(279d) Role of Convection and Viscoelastic Phase Separation (VPS) in the Crystal

An automated drop on demand (DoD) system was utilized to deposit droplets of dopamine hydrochloride solution onto hydroxypropyl cellulose film.. During the evaporation process we have observed the transformation of a studied system from one phase to another. Furthermore, we have shown that the observed 1st order phase transition can be manipulated via interfacial characteristics of substrates to produce amorphous or crystalline structures upon droplet impingement. In particular, the final structure of dopamine hydrochloride droplets appeared to be dependent upon not only the interfacial characteristics of the substrate but also a phenomenon known as viscoelastic phase separation(VPS). In general we found, that for conditions where the interfacial strength was less than Van der Waals forces the evaporation and mass transfer of the fluid solution are affected by VPS. During VPS the formation of a transient gel occurs and water diffuses quickly into the surrounding air leading to movement of water loaded with API toward the center of the drop and the eventual crystallization of the API.