Three-Dimensional Numerical Simulation on Marangoni Convection in a Sessile Water Droplet Evaporating in its Vapor at Low Pressure

In order to understand the effect of Marangoni convection on the evaporation rate and the flow pattern, we performed a series of three-dimensional numerical simulations on evaporation process of sessile water droplet by introducing a kinetic model. Substrate temperature and the ratio of vapor pressure varied from 299 K to 308 K and from 0.92 to 0.99, respectively. The variation range of contact angle θ was between 9.15° and 120°. Results show that the axisymmetric ring-shape temperature distribution on droplet free surface transforms into a serrated temperature distribution because of the enhancing Marangoni convection with the increase of substrate temperature. In addition, with the decrease of vapor pressure, the total evaporation rate on free surface will be increased and the Marangoni effect will be enhanced. The flow pattern is axisymmetric at 90o ≤ θ ≤ 120o when the substrate temperature is fixed. However, it shifts to a multi-cellular pattern with the decrease of contact angle as a result of the competition between Marangoni flow and the evaporation. Additionally, the total evaporation rate increases with the increase of contact angle when contact angle is above 60o, but when the contact angle is below 60o, the trend is reverted. The temperature distribution becomes distinct at a small contact angle.