Nanoscale Interferometric Sensors to measure instantaneous wall shear and normal pressure in high Re in turbulent boundary layers

Measuring instantaneous wall shear and pressure distributions in a high Re_\tau (>9,000) turbulent boundary layer is of critical importance in understanding the role of near wall flow structures (e.g. super structures) interacting with wall. Due to a wide range of scales in high Re boundary layers and the magnitude disparity between pressure and shear stresses, measuring the instantaneous wall stresses at sufficient resolution remains a challenge. To address the needs, we have developed Nanoscale Interferometric Sensor (NIS) suitable for measuring instantaneous wall stresses at high spatiotemporal resolution. Each NIS sensor consists of an array of 200um (D) x 1mm (H) wells that are flush filled with a polymeric mixture (Polydimethylsiloxane & gels) with tunable elastic moduli (E), ranging from 10kPa to 2 MPa. Each well is topped with a ~100 nm (H) x 100um (D) nanobump made of silicon nitride. Under the flow stresses, the polymeric sensor deforms readily in wall normal direction (pressure), while the hydrodynamically smooth optically rough nanobump amplifies the shear deformation (shear). These nm deformations are captured by microscopic Mach-Zehnder Interferometry system and wall stresses are obtained with calibration.