Probing the interfacial rheology of complex emulsions using microfluidic droplet deformation dynamics

Chemically stabilized emulsions can often be difficult to separate, since diffusion of surfactant molecules from the bulk and their subsequent adsorption at the liquid-liquid interface reduces the interfacial tension between the two phases, leading to increased emulsion stability. Additionally, surfactant molecules can render the interface viscoelastic, posing a challenge for coalescence of droplets. Here we present a microfluidic platform for investigating the fundamental physical properties affecting coalescence of surfactant-stabilized liquid-liquid interfaces. In previous work, a dynamic microfluidic tensiometer was used to measure the time-dependent interfacial tension from deformation of droplets traveling through a contraction geometry. To further investigate the interfacial rheology of surfactant-stabilized emulsions, a microfluidic platform is used to perturb droplet shape. The shape relaxation dynamics can be correlated with interfacial moduli on length and time scales relevant to emulsion separation applications. The results of this work will be used to infer properties of complex emulsions which can impact the coalescence behavior of micron-scale droplets and the subsequent separation of these emulsions.