Production of W/O/W double emulsions. Part I: Visual observation of deformation and breakup of double emulsion drops and coalescence of the inner droplets

We investigate deformation and breakup of double emulsion drops, in a system consisting of an aqueous continuous phase and double emulsion oil drops containing smaller droplets of water. The system is stabilized by the presence of the surfactant polyglycerol polyricinoleate (PGPR). Extensional flow generated in a 4-roll mill is used for these investigations. Steady state deformation is not influenced by the concentration of the innermost water droplets, and thus, in the range of strain rates studied, is not affected by the rheological properties of the drop phase. Moreover, drop shapes and relaxation behavior also do not depend on the disperse phase concentration of the water droplets. Finally, the critical Capillary number for drop breakup is independent of viscosity ratio. These observations seem to be at least partly the result of immobilized drop interfaces under the flow conditions that were studied. An important phenomenon is loss of the innermost water droplets by coalescence with the outer water phase. This is only observed in this system if a hydrophilic surfactant is dissolved in the continuous phase. Moreover, if coalescence occurs, coalescence phenomena are observed to be independent of the deformation and breakup of the double emulsion drop. Coalescence phenomena do not only take place within seconds but can be observed over several minutes. This implies that instabilities in multiple emulsions are not mainly governed by process conditions, but are rather an intrinsic consequence of the formulation.