Soft sensor for measuring instantaneous wall stress distributions in turbulent wall flows

Obtaining wall stress distribution in turbulent flow have overarching implications. Past technologies based on piezo-electric materials resulted in costly and bulky sensors with low frequency responses and measurement sensitivities. Enabled by our recent success in synthesizing wrinkle-free nm metallic thin film encased in polymers (i.e. flexible mirror), we are developing a technique capable of measuring non-intrusively and simultaneously pressure and shear stress distributions at high spatial resolutions. To achieve such capabilities, we create an array of polymer filled mWells in a substrate with only their top surfaces open to flows. The nm-scale 3D deformations within each mWell (pixel), caused independently by its interactions with local flow shear and pressure, is obtained by measuring remotely deformation of a nm thin film mirror embedded horizontally within each mWell by microscopic interferometry. Using interfacial jamming by nanoparticles, we have successfully fabricated miniaturized polymer encased flexible mirrors and microscopic interferometry. Initial assessment shows the sensitivity of ~4Pa and response frequency of 170 kHz. Additional validations are ongoing using our benchtop shear facility.