Properties of emulsions such as stability, viscosity or color can be influenced by the droplet size distribution. High-pressure homogenization (HPH) is the method of choice for emulsions with a low to medium viscosity with a target mean droplet diameter of less than 1 µm. During HPH, the droplets of the emulsion are exposed to shear and extensional stresses, which cause them to break up. Ongoing work is focused on better understanding the mechanisms of droplet breakup and relevant parameters. Since the gap dimensions of the disruption unit (e.g., flat valve or orifice) are small (usually below 500 µm) and the droplet breakup also takes place on small spatial and time scales, the resolution limit of current measuring systems is reached. In addition, the high velocities impede time resolved measurements. Therefore, a five-fold and fifty-fold magnified optically accessible coaxial orifice were used in this study while maintaining the dimensionless numbers characteristic for the droplet breakup (Reynolds and Weber number, viscosity and density ratio). Three matching material systems are presented. In order to verify their similarity, the local velocity profiles of the emerging free jet were measured using both a microparticle image velocimetry (µ-PIV) and a particle image velocimetry (PIV) system. Furthermore, the influence of the outlet geometry on the velocity profiles is investigated. Similar relationships were found on all investigated scales. The areas with the highest velocity fluctuations were identified where droplets are exposed to the highest turbulent forces. The Reynolds number had no influence on the normalized velocity fluctuation field. The confinement of the jet started to influence the velocity field if the outlet channel diameter is smaller than 10 times the diameter of the orifice. In conclusion, the scaling approach offers advantages to study very fast processes on very small spatial scales in detail. The presented scaling approach also offers chances in the optimization of the geometry of the disruption unit. However, the results also show challenges of each size scale, which can come from the respective production, measurement technology or experimental design. Depending on the problem to be investigated, we recommend conducting experimental studies at different scales.

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual industrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely difficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To overcome this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in conjunction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion.

For the research on droplet deformation and breakup in scaled high-pressure homogenizing units, a pressure stable inline droplet generator was developed. It consists of an optically accessible flow channel with a combination of stainless steel and glass capillaries and a 3D printed orifice. The droplet size is determined online by live image analysis. The influence of the orifice diameter, the mass flow of the continuous phase and the mass flow of the disperse phase on the droplet diameter were investigated. Furthermore, the droplet detachment mechanisms were identified. Droplet diameters with a small diameter fluctuation between 175 µm and 500 µm could be realized, which allows a precise adjustment of the capillary (Ca) and Weber (We) Number in the subsequent scaled high pressure homogenizer disruption unit. The determined influence of geometry and process parameters on the resulting droplet size and droplet detachment mechanism agreed well with the literature on microfluidics. Furthermore, droplet trajectories in an exemplary scaled high-pressure homogenizer disruption unit are presented which show that the droplets can be reinjected on a trajectory close to the center axis or close to the wall, which should result in different stresses on the droplets.

High‐pressure homogenization is a commonly used process to produce emulsions with a droplet size of less than 1 μm. During the process, a pre‐emulsion is pumped with a pressure of several mega Pascal through a disruption unit, where the droplets are deformed and subsequently broken up in the turbulent area of the disruption unit. The scope of this investigation is to determine the influence of the droplet trajectory on the droplet size distribution of emulsions of different viscosity ratios or interfacial tension. Measurements of the droplet deformation prior to the droplet breakup using image‐processing tools complemented the observations. In addition, computational fluid dynamics simulations were performed to determine the stress history on the droplet trajectories. It was found that droplets on a trajectory close to the wall are more deformed when leaving the disruption unit compared to droplets on the centreline. The deformation of droplets at the edge of the jet increases downstream the disruption unit until it is finally disrupted. The simulation results support the experimental data, as it can be shown that shear and strain stresses on the trajectories close to the wall significantly exceed the stresses on the trajectories on the centreline. For an emulsion with a viscosity ratio greater than 3, droplets on a trajectory close to the wall resulted in smaller droplets and narrower droplet size distribution, while no significant influence was found for smaller viscosity ratios. Lowering the interfacial tension results in a stronger deformation, which was more pronounced for lower viscosity ratios (λ ≈ 3).

High-pressure homogenization is a popular method to produce emulsions of droplet sizes smaller than 1 mm. Regarding the role of cavitation in the process, it is still discussed controversially whether it promotes or hinders droplet breakup during high-pressure homogenization. In this study droplet breakup in a cavitating flow of an optically accessible highpressure homogenizer orifice is visualized using a high-speed camera setup. The cavitation regime is characterized using laser light induced luminescence. Droplet deformation and breakup events are compared to a cavitation-free flow with the same specific energy input. Once the jet cavitation regime is reached, droplet breakup took place further downstream the orifice exit, and the larger droplets are measured in the final product.