Emulsion processing—from single-drop deformation to design of complex processes and products
Introduction
Dispersion processing of a liquid in another immiscible liquid in order to produce emulsion-based products is a major field of interest for pharmaceutical, food, cosmetics and chemical industries. Related processes are in general continuous flow processes in which a disperse microstructure is generated under laminar and/or turbulent flow conditions. To tailor-make emulsion-based microstructures with distinct functional properties, the material behavior under processing and application conditions has to be known. As a consequence, single- or multi-drop deformation behavior and structure building under process-related flow conditions (1) the impact of process design and processing parameters on the microstructure–rheology relationships (2) and the final product microstructure-based quality characteristics (3), have to be known. Typical flow-processing operations for drops/emulsions are disperse mixing, pumping, spraying and extruding in which laminar and/or turbulent flow conditions are present. Under laminar flow conditions that are applicable for highly viscous systems, mixed shear and elongation flow fields act most frequently. The droplets are deformed, broken up (dispersing) and possibly shape-fixed (micro-structuring, encapsulation). Drop size and size distribution, interfacial properties and structuring elements in the continuous fluid phase (e.g., macromolecules) may contribute to the formation of a complex network structure in emulsion systems. This structure determines the material behavior of the final product under various possible application conditions and is consequently closely related to product quality characteristics.
Deformation and breakup behavior of the disperse drops are strongly dependent from the rheology of the disperse and continuous fluid phases as well as from the interface properties.
A detailed description of the velocity field in the dispersing process can be received from flow visualization experiments and from computational fluid dynamics (CFD). The coupling of local velocity field information and rheological material functions, which are received from rheometric shear and elongation experiments, allow for determining the related local shear and normal stress distributions. Microstructuring is triggered by the stresses acting locally in the flow field and occurs if critical structure specific stresses are exceeded. For strong deformation and related breakup deformable disperse units have to experience a critical total deformation along their flow tracks with respect to the overlaid relaxation. This requires sufficiently high deformation rates and residence times.
Section snippets
Emulsions and drop interfaces
Emulsions are in the simplest case two-phase systems consisting of a polar, hydrophilic and a non-polar hydrophobic fluid phase. Depending on the disperse phase, the related systems are denoted as O/W (oil in water) and W/O (water in oil) emulsions. Depending on the mean drop diameter, one divides into micro- (10–100 nm), mini- (100–1000 nm) and macro-emulsions (0.5–). Both 2 and 3 are thermodynamically unstable, thus tending to drop re-coalescence and phase separation.
Emulsion drop
Conclusions
The introduced coupled scheme of flow experiments, numerical flow simulation and modeling have proved to be a powerful toolbox for improved dispersing apparatus design, scale-up and optimization as well as for tailor-made functional product design.
Acknowledgements
The authors would like to thank their co-workers, in particular the Ph.D. students S. Kaufmann, C. Cramer and T. Hövekamp and the mechanical workshop of the LMVT laboratory at ETH Zürich for their contributions to this work, as well as the Swiss National Research Foundation (SNF), the European Union (EU) and the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AIF, Germany) for their financial support.
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