Single Droplet Drying: Transition from the Effective Diffusion Modelto a Modified Receding Interface Model

摘要

Drying experiments on single droplets of aqueous amorphous polymer solution showrnmorphological changes towards the end of drying that result in an under-prediction ofrnthe drying rate using an effective diffusion based model. Alternately, otherrnresearchers argue that the receding interface model more accurately reflects thernphysics of drying by predicting a fixed droplet radius once a specified surfacerncondition is reached, usually the saturation concentration. However, this surfacerncondition is not adequate for many skin forming materials. The conditions at whichrndroplet radial contraction ceases will be determined by the balance between internalrnmoisture loss causing a collapsing pressure and the mechanical strength of the surfacernskin. Because measurements and prediction of surface stress are difficult, it isrnproposed that they are related to the state of the polymer solution at the surface whichrnis defined by the proximity of the surface temperature to its glass transitionrntemperature,rn(T–T ). G In this work, an effective diffusion model is used to predict idealrnshrinkage until a critical temperature difference orrn(T–T ) g crit is reached where thernsurface area of the droplet becomes fixed and the skin grows towards the dropletrncentre, that is, as a receding interface. For maltodextrin DE5, arn(T–T ) g crit of 20°C wasrnfound to provide an accurate prediction of the drying rate. While these results showrn(T–T ) g crit is indicative of mechanical stress development, it points to a need for furtherrnunderstanding of mechanical stress development in skin forming polymers duringrndrying.