Direct measurement of vorticity using tracer particles with internal markers *

We demonstrate an optical imaging technique to obtain a direct measurement of 3D vorticity in a flow field, based on the measurement of the instantaneous rotational rate of microscale tracer particles. The tracer particles of ~50 µm with internal markers (~2 µm) are fabricated using a flow-focusing microfluidic device. Digital inline holography (DIH), which consists of a collimated coherent light beam and a digital camera that capture the diffraction signals (holograms) from the objects within the beam path, is employed to image several tracer particles within a field of view of centimeter scale. The holograms are then processed using an inverse reconstruction approach to obtain the 3D positions of each internal marker within a tracer particle. The translation and rotation of the particles are then derived from the time-resolved 3D positions of internal markers. The proposed approach is calibrated using a solid-body-rotational flow system and will be applied to probe the small-scale vorticity dynamics in different turbulent flows.