substrate temperatures only in the absence of Marangoni flow. When thermocapillarity is accounted for, strong surface flows shape the liquid velocity field during most of the droplet lifetime and the model starts to overestimate evaporation rates with increasing substrate temperature.

中文翻译：

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固定水滴蒸发过程中传输机制的相互作用

由于其在不同科学和技术领域的新兴应用，液滴蒸发在过去几十年中得到了深入研究。然而，运输机制的作用一直是激烈辩论的主题，尤其是水滴中马兰戈尼流的存在。这项工作旨在通过开发一个综合模型来解释在存在和不存在热毛细现象的情况下干燥固定水滴内部的转换传输机制，该模型解释了两个相中的所有相关物理以及界面处的界面现象. 该模型通过揭示从浮力引起的瑞利流到导致咖啡环效应的径向流的过渡，揭示了迄今为止尚未探索的混合径向和浮力流。只有在没有 Marangoni 流的情况下，模型的预测才能很好地匹配先前在不同基板温度下的实验结果。当考虑到热毛细作用时，在液滴寿命的大部分时间里，强烈的表面流动会塑造液体速度场，并且该模型开始高估基板温度升高时的蒸发率。