Highly concentrated emulsions: role of droplet size

Abstract

The problem of droplet size dependence on rheological properties of highly concentrated emulsions was studied. Changes in droplet size distribution were achieved by multi-pass flow through a small orifice set as the outlet of a piston-chamber pumping instrument, and extended elongation of droplets combined with the transient-transition flow regime (laminar/turbulent) at the outlet of the orifice provided the shift of the droplet sizes to the smaller-size side of the distribution. Their distributions were wider and of the logarithmic-normal distribution type. Two models were proposed and used to fit the refinement evolution and the width of distributions respectively. The droplet size versus number of pumping cycles was expressed by the fitting parameters, θD, θω, Dcrit., ωcrit., and C, which, in turn, gave a clear estimation of factors that influence refinement during pumping. These factors involved surfactant type and surfactant concentration, where higher surfactant concentration induced higher efficiency with regard to droplet disruption. The most important part of the result of the investigation is, however, the experimental proof that the shear modulus versus droplet size is not necessarily expressed by a linear reciprocal dependence, but by an exponent equal to 2. It was shown that the stability of highly concentrated emulsions in shearing is determined by some critical value of deformation with reference to the yielding conditions. This value on average equalled 0.07 for all samples under investigation.