Single droplet drying: Transition from the effective diffusion model to a modified receding interface model

Drying experiments on single droplets of aqueous amorphous polymer solution show morphological changes towards the end of drying that result in an under-prediction of the drying rate using an effective diffusion based model. Alternately, other researchers argue that the receding interface model more accurately reflects the physics of drying by predicting a fixed droplet radius once a specified surface condition is reached, usually the saturation concentration. However, this surface condition is not adequate for many skin forming materials. The conditions at which droplet radial contraction ceases will be determined by the balance between internal moisture loss causing a collapsing pressure and the mechanical strength of the surface skin. Because measurements and prediction of surface stress are difficult, it is proposed that they are related to the state of the polymer solution at the surface which is defined by the proximity of the surface temperature to its glass transition temperature, (T − Tg). In this work, an effective diffusion model is used to predict ideal shrinkage until a critical temperature difference or (T − Tg)crit is reached where the surface of the droplet becomes fixed and the skin grows towards the droplet centre, that is, as a receding interface. For maltodextrin DE5, a (T − Tg)crit of 20 °C was found to provide an accurate prediction of the drying rate. While these results show (T − Tg)crit is indicative of mechanical stress development, it points to a need for further understanding of mechanical stress development in skin forming polymers during drying.