Optical Fiber Probe Viscometer Based on Hollow Capillary Tube

Viscosity measurements of a solution are crucial for many processes involving fluid flows. The current optical fiber viscometers are complex and, in some cases, provide indirect measurements of viscosity through other non-optical effects. We developed a miniaturized optical fiber probe capable of providing an optical interferometric measurement of the viscosity of small volumes of a liquid viscous medium (less than 50 pL). The probe consists of an air cavity with a small access hole for fluids, which resulted from a simple post-processing of a hollow capillary tube. The structure behaves as a two-wave interferometer, where the intensity of the signal is sensible to the position of the air-fluid interface inside the cavity. The fluid displacement over time is obtained by monitoring the signal intensity variations, at 1550 nm, during the process of removing the sensing head from a fluid solution. Multiple sucrose solutions with viscosities ranging from 2.01 to 16.1 mPa·s were used for calibration. The viscosity of the solution is measured through the fluid evacuation velocity in the first 300 ms of resolved oscillations during the evacuation process. Reproducibility measurements, the influence of temperature, and the access hole dimensions are also addressed. The application to biological fluids is important to be considered in future studies.