Modeling the solidification of O/W-emulsion droplet in solvent evaporation technique

The removal of organic solvent plays an important role in the fabrication of polymer spheres by emulsion-based solvent evaporation technique. Mathematical model describing the mass transfer of fluorobenzene (FB) inside the O/W emulsion droplet, from the droplet to the continuous phase and from the continuous phase to the atmosphere to produce millimeter-sized polystyrene (PS) spheres was established. A novel approach based on finite volume method was developed to numerically solve the mathematical equations concerning the shrinkage of the solidifying droplet. Using this method, the variations of the droplet size and the concentration field inside the droplet were captured. The details of the solidification process which cannot be obtained using experimental methods were revealed. Several factors influencing the curing rate and mass transfer process, such as droplet number, initial droplet diameter, initial concentration and addition of FB in the continuous phase, were investigated in detail. The simulation results indicated that changing the initial concentration or changing the initial diameter of the droplet to tune the solidified particle size were essentially the same considering their effects on the details of the solidification process. When adding FB in the continuous phase to reduce the curing rate and concentration gradient, dispersing the added FB into droplets with the same diameter to that of the PS/FB droplet to be solidified has the most significant effect.