Different Shapes of Menisci Emerging during Evaporation: Picolitre Droplets Showing Diversities in Cylindrical Wells.

This work is to investigate the evaporation of a printed droplet in a cylindrical well so as to understand the evolution of height profile of a droplet and the possible reasons for the appearance of a dynamic meniscus. The radii and depths of the cylindrical wells in this study range from 25.0 to 75.0 micrometers and 1.00 to 4.00 micrometers, respectively. A parameter that represents the impact from the property of a droplet and its host well is found effective to predict the shape of a meniscus emerging from an evaporating pure droplet. For binary droplets, four new shapes of menisci are found, M-shaped profile, bell-shaped profile, lopsided profile and spreading profile, the appearance of which depend on the surface tension difference between the two solvents in a binary droplet. M-shaped profile and spreading profile appear when the more volatile solvent has the lower surface tension, while bell-shaped profile and lopsided profile present when the more volatile solvent has the higher surface tension. In addition, the non-ideality of a binary system also has a significant impact on a shape as the evaporation rate of a supposed less volatile solvent may become the more volatile one when it is dilute, such as the mixture of n-butanol and toluene. The evaporation rate of n-butanol is higher than that of toluene when n-butanol is dilute.