Sunlight does not always speed up evaporation

It is often assumed that the presence of sunlight expedites the evaporation of water droplets. However, in a study of the effects of sunlight, Xiongjiang Yu and Jinliang Xu found the counterintuitive result that due to competing effects, the presence of sunlight hinders evaporation in the initial stage.

Yu and Xu used a high-speed camera to capture the behavior of a droplet under xenon light, which resembles sunlight.

According to the authors, the process of evaporation under sunlight irradiation can be separated into two competing regimes governed by different mechanisms. When a droplet is exposed to sunlight, the temperature difference between the bottom of a droplet and its apex decreases, which weakens Marangoni flow within the droplet and initially slows down the heat transfer from the substrate to the droplet, leading to slower evaporation. Later, as light reflection is reduced with a flattened droplet surface caused by the initial step, the process is accelerated, and sunlight-irradiated evaporation eventually catches up and surpasses evaporation in natural conditions.

In addition to their significance to solar energy, these findings also have impacts on all sorts of climates, ranging from dry areas to bodies of water.

“On a large water surface such as a river, lake or sea, water evaporation is faster with sunlight radiation compared to that without sunlight irradiation,” said Xu. “Because sunlight irradiation hinders droplet evaporation in the short term, cloudy weather is preferable to keep dew droplets on a plant leaf or soil for a longer time, thus the water absorption by a plant leaf or soil is enhanced. This implication is important in desert regions.”

Source: “Does sunlight always accelerate water droplet evaporation?,” by Xiongjiang Yu and Jinliang Xu, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/5.0012700 .