Kinetics of formation of polysaccharide-covered micrometric oil droplets under mechanical agitation

Abstract

• Amphiphilic dextrans were used for preparing emulsions under mechanical agitation. • Kinetics of droplet formation was investigated by in situ rheo-optical measurements. • Droplet rupture was controlled by the ratio of shear forces to Laplace pressure. • Oil volume fraction had no effect on the kinetics of droplet formation. Hexadecane-in-water emulsions were prepared by mechanical agitation in vane geometry using amphiphilic derivatives of dextran (nonionic bacterial polysaccharide) as stabilizers. Thanks to a laboratory-made experimental device coupling rheometry and back-scattered light analysis, we investigated the kinetics of droplet formation and the simultaneous variation of global shear stress. Oil volume fraction was varied between 10 and 50% and rotation speed between 40 and 100 rad s −1 . The viscosity of aqueous phase was controlled by addition of sodium alginate and thus exhibited non-Newtonian rheological behavior. After forming a coarse emulsion, droplet rupture led to emulsions with volume-average droplet radius between 5 and 10 μm provided that rotation speed and aqueous phase viscosity were high enough. Monitoring was consistent with droplet rupture occurring under shear forces such that capillary number remained approximately constant and close to its critical value. Although rotational speed and sodium alginate concentration had strong effect on emulsification kinetics, oil volume fraction had no significant effect.