The formulation of high quality emulsions is a key challenge in many industrial applications. The premix emulsification process in porous membranes enables the generation of tailored emulsions with fine and narrow droplet size distributions under low shear and energy input. However, the droplet deformation and breakup process within porous structures is a complex mechanism and single breakup events are hard to relate to the local stress conditions and the pore geometry. This relation however is required for the proper design of membrane structures with specific emulsification behavior (i.e., avoidance of stress peaks). Thus, in this contribution, the stress residence time behavior of single droplets during deformation and breakup in idealized micro-pores is investigated for different Capillary numbers and droplet sizes. The interface stress induced droplet deformation and breakup process is to be analyzed in a generic flow configuration. The results show that interface stresses are applied by the wall interface (wall-droplet interface) and by the liquid-liquid (continuous-droplet interface) interface and that both stress contributions have to be considered separately in order to understand the droplet deformation and breakup process. Only at the liquid-liquid interface, stress induced deformation is possible. The analysis of the stress conditions delivers a correlation between the stress residence time behavior and the interface deformation, which can be directly related to the pore geometry. As a result, main deformation and breakup trends are derived. This enables better opportunities for proper membrane design and handling of shear sensitive media in the premix emulsification process.
Multiple liquid emulsions of the water in oil in water (W1/O/W2) type are used in a variety of consumer or technical applications, for instance in the encapsulation of certain active ingredients. The encapsulation process and release mechanisms of the inner phase of the carrier drops are important in order to properly process and formulate such liquid‐liquid systems. In this work the stability and breakage of multiple W1/O/W2 emulsions under mechanical shear stress are investigated for emulsions with different surfactants and surfactant concentrations of the internal emulsion. Stressing the emulsions in a mechanical stirring process is compared to the membrane emulsification process. The membrane emulsification process results in higher encapsulation efficiencies than the stirring process. The emulsion droplets were subjected to shear stress below and above the critical capillary number for drop breakup. The results show that stable inner emulsions with sufficient surfactant concentrations increase the overall encapsulation efficiency for multiple emulsions subjected to shear stress, although the effect is not prominent. The depletion of the carrier oil droplets could be achieved for Ca numbers below the critical limit, reducing the encapsulation efficiency below 10 %. This shows that even a low shear stress can result in content release from the internal droplet phase. The experimental emulsion release study is supported by a numerical simulation of drop deformation and break‐up under shear stress.
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