Elastic Filament Velocimetry (EFV) is a highly sensitive technique for measuring fluid velocity. The patent pending sensing technology relies on a freestanding, electrically conductive nano-ribbon that is fixed at both ends. Since the governing dimension of the nano-ribbon is on the order of a micrometer, the forces acting on it are dominated by viscosity. The viscous drag from the passing fluid flow elongates the wire and can be measured as resistance change via a strain gauge effect. For a given wire and fluid, the flow velocity can be directly correlated to the resistance of the wire.

The sensor chip
A closeup of the nanoribbon

If the aspect ratio of the nano-ribbon is kept high (prototype design is approx. 7000 times longer than it is thick), the nano-ribbon can exhibit large deflections compared to its thickness. This previously unexplored limit allows a measurable resistance change due to the strain in the material. This can be correlated to the flow rate with very high precision. The picture to the left shows the nano-ribbon in a prototype sensor designed for water and air measurements. Since the liquids used in infusion therapy typically have viscosities as high as water or higher, this novel mode of flow sensing can be used to detect flow rates as low as 0.5ml/hour—smaller than any available technology.

Fu, M. K., Fan, Y., Byers, C. P., Chen, T.-H., Arnold, C. B., & Hultmark, M. (2017). Elastic filament velocimetry (EFV). Measurement Science and Technology, 28(2), 25301.