INFLUENCE OF THE DEVELOPMENT OF PARTICLE MORPHOLOGY UPON RATES OF LOSS OF VOLATILE SOLUTES DURING DRYING OF DROPS

An electron-capture detector was used to monitor instantaneous rates o f loss of SF6 during drying of suspended drops of several different aq ueous solutions. Simultaneously the appearance of the drop was recorde d with a videocamera, thereby enabling the rate of evolution of SF6 to be related to changes in particle morphology. Early in drying, rates of loss of SF6 decrease toward zero as selective diffusion develops. F or coffee extract frothy arms of liquid protrude from the drop surface later in drying and produce surges of SF6 evolution. For maltodextrin solutions there was substantial loss of SF6 due to repeated expansion , bursting, and collapse, as bubbles formed repeatedly within a drop. Drops of nonfat milk remained smoother and more spherical in appearanc e, with much less tendency for surface disruptions. Nonetheless, at hi gher temperatures milk drops showed periods of substantial SF6 loss la ter in drying.