Droplet size distributions derived from evolution of oil fraction during phase separation of oil-in-water emulsions tracked by electrical impedance spectroscopy

Abstract

Oil-in-water emulsions with dispersed phase volumetric fraction up to 0.2 are prepared in a vertical cylindrical vessel via catastrophic phase inversion emulsification using the non-ionic surfactant Ethylan 1008. Two surfactant concentrations are selected in order to generate emulsions with different droplet size distributions as determined by optical microscopy. Electrical resistance tomography (ERT) and electrical impedance spectroscopy (EIS) measurements are carried out at four different heights along the vessel during phase separation to track the evolution of oil volumetric fraction. It is seen that EIS captures the physical phenomenon of cream and aqueous phase separation with higher accuracy and sensitivity than ERT. Thus, the oil fraction evolution curves derived by EIS at different heights are mathematically treated, using an innovative technique developed here. This delivers droplet size distributions that are directly associated with droplet motion and, therefore, characteristic of phase separation in the emulsion. The droplet size distributions retrieved from EIS at different heights (i) take into account a very large portion of the emulsion volume, (ii) are not subject to sampling and image analysis errors, (iii) are user unbiased and highly repeatable, (iv) are obtained on-line and non-intrusively.