Dynamics of evaporating drop on heated surfaces with different wettabilities

Abstract

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.