Our company developed a unique miniature component, the Optical Micro-Spring™ (OMS), a miniature load cell capable of measuring high loads in a wide dynamic range while preserving its dimensions within a micrometer under pressure. OMS is based on the "Whispering Gallery Mode" (WGM) technology. WGM employs dielectric micro-resonators with light captured inside. A minute change in the size of the micro-resonator (such a glass sphere or crystal disk) that is subjected to external force alters the resonator spectrum and manifests itself as a shift in the resonant frequency.

OMS is a 100-500 micron resonator (micro-sphere) coupled with an optical fiber and pressing walls. Uniqueness of WGM resonator is its miniature size and extremely high quality factor that allows for detection of displacements of the order of 1 nanometer. Such a miniscule measurable displacement allows for a design of a unique shear stress sensor that has a infinitesimally small gap between wall and floating element (less than 1 micron). Such a small gap makes sensing surface virtually continuous with the wall thus the sensor does not interfere with the flow while measuring a shear force in a wide range.

Measurement of wall shear stress is a long standing problem in fluid mechanics. For non-Newtonian fluids and mixing flows it is a critical parameter describing interaction of fluid and boundary. Wall shear stress indicates conditions of airflow over an aircraft's wing, consistency of toothpaste, or amount of plaque buildup in an artery. Too much shear stress can damage delicate biological cell structures as they are being produced, while too little shear stress will negatively affect the production of inks and paints, chocolate or cosmetics. The chemical and pharmaceutical industries are suffering from the inability to scale and predict characteristics of processing equipment ("High Shear Mixers: Still Widely Misunderstood").

Various designs of shear stress sensors are known, but none of them found commercial applications, since they all suffer from the requirement for the floating element to move for a significant distance before a displacement is detected (>0.1 mm). The gap between the sensor floating element and the walls either requires a cover that jeopardizes the shear stress measurement or creates leakage of the fluid into the sensor that does the same. Using Optical Micro-Spring™ Lenterra was able to design a wall shear stress sensor that overcomes the problem.


List of related publications