液滴在不同润湿性表面上蒸发时的动力学特性

Dynamics of evaporating drop on heated surfaces with different wettabilities

基于润滑理论,采用滑移边界条件建立了二维液滴厚度的演化模型和移动接触线动力学模型,利用数值计算方法模拟了均匀加热基底上固着液滴蒸发时的动力学特性,分析了液-气、固-气和液-固界面张力温度敏感性对壁面润湿性和液滴动态特性的影响.结果表明,液滴的运动过程受毛细力、重力、热毛细力和蒸发的影响,重力对液滴铺展起促进作用,而毛细力、热毛细力则起抑制作用;通过改变界面张力温度敏感性系数,可使液滴蒸发过程中的接触线呈现处于钉扎或部分钉扎模式,且接触线钉扎模式下的液滴存续时间低于部分钉扎模式;提高液-气与液-固界面张力温度敏感系数均可改善壁面润湿性能,加快液滴铺展速率;而增大固-气界面张力温度敏感系数则导致壁面润湿性能恶化、延缓液滴铺展过程;通过改变固-气界面张力温度敏感系数更有利于调控处于蒸发状态下的液滴运动.

The dynamics of evaporating sessile drop on a uniformly heated, horizontal, solid substrate is considered. On the basis of lubrication theory and Navier slip condition, an evolution equation for the height of the two-dimensional drop is established. The numerical results show that the drop evolution is governed by capillary force, gravity, thermal capillary force and evaporation. Gravity exerts a promoting effect on drop spreading, while capillary force and thermal capillary force inhibit drop spreading. The typical dynamic features including contact line pinning or partial pinning modes during the drop evaporation are illustrated by changing the temperature-sensitive coefficient in the present model, and the drop lifetime of contact pinning mode is found to be shorter than that of contact line partial pinning mode. Under the same temperature-sensitive coefficient of three interfaces, the drop evolution is indicated with three typical stages： 1） spreading stage, 2） contact line pinning stage, and 3） both contact line and contact angle decreasing stage. As interface tension of liquid-gas or liquid-solid is more sensitive to temperature, the drop evolution is divided into two typical stages： 1） spreading stage and 2） contact line pinning stage. The equilibrium contact angle tends to be smaller and the substrate wettability is improved, leading to the increased spreading speed, the prolonged time of the contact line to reach pinning： the faster the evaporation rate, the shorter the lifetime of drop is. Additionally, the same effect of sensitivity of liquid-gas and liquid-solid interface tension to temperature on the wettability of substrate is observed. When the interface tension of solid-gas is more sensitive to temperature, the drop evolution is characterized in four typical stages： 1） spreading stage, 2） contact line pinning stage, 3） contact line de-pinning and constant contact angle stage, and 4） both contact line and contact angle decreasing stage. The equilibrium contact angle tends to be greater and the substrate wettability is deteriorated, leading the spreading speed to decrease. Hence, it is more effective to manipulate the drop movement in the presence of evaporation by regulating the temperature-sensitive coefficient of the solid-gas interface.