On the visualization of droplet deformation and breakup during high-pressure homogenization

The properties of emulsions are strongly influenced by the size distribution of the droplets. In order to achieve droplets on a microscale, high-pressure homogenization is used to transfer stresses to the droplet surface in the flow field upstream, in and downstream the disruption unit of the homogenizer. The droplets are deformed and eventually break up when exceeding critical values. Inline measurement techniques are still very challenging, due to highly complex flow conditions on microscales, high process pressures and large velocities. In this work, the optical flow measurement technique micro particle image velocimetry (μPIV) is used to quantify the flow field, the local stresses as well as droplet deformation and breakup. A special homogenization orifice which is optical accessible enabled the visualization in the whole area of interest before, in and after the restriction up to 80 bars homogenization pressure. The study of the single-phase flow with particular focus on the local stresses showed laminar and transitional conditions at Re number ranging from 285 to 1280. Droplets of two different viscosities are then examined at these conditions while passing the orifice. At the inlet, their size, deformation and position are investigated by an automated image processing algorithm and correlated with the local velocity gradients. At the outlet and downstream, deformation and breakup of droplets are shown within the possibilities of the μPIV and discussed in relation to known droplet breakup mechanisms. Finally, the droplet size distributions offline obtained by static light scattering are compared with observed phenomena of the individual drops in order to gain insights into droplet disruption in high-pressure homogenization.