Droplet breakup mechanisms at different droplet trajectories during high-pressure homogenization

Emulsions have a wide range of applications. Among others, they are used in the cosmetics, pharmaceutical and food industries. The properties such as stability, rheological properties or color can be influenced by the drop size distribution (DSD). The process of high-pressure homogenization is mostly used for emulsions with low to medium viscosity of 1-200 mPa∙s and allows the generation of droplets with diameters less than 1 µm. A high-pressure homogenizer consists of a high-pressure pump and a subsequent comminution unit. During the process, a pre-emulsion, which still contains large droplets, is pumped through the disruption unit at a pressure of several hundred bar. In the disruption unit, a cross-sectional reduction causes a strong acceleration of the flow. At the outlet, the emulsion leaves the disruption unit as a confined free jet. As it flows through the disruption unit, the droplets of the emulsion are subjected to shear stresses and elongational strain as well as cavitation, causing the droplets to break up.
Droplet breakup has been extensively studied under steady-state laminar conditions. However, these findings cannot be applied to the process of high-pressure homogenization, since the stresses on the droplets change continuously during their passage through the breakup unit, and thus steady-state conditions are not achieved. ... mehrThe influence of process parameters and geometry on the resulting stresses and the effect of these stresses on drop breakup are the focus of current research.
Previous experimental work has shown that even small changes in geometry cause significant change in the DSD. Studies on step orifices have shown that drop breakup, also is influenced by the type and duration of the preceding deformation. This work focuses on the measurement of local flow conditions and their influence on droplet deformation and breakup in coaxial pinhole orifices. A µ-PIV measurement system is applied to determine the local velocity profile. In addition, the high-speed camera of the measurement system is used for visualization of fluorescence-labeled droplets. We work with two differently scaled systems. In addition to a full-scale system of a high-pressure orifice, we apply a 5-fold magnified system in order to be able to investigate, among other things, the influence of the trajectory on droplet deformation and breakup.
The presentation will show the setup of the measurement systems, boundary conditions for the scaling as well as results on the influence of the droplet trajectory on the droplet breakup.