An optofluidic flow meter to determine a rate of fluid flow in a flow member includes: the flow member; a primary fluid conduit disposed in the flow member and that receives a fluid; a secondary fluid conduit disposed in the flow member; and a fiber optic comprising a fiber Bragg grating interposed between a first flow region of the primary fluid conduit and a second flow region of the secondary fluid conduit and that: physically distorts relative to a pressure differential between the primary fluid conduit and the secondary fluid conduit; and produces a shift in a Bragg wavelength in response to a physical distortion due to the pressure differential.
Optofluidics is the marriage of microfluidics and optical technology. The NIST optical flow meter uses a photonic sensor to detect pressure changes inside a microfluidic channel, which is then used to calculate volumetric flow rates. It provides on-chip assessment of flow and heat transfer resulting in improvement in fluid metrology and advances in biological sensing. Accurately measuring flow rates is critical to various microfluidic applications such as droplet formation, particle sorting, flow cytometry and mixing.
Most current pressure measurements rely on external pressure transducers. However, due to pressure dissipation and delays in transmission, it is difficult to accurately measure local pressure in a microfluidic chip using that approach. In addition, many pressure and flow meters are not compatible with liquids or biofluids.
The NIST invention, by contrast, integrates a pressure sensor into a microfluidic chip and provides measurement of microscale forces (pressure). Flow is calculated by dividing pressure drop by the fluidic resistance of the system. A fiber optic with a fiber Bragg grating in its core that reflects selected wavelengths of light is attached to a flexible membrane of 180 µm thickness. The membrane is mounted between the fluid channel and a rounded receptable above the membrane.
A column of temperature-controlled water, or alternatively air pressure above the column, determines pressure in the fluid conduit. As pressure increases or decreases, it strains the membrane and thus the fiber optic. That displacement shifts the Bragg wavelength. The Bragg shift is monitored either by measuring the change in the peak wavelength or the signal intensity (of either the light reflected from the Bragg grating or peak absorption of the light transmitted through the grating at the exit of the fiber optic). After calibrating the Bragg shift to pressure (for example, using changes in height of a column of water), then it can also be used to report volumetric flow rate when scaled by the fluidic resistance of the system.