All fiber-optic viscosity, density, and temperature measurements of liquids using a photothermally actuated cantilever

An all-fiber-optical method is presented to monitor densities, viscosities, and temperatures of Newtonian liquids. The actuation is performed by photothermally heating the base of a steel cantilever with an intensity-modulated 808 nm diode laser. The cantilever vibrations are measured with an in-fiber Fabry–Pérot cavity sensor attached along the length of the cantilever. When immersed in a viscous fluid, the cantilever response can be related to the fluid properties: a shift in the resonance frequency corresponds to a change in fluid density, and the width of the resonance peak gives information on the dynamic viscosity after calibration of the system. Aqueous glycerol and sucrose samples in the density range of 0.997–1.17 g cm⁻³ and in the viscosity range of 0.89–8.49 mPa s were used to investigate the limits of the sensor. Representative beverage samples were also analyzed as unknowns.