Droplet evaporation on functional surfaces

Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a fixed time interval. However, the transient nature coupled with the significant mass-transfer governed gas-dynamics occurring at the droplet three-phase contact line make the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines make the decoupling of these processes impossible with classical transient methods. Here, we use our recently developed spatially-steady method to characterize the rate of evaporation of sessile droplets on functional surfaces. By utilizing a piezoelectric dispenser to feed microscale droplets ( ≈ 9 µm) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface, and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using the spatially-steady technique, we studied water evaporation of droplets having base radii ranging from 30 µm to 270 µm on surfaces of different functionalities (45 ≤ _a,app ≤ 162, where _a,app is the apparent advancing contact angle) under different substrate temperature conditions (30℃ ≤ _s ≤ 60℃, where _s is the functional surface temperature). Our work shows that the rate of evaporation increases linearly for increasing droplet size, and the surface functionality halts its important role at elevated temperatures.