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Current experiment techniques for vorticity measurement suffer from limitedspatial and temporal resolution to resolve the small-scale eddy dynamics inturbulence. In this study, we develop a new method for direct vorticitymeasurement in fluid flows based on digital inline holography (DIH). The DIHsystem utilizes a collimated laser beam to illuminate the tracers with internalmarkers and a digital sensor to record the generated holograms. The tracersmade of the polydimethylsiloxane (PDMS) prepolymer mixed with internal markersare fabricated using a standard microfluidic droplet generator. A rotationmeasurement algorithm is developed based on the 3D location reconstruction andtracking of the internal markers and is assessed through synthetic holograms toidentify the optimal parameter settings and measurement range (e.g., rotationrate from 0.3 to 0.7 rad/frame under numerical aperture of imaging of 0.25).Our proposed method based on DIH is evaluated by a calibration experiment ofsingle tracer rotation, which yields the same optimal measurement range. Usingvon K\'arm\'an swirling flow setup, we further demonstrate the capability ofthe approach to simultaneously measure the Lagrangian rotation and translationof multiple tracers. Our method can measure vorticity in a small region on theorder of 100 ${\mu}$m or less and can be potentially used to quantify theKolmogorov-scale vorticity field in turbulent flows.

Direct measurement of vorticity using tracer particles with internal markers