Combined effect of relative humidity and substrate temperature on evaporation of methanol droplet

Understanding and manipulating the mechanisms involved in the evaporation of organic solvent droplets in a humid environment are of great importance for numerous industrial and biological applications. The combined effect of substrate temperature and relative humidity of the environment on evaporation of pure methanol droplets has been experimentally investigated. A drop shape analyzer was used to record the evolution of contact angle, base diameter, and volume of the droplet. At low relative humidity, the droplet experiences three stages of evaporation: constant, slightly increasing, and sharply decreasing constant angle. Substrate temperature has no effect on the qualitative behavior of the droplet at low relative humidity. At high relative humidity, droplet evolution is influenced by substrate temperature. At relatively high substrate temperature, the droplet undergoes two stages of evaporation where the contact angle increases to a maximum then decreases until the end of evaporation. At low substrate temperature, the droplet experiences a single stage where it reaches an equilibrium state with finite volume, diameter, and contact angle. In humid environment, water vapor gets adsorbed–absorbed and/or condensed on the droplet. Decreasing substrate temperature causes the temperature at the liquid–gas interface to fall below the dew point which enhances the water condensation. The evaporation rate diminishes as the relative humidity rises and its effect is more pronounced for lower substrate temperature. Relative humidity of the surroundings seems to have a negligible effect on the initial evaporation rate which is mainly attributed to the evaporation of methanol. A regime map is proposed based on the different evolution of droplets under different environmental conditions as well as substrate temperature.