Evaporating thin film profile near the contact line of a partially wetting water droplet under environmental heating

Abstract

To understand the thin film evaporation in a partial wetting system, water morphology near the receding contact line driven by evaporation has been investigated experimentally. A state-of-the-art tapping-mode atomic force microscopy (TM-AFM) was used to obtain the nanoscale film profile. The test section first reached equilibrium in an enclosed chamber. Then a small amount of heat was supplied to increase the gas temperature to achieve an environmental heating. The droplet was evaporating and the receding velocity of the contact line was below 20nm/s. The film profiles were found following the macroscopic profile, which was about linear, till about 2nm thickness from the contact line which was already the limitation of the measurement. The results indicate that the microscopic contact angle at nanoscale was approximately equal to the optically detected macroscopic contact angle under this weak environmental heating.