Near-Critical Fluid Jets Investigated With a Polarized Structured Illumination Approach

In this work we present a modified 2p-SLIPI setup and its first application to near-critical fluid jets. The setup allows the suppression of multiple scattering while simultaneously enabling the detection of the (de)polarization (Mueller) matrix for vertically and horizontally polarized incident light. The latter is a key requirement for enabling droplet sizing based on polarized Lorenz-Mie scattering and for classifying the type of non-equilibrium phase transitions occurring close to the critical point, respectively. The application of structured illumination in both the context of polarized Lorenz-Mie measurements and near-critical jets is not straightforward. This requires the development of a technically modified optical setup compared to the commercially available solution. Its main requirements are to introduce a distinct but adjustable state of polarization and to provide a high spatial resolution. The latter is essential in order to resolve the small-scale structures of the thermodynamic non-equilibrium phenomena close to the nozzle exit. With the final configuration, we achieved a structured illumination of 18 lp/mm in a field of view of 7.7 mm × 6.5 mm with a pixel resolution of 6.5 μm. With the new setup, we investigated heated n-hexane injections into a quiescent nitrogen ambiance, and for the first time, we present multiple scattering corrected images of near-critical fluid jets. The corrected images show a considerable improvement compared to the conventional illumination. To contextualize the scattering data, an overlay with simultaneously acquired shadowgrams provides the morphological details of the fluid jets.