Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , 1206 W. Green St., Urbana, Illinois 61801, United States.
International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University , 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Langmuir. 2017 Oct 31;33(43):12007-12015. doi: 10.1021/acs.langmuir.7b02821. Epub 2017 Oct 10.
Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a given time interval. However, the transient nature coupled with the significant mass-transfer-governed gas dynamics occurring at the droplet three-phase contact line makes the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines makes the decoupling of these processes impossible with classical transient methods. Here, we present a method to measure the rate of evaporation of spatially and temporally steady droplets. By utilizing a piezoelectric dispenser to feed microscale droplets (R ≈ 9 μm) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using our steady technique, we studied water evaporation of droplets having base radii ranging from 20 to 250 μm on surfaces of different functionalities (45° ≤ θ ≤ 162°, where θ is the apparent advancing contact angle). We benchmarked our technique with the classical unsteady method, showing an improvement of 140% in evaporation rate measurement accuracy. Our work not only characterizes the evaporation dynamics on functional surfaces but also provides an experimental platform to finally enable the decoupling of the complex physics governing the ubiquitous droplet evaporation process.
液滴蒸发是控制许多人为和自然过程的重要现象。在过去的一个世纪里，准确地表征蒸发速率引起了众多科学家的关注。传统上，研究人员通过观察给定时间间隔内液滴尺寸的变化来研究蒸发。然而，由于在液滴三相接触线处发生的瞬态性质和显著的传质控制的气体动力学，传统方法很粗糙。此外，内部和外部流动、蒸发动力学、热毛细现象、二元混合物动力学、曲率和移动接触线之间的复杂平衡使得这些过程不可能通过经典的瞬态方法解耦。在这里，我们提出了一种测量空间和时间稳定液滴蒸发速率的方法。通过利用压电分配器以规定的频率将微尺度液滴（R ≈ 9 μm）输送到较大的蒸发液滴上，我们可以在任何表面上创建具有不同尺寸的液滴，并通过调节压电液滴添加频率来研究它们的蒸发速率。使用我们的稳定技术，我们研究了在不同功能表面（45°≤θ≤162°，其中θ是表观前进接触角）上基半径从 20 到 250 μm 的液滴的水蒸发。我们用经典的非稳态方法对我们的技术进行了基准测试，显示蒸发率测量精度提高了 140%。我们的工作不仅描述了功能表面上的蒸发动力学，还提供了一个实验平台，最终能够解耦普遍存在的液滴蒸发过程所涉及的复杂物理。
