Lenterra, Inc. is developing an all-optical technology for temperature, force and wall shear stress measurement which relies on monitoring optical resonances commonly known as Whispering Gallery Modes (WGMs) in a spherical micro-resonator. An optical resonance occurs when the optical path to complete a round trip inside a resonator is an integer multiple of the light wavelength. By scanning the wavelength of this light, the WGM spectrum is obtained by measuring the intensity of the light decoupled from the resonator.

Optical micro-cavity resonators have very high quality factors (Q = λ/δλ where δλ is the resonance linewidth). Q-factors on the order of 10⁷ are routinely obtained in our development facility. WGM resonances are highly sensitive to morphological changes of the microsphere (such as the size, shape, or refractive index) caused by temperature changes or applied forces. Micro-sphere radius variations as small as 0.01 nm can be detected which leads to the high sensitivity and very large (~10⁶) dynamic measurement range of these sensors.

WGM Technical Attributes and Competitive Advantages

• Direct, real-time measurement
• Miniature size (can be as small as 3 mm dia.)
• High sensitivity (7×10^{-4 o}C for temperature, 6×10^-4 N for force, 6 Pa for wall shear stress)
• Dynamic range of 10⁶
• Can work with extremely high loads
• No moving parts
• Insensitive to electromagnetic interference
• Fast response (up to 1 kHz)
• Can operate in harsh environments
• Can be deployed as a sensor array

Shear Stress Sensor

The Optical Micro-Spring™ (OMS) is a miniature load cell based on WGM technology that is capable of measuring high loads in a wide dynamic range while preserving its dimensions within a micrometer under pressure. An OMS is a 100-500 micron diameter microsphere resonator in contact with force-applying surfaces and optical fiber. The unique strengths of WGM resonators are their miniature size and extremely high quality factors that allow for detection of displacements on the order of 1 nanometer. Such miniscule measurable displacements allow for the design of a shear stress sensor that has an infinitesimally small gap between wall and floating element (less than 1 micron). Such a small gap makes the sensing surface virtually continuous with the wall thus the sensor does not interfere with the flow while measuring a shear force over 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 are found in commercial application, since they all suffer from the requirement that the floating element be able to move over 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 the Optical Micro-Spring™ Lenterra has been able to design a wall shear stress sensor that overcomes this problem.

WGM Sensor Applications

• Monitoring wall shear stress and temperature in pharmaceutical and chemical processing
• Measuring local pressure drop in utility pipelines
• Monitoring blood and respiratory flows within the body
• Aeronautical stall detection
• Ultrasensitive detection of pathogens in bioassays

Related publications can be found here.