Shear Induced Rupturing of Nanoemulsion Droplets in Dilute and Concentrated Surfactant Solutions

We use high-pressure microfluidic injection to rupture silicone oil-in-water droplets repeatedly down to diameters below 100 nm, thereby creating ``nanoemulsions.'' These droplets are stabilized against coalescence by the surfactant sodium dodecyl sulfate (SDS). We systematically increase the SDS concentration, $C$, from 8 to 1000 mM, and we find a decrease in the droplet radius that follows a power law form:$\left\langle {a_s \left( C \right)} \right\rangle \sim C^{-\alpha }$, where $\alpha $= 1/3, over several decades in $C$ down to an average radius of $\left\langle {a_s } \right\rangle $ = 18 nm. The larger droplet radius at small $C$ may be due to reduced coverage of the deformed droplet surfaces by the surfactant, thereby facilitating shear-induced coalescence. Our observed decrease in the droplet radius deviates from the classical prediction that the radius is inversely proportional to the viscosity of the continuous phase.