Determination of permeability characteristics from centrifugal flotation velocity of deformable oil droplets in O/W emulsions

The permeability characteristics of highly concentrated O/W emulsions were clarified on the basis of the kinetic data of the centrifugal flotation experiments with the aid of an analytical centrifuge which could monitor timed near-infrared (NIR) transmission profiles of the samples in the centrifugal field, and then the permeability data of the soft particles of oil droplets were compared with those of the rigid particles of silicon dioxide in centrifugal sedimentation. The dependence of centrifugal flotation velocity of concentrated emulsion on the emulsion concentration was not able to be described by the conventional Richardson–Zaki equation. By considering an analogy between the flotation of oil droplets in a solvent and the permeation of a solvent through the granular bed of oil droplets, the specific flow resistance of the oil droplets was determined as a function of the porosity from the data of centrifugal flotation velocity measured for a variety of initial porosity of O/W emulsion. Moreover, the dependence of the effective specific surface of the oil droplets on the porosity of O/W emulsion was derived using the Happel's cell model, which accurately describes the flow properties in the concentrated colloidal dispersions. As a consequence, it was shown that the effective specific surface of the oil droplets markedly decreased with decreasing porosity of emulsion in the range of the relatively high concentration. By contrast, it is significant to note that the effective specific surface remained almost constant in the case of centrifugal sedimentation of particulate suspension of silicon dioxide. These results provide important new insight into the mechanisms involved in the solid–liquid separation of the deformable particles.