Effects of droplet size and solute concentration on drying process of polymer solution droplets deposited on homogeneous surfaces

An experimental setup is constructed to investigate the drying process of a micro-scale polymer solution droplet deposited on homogeneous surfaces. The contact angle during evaporation does not agree with the contact angle hysteresis measured using an inclined plate method due to the size effect. The average polymer concentration at which the contact line is pinned is independent of the initial solution concentration. This fact reveals that the free convection plays an impotent role before pinning. The pinning time, or the receding distance, is moreover found to be a key factor that dominates the shape and dimensions of the polymer films.