Does sunlight always accelerate water droplet evaporation?

Abstract

We investigate droplet evaporation, which is a natural phenomenon but the mechanism is not well understood. We are surprised to find that sunlight irradiation does not always enhance droplet evaporation, which is against the common sense that “Sun accelerates water evaporation.” This is true at least for short-time evaporation. A whole droplet lifetime consists of two regimes of evaporation: a light induced deterioration regime and an acceleration regime. The deterioration regime is explained by the decreased temperature difference from the droplet bottom to apex, weakening Marangoni flow to hinder conduction heat transfer from the substrate to the droplet. The enhanced regime is explained by the reduced light energy reflection via the droplet surface. The substrate conduction heat transfer and radiation heat transfer of light are coupled to dominate evaporation. The two mechanisms create opposite contributions, resulting in a constant evaporation rate for sunlight irradiation on a droplet. However, natural light decreases the evaporation rate vs time. Hence, evaporation rates with and without sunlight irradiation cross at a specific time. Our work enhances the fundamental understanding of droplet evaporation and provides a useful guideline for efficient solar energy utilization.