A measure of flow vorticity with twisted beams of light

The measurement of vorticity, a parameter providing local measurements of rotation at every point in a flow, would greatly assist research fields as diverse as biology microfluidics, complex motions in the oceanic and atmospheric boundary layers, and wake turbulence on fluid aerodynamics. However, the precise measurement of flow vorticity is difficult. Here, we devise an experiment in which the local vorticity of a flow can be estimated by probing the fluid with Laguerre-Gauss beams, optical beams that show an azimuthal phase variation that is the origin of its characteristic non-zero orbital angular momentum. The key point is to make use of the transversal Doppler effect of the returned signal that depends only on the azimuthal component of the flow velocity along the ringshaped observation beam. We found from a detailed analysis of the experimental method that probing the fluid with LG beams is an effective and simple sensing technique capable to produce accurate estimates of flow vorticity.