An electron-capture detector was used to monitor instantaneous rates of loss of SF6 during drying of suspended drops of several different aqueous solutions. Simultaneously the appearance of the drop was recorded 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. For 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 appearance, with much less tendency for surface disruptions. Nonetheless, a t higher temperatures milk drops showed periods of substantial SF6 loss later in drying.
ABSTRACT:The spontaneous spreading of emulsions of water dispersed in silicone oil onto glass surfaces is examined using differential interference contrast (DIC) microscopy. Spreading occurs via a precursor film from which the emulsion droplets are excluded. The radius of the interline of the bulk drop is found to vary as (time) 1/10 , as is commonly observed for the spontaneous spreading of pure liquids. The spreading rate constant decreases linearly with the volume percent of the dispersed phase, but drops suddenly to zero at approximately 73% dispersed phase. The width and spreading rate of the precursor film also is found to decrease with dispersed phase concentration. A fingering type of instability is evident at the leading edge of the precursor film, yet has little effect on the spreading rate of either the precursor film or the droplet interline.
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