Experimental study on evaporation characteristics of micro droplets affected by irregular surface: Ugli case

Droplet evaporation can be commonly found on the fruits’ surface, which is critically related to the efficiency of pesticide spraying, storage quality with temperature fluctuations and so on. Numerous works have demonstrated the coupling effects between the droplet behavior and the substrate characteristics. However, the droplet performance on fruits surface have been less focused on compared to the substrate of metal, concrete and chemical materials. Therefore, the present works aim to investigate the droplet behaviors affected by the biological substrate experimentally. The deionized water and Ugli skin have been selected as droplet and substrate, respectively. 3D video microscope, Drop shape analyzer and infrared camera have been employed to measure and observe the micro-structure, evaporation performance and thermal effects of droplet with constant initial volume of 2ul. The results show that Ugli skin features strongly irregular roughness and hydrophilic properties as the initial contact angle is below 90 ^o . Linearly decreased variation of contact angle and volume versus time has been observed, even though the irregular surface roughness and outflow components from the Ugli skin can bring difficulties to the accurate measurement. Meanwhile, three phases have been observed during the whole evaporation process, including spreading, constant area and shrinkage. Moreover, lower values of temperature near the top center of droplet can be always found during the whole evaporating process with peak values of temperature gradient near the triple-phase region.