Abstract
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.
Highlights
The research on high-pressure homogenizers started in the late 19th century with the first patent of a high-pressure homogenizer [1]
The principle of high-pressure homogenizers has not changed since the first patent, droplet breakup mechanisms are still under investigation to this day [6,7,8,9,10,11,12,13,14,15,16,17]
An overview of possible breakup mechanisms can be found in [2]. They include end-pinching, binary breakup, Rayleigh–Plateau-instabilities and tip streaming. These findings from defined stationary flows cannot be directly transferred to droplet breakup during high-pressure homogenization, as the flow field in the disruption unit of the high-pressure homogenizer is complex with fast changing stresses from shear stress and elongational strain to turbulent stresses acting on the droplets during their passage through the disruption unit [6,22,23,24]
Summary
The research on high-pressure homogenizers started in the late 19th century with the first patent of a high-pressure homogenizer [1]. They include end-pinching, binary breakup, Rayleigh–Plateau-instabilities and tip streaming These findings from defined stationary flows cannot be directly transferred to droplet breakup during high-pressure homogenization, as the flow field in the disruption unit of the high-pressure homogenizer is complex with fast changing stresses from shear stress and elongational strain to turbulent stresses acting on the droplets during their passage through the disruption unit [6,22,23,24]. Innings and Trägårdh [7] built the first optically accessible high-pressure homogenizer disruption unit with a capillary in front of the smallest cross section to introduce the disperse phase They injected either a pre-emulsion with a wide droplet sizes distribution (DSD) (5–50 μm) or the pure oil disperse phase through the capillary. IInn tthhiiss ppaappeerr ccoonnssttrruuccttiivvee ddeettaaiillss aanndd ppaarraammeetteerrss wwiitthh wwhhiicchh ddrroopplleett ssiizzee aanndd ttrraajjeeccttoorryy ccaann bbee iinnfflluueenncceedd aarree rreeppoorrtteedd
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