Holographic Monitoring of Coarsening in Binary Oil-in-Water Emulsions*

We use holographic characterization to monitor changes in the diameters and compositions of individual emulsion droplets during coarsening of binary emulsions. When the initial droplets are composed of two distinct but miscible fluids, the increase in droplet size is accompanied by compositional mixing, with the end result being a homogeneous emulsion of stoichiometrically mixed droplets. Holographic characterization can distinguish Ostwald ripening from coarsening dominated by collisional coalescence. Our measurements reveal that model oil-in-water emulsions composed of slightly soluble aromatic oils coarsen almost exclusively through ripening. Insoluble silicone oil emulsions, by contrast, are kinetically stable, but can be made to coalesce by mixing or sonication. The ability to track both size and composition provides insights into the aging process in heterogeneous emulsion that cannot be obtained in other ways.