Video-microscopic investigation of the coupling of reversible flocculation and coalescence

Abstract

A theory for the coupling of flocculation, coalescence and floc fragmentation in dilute, polydisperse oil-in-water emulsions with narrow droplet size distributions is proposed. For the first time, the strong decrease in the total droplet concentration in dilute oil-in-water emulsions at singlet–doublet quasi-equilibrium and at negligible multiplet concentration is measured by combining video-enhanced microscopy (VEM) with a microslide preparative technique. The possibility to investigate coalescence in the simplest model emulsion, namely a singlet–doublet quasi-equilibrium, is thereby verified. The coalescence process is observed and the rate of coalescence is evaluated over a rather broad range of electrolyte concentrations, surface charges, droplet dimensions and concentrations corresponding to the technically important case of a long coalescence time, caused either by the electrostatic retardation of droplet coagulation and/or by the high stability of the thin emulsion film with an almost saturated adsorption layer of surfactant.