Wall shear stress sensor based on the optical resonances of dielectric microspheres

We report an optical wall shear stress sensor based on the whispering gallery mode (WGM) shifts of dielectric micro-resonators. The optical resonators are spheres with a typical diameter of several hundred microns and they serve as the sensing element. The wall shear force acting on a movable plate is transmitted mechanically to the microsphere. As a result of the applied force, the shape of the resonator is perturbed leading to a shift of the optical resonance (WGM). The one-dimensional wall shear stress is measured by monitoring these WGM shifts. Shape perturbations of the order of a nanometer can be detected with this optical method. The measurement resolution and range can be optimized by using dielectric sphere materials of different stiffness covering a wide range of flows. Prototype sensors using PDMS spheres have been built and validated in a laminar Poiseuille flow and in a plane wave acoustic tube.