Investigation of the effect of a simple salt on the kinetics of gravity induced coalescence for a viscosity matched emulsion system

In this work, we study the effect of a simple salt such as sodium chloride on the kinetics of gravity driven coalescence for a viscosity matched oil and water system. The time it takes for a drop to coalesce with a macroscopic interface is experimentally determined for oil drops in water and vice-versa. For a given set of fluids, the viscosities of the phases are matched and the drop sizes of the oil and water are controlled such that they are close. This ensures a similarity of buoyancy and gravitational forces for both cases. In the absence of interfacial effects, the coalescence of both water and oil drops at the interface should have been equal, due to the similarity of hydrodynamic conditions. It is observed that the presence of salt in the continuous phase retards the flow at the interface, thereby increasing the coalescence time. Such a phenomena was hitherto unobserved in liquid–liquid systems and will significantly influence the coalescence rates in emulsions. The experimental data are compared with existing theory. The results are explained on the basis of the creation of an interfacial tension gradient due to the presence of the salt molecules at the interface. The adsorption coefficient of the salt at the interface has been back calculated by including the force opposing the drainage in the film due to the interfacial tension gradients. The phenomenon is also discussed in relation to some of the observations that have been already reported in the literature.