Abstract:

The dynamics of the three-phase contact line for deionized water and gold nanofluid droplets with solar heating were experimentally investigated using substrates of various wettability. 2μL deionized water and nanofluid droplets, irradiated by a certain power solar simulator, were added to the hydrophilic or hydrophobic surfaces. A high-speed camera was used to record the evaporation process of the droplets on different surfaces in real time. The images were processed by the MATLAB program to obtain the dynamic characteristics of the droplets on different surfaces during evaporation, including the contact angle and contact area diameter. It has been found that droplets have different dynamics of the three-phase contact line on the hydrophilic and hydrophobic surfaces. Deionized water droplets evaporation on the hydrophilic surface was controlled by the constant contact area mode and the constant contact angle mode sequentially. Deionized water droplets exhibit “stick-slip” evaporation characteristic on hydrophobic surface. Initially, the droplet evaporates retaining a constant contact area. After the evaporation proceeds and minimum contact angle reached, then the three-phase contact line “slips” to a more energetically favorable position suddenly leading to a new, smaller contact radius and a larger contact angle. This cycle was repeated until the droplet dry-out. The evaporation process of nanofluid droplets on the hydrophilic surface was mainly controlled by the constant contact area evaporation mode. The “stick-slip” evaporation process was also observed on the hydrophobic surface. From the perspective of the surface energy, the “pinning” and “de-pinning” of droplet contact line was analyzed in detail. It was concluded that the wettability of the substrate and the deposition of nanoparticles affect the dynamics of the three-phase contact line on different surfaces. It is important for people to understand the droplet evaporation process in nature, industry and life.

Key words: solar energy, droplet, evaporation, nanofluid, mass transfer, wettability

摘要：

实验研究了亲水和疏水表面上太阳能加热去离子水及金纳米流体液滴三相接触线动力学。在亲水和疏水表面滴加2μL去离子水和纳米流体液滴，用一定功率太阳能模拟器照射液滴使其蒸发，期间采用高速摄像机实时记录液滴在不同表面上的蒸发过程。由MATLAB程序处理图像得到液滴在不同表面上蒸发过程中接触角和接触圆直径的动态变化过程。发现液滴接触线在不同亲疏水表面上存在不同运动特性。去离子水液滴在亲水表面上常接触面积模式和常接触角模式依次控制蒸发过程。去离子水液滴在疏水表面上都呈现出“黏-滑”蒸发特性，即液滴先以常接触面积模式蒸发，之后接触线快速滑动，接触线固定后再以常接触面积模式蒸发，依次往复。纳米流体液滴在亲水表面上主要以常接触面积蒸发模式为主，在疏水表面上同样呈现“黏-滑”蒸发特性。从液滴表面能角度出发，对液滴接触线“钉扎”和“去钉扎”过程进行详尽分析，得出基底润湿性和纳米颗粒沉积是影响液滴接触线在表面上运动的重要因素。

关键词: 太阳能, 液滴, 蒸发, 纳米流体, 传质, 润湿性