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The use and production of high quality emulsion with a fine and homogeneous droplet size distribution is a key factor in several industries, i.e. pharmaceutical and food technology. In particular the premix emulsification process allows the adjustment of the droplet size distribution according to the membrane and pore structure. Furthermore it is a low shear process suitable for shear sensitive media (i.e. biological systems). The influence of the process parameters on the final product (droplet size distribution) has already been well investigated in literature, whereas the mechanisms regarding the droplet deformation and breakup within the membrane structure are still unknown. The deformation and breakup process is determined by local shear and strain stresses. Local stress peaks, but also frequently stressing (stress-residence-time) of the droplet interface may cause breakup of the droplet. Furthermore shear sensitive emulsifiers (i.e. proteins) are affected by local and time dependent stress distributions, which may have an effect on the quality of the final product. Hence, in this work numerical investigations with the Volume-of-Fluid-Method were performed to understand stress related breakup mechanisms in microporous structures.
A Volume-of-Fluid method formulation within OpenFOAMs InterFoam solver was implemented. This model enables the calculation of local shear and strain conditions (stress tensor) at the interface of a liquid-liquid system. With this extension the stress residence time behavior at the liquid-liquid interface in idealized pore structures and elementary flow constrictions has been analyzed. Parameters were varied by the pore geometry, capillary number, contact angle and droplet size. The parameters influence on the droplet deformation and breakup process is analyzed to derive the main stress related mechanisms for droplet breakup [1]. The simulation procedure was extended to real membrane pore geometries (CT-scans) and compared to the stress conditions and break up mechanisms received from the idealized pore geometries. Local and integral stress conditions gives insight of the geometry and time dependent droplet deformation and breakup process enabling new approaches in membrane design.
This project has been funded by the German Research Foundation (DFG) within the SPP 1934 “DiSPBiotech” (Tobias Wollborn) and the MIMENIMA Graduate School (Laura Luhede and Alexander Schulz) at the University of Bremen. The resources for the numerical calculations have been provided by The North-German Supercomputing Alliance (HLRN).
[1] T. Wollborn, L. Luhede, and U. Fritsching, “Evaluating interfacial shear and strain stress during droplet deformation in micro-pores,” Phys. Fluids, vol. 31, no. 1, p. 012109, 2019.
