Coalescence coupling with flocculation in dilute emulsions within the primary and/or secondary minimum

The simplest singlet–doublet emulsions (SDE) exhibit singlet–doublet quasi-equilibrium (s.d.e.) and slow coalescence between doublets. Orthokinetic coagulation and creaming may be eliminated using low-density contrast emulsions. The investigation of the evolution in time of the total number of droplet in such emulsions at SDE was recommended as a standard method to determine the characteristic times of the elementary acts of coalescence (τc) and doublet fragmentation (τd) [J. Disp. Sci. Technol. 19 (1998) 311]. Further improvement of this method and its automation requires discrimination between singlets and doublets in the bulk, which is possible for droplets of approximately 5 μm in size. Simultaneously, the droplet dimension must not be too large as the fragmentation time increases rapidly with the droplet dimension, while it has to be very small in compare to the Smoluchowski time (τsm) to preserve the condition for SDE. It is shown here that this controversy in the demand to the droplet size can be satisfied by a simultaneous decrease in electrolyte concentration, increase in surface potential and decrease in droplet volume fraction. The calculation of the fragmentation time in a wide range of electrolyte concentration, surface potential and particle radius shows that τd may be very small in compare to τsm (increases at small volume fractions) at such large mini-emulsion droplet dimension as it is necessary for a singlet and doublet discrimination. The second experimental difficulty is the discrimination between doublets in the primary (PD) and secondary minimum (SD) using video microscopy, while there is a huge difference in their kinetic behavior in respect to fragmentation and coalescence. The analysis of the domains for PD only, for SD only, and for a coexistence of SD and PD shows that the characterization of mini-emulsions has to be accomplished using experimental conditions corresponding to the domain of SD only.