Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
A mathematical model is proposed to describe the fluid dynamics, mass and heat transfer in a solution droplet evaporating on a flat surface during drying process. A decrease of droplet volume due to evaporation of a solvent, evaporation latent heat generated on the free surface, and an increase of a solute on the free surface are considered in the model. Governing equations are numerical solved using a finite element method. A Lagrangian method is applied to predict the deformation of an evaporating droplet. Firstly, the outward flow caused during selfpinning of the contact line are examined under an ideal condition. The calculated velocities agree well with calculated results using a one-dimensional model. Secondly, the drying process of a polystyrene/anisole solution droplet with the equivalent diameter of 20 µm are estimated. The migration of the solute at the contact line is found to be finished by the instant when a thin liquid film with a low solute concentration are still remained at the center part. As a result, a ring structure develops on the periphery of the dried film. Lastly, the effect of fluid viscosity is investigated. A high viscosity essentially decreases the fluid velocity, resulting in vanishing the ring structure. The effect of viscosity on the configuration of the film agrees with empirical results.