Leveraging optical activity in visualizing particle-laden flows

In this research, visualization of particles is achieved by leveraging optical activity that is inherent in the helical and crystalline structure of many substances. Optically active particles (OAPs) modify the polarization of the incident light transmitted through them. Here, we propose to use an inline optical setup to visualize OAPs by imaging their forward transmitted light while blocking the high intensity background illumination that would otherwise saturate the camera. We show that the background illumination can be effectively blocked by placing OAPs between either two perpendicular linear polarizers or two properly paired couples of a linear polarizer and a quarter wave plate. We show that in such a setup, there is no need for powerful lasers like those used in dominant side-scattering based techniques (e.g., particle image velocimetry), and less intense light emitting diodes may be used. The proposed technique was validated against detailed digital holographic measurements of the translational velocities and orientations of (optically active) nylon fibers suspended in a fully developed turbulent channel flow. The results using both techniques compared well showing the capability of this technique in the field of particle-laden flows.