Barmi Meysam R, Meinhart Carl D
Department of Mechanical Engineering, University of California Santa Barbara , Santa Barbara, California 93106, United States.
J Phys Chem B. 2014 Mar 6;118(9):2414-21. doi: 10.1021/jp408241f. Epub 2014 Feb 24.
The evaporation rate and internal convective flows of a sessile droplet with a pinned contact line were formulated and investigated numerically. We developed and analyzed a unified numerical model that includes the effects of temperature, droplet volume, and contact angle on evaporation rate and internal flows. The temperature gradient on the air/liquid interface causes an internal flow due to Marangoni stress, which provides good convective mixing within the droplet, depending upon Marangoni number. As the droplet volume decreases, the thermal gradient becomes smaller and the Marangoni flow becomes negligible. Simultaneously, as the droplet height decreases, evaporation-induced flow creates a large jet-like flow radially toward the contact line. For a droplet containing suspended particles, this jet-like convective flow carries particles toward the contact line and deposits them on the surface, forming the so-called "coffee ring stain". In addition, we reported a simple polynomial correlation for dimensionless evaporation time as a function of initial contact angle of the pinned sessile droplet which agrees well with the previous experimental and numerical results.
对具有固定接触线的静止液滴的蒸发速率和内部对流流动进行了公式化并进行了数值研究。我们开发并分析了一个统一的数值模型，该模型包括温度、液滴体积和接触角对蒸发速率和内部流动的影响。气/液界面上的温度梯度由于马兰戈尼应力而导致内部流动，这取决于马兰戈尼数，在液滴内提供了良好的对流混合。随着液滴体积减小，热梯度变小，马兰戈尼流动可忽略不计。同时，随着液滴高度减小，蒸发诱导流动产生一个朝向接触线径向的大的射流状流动。对于含有悬浮颗粒的液滴，这种射流状对流流动将颗粒带向接触线并将它们沉积在表面上，形成所谓的“咖啡环污渍”。此外，我们报告了一个简单的多项式相关性，用于描述无量纲蒸发时间作为固定静止液滴初始接触角的函数，该相关性与先前的实验和数值结果吻合良好。
