Effect of rheology on coalescence rates and emulsion stability

The effect of continuous-phase rheology on the coalescence time of single water drops at an organic/aqueous interface was investigated experimentally. For Newtonian fluids, the coalescence time increases monotonically with the continuous-phase viscosity at a fixed drop diameter and with the drop diameter at a fixed continuous-phase viscosity. Elasticity in the continuous phase caused significant increases in the coalescence times for drops of less than 1 mm diameter in the systems studied, but had no discernible effect on the coalescence of larger drops. Estimates of the interfacial shear rates were used to demonstrate that the elasticity of the continuous phase becomes significant in the coalescence process as the drop size decreases. These results indicate that emulsion stability may be increased without the use of surfactants by adding a suitable polymer to the continuous phase and control of the dispersed-phase drop size.