Coalescence and compression in centrifuged emulsions studied with in situ optical microscopy

We report an experimental method to investigate droplet dynamics in centrifuged emulsions and its application to study droplet compression and coalescence. The experimental setup permits in situ monitoring of an ensemble of droplets in a centrifuged monolayer of monodisperse emulsion droplets using optical microscopy. We studied a hexadecane-in-water emulsion stabilized by the ionic surfactant sodium-n-dodecyl sulfate as a model system. With a microfluidic T-junction emulsion droplets of 97 mm diameter are produced which are subsequently inserted into a rectangular glass chamber of 100 mm height. Using an emulsion which is stable against coalescence, we measured the steady-state oil volume fraction in the compressed layer as a function of the compressive force induced by centrifugation, and quantified the deformation of droplets upon compression. To induce coalescence, we decreased the SDS bulk concentration by dilution of the continuous phase with water before the start of centrifugation. The growth rate of the separated oil phase, which forms on top of the emulsion, and the extent of drop–drop coalescence in the droplet layer upon centrifugation were evaluated as a function of the radial acceleration. The potential of the method for studies in emulsion science and possible improvements of the experimental setup are discussed.