Tunable local sensitivity in ultrasound waveguides via selective tapering: Newtonian viscosity sensing

This work enables reliable in situ viscosity sensing in highly viscous fluids using a selectively tapered torsional ultrasound waveguide, which spatially localizes viscous fluid-induced attenuation and overcomes the inherent sensitivity–attenuation trade-off of deep-immersion waveguide sensors. Large tanks, such as polymer reactors and food processing systems, require immersion depths exceeding 0.5 m, where significant attenuation limits practical implementation. For example, approximately 4% signal loss occurs at 25 mm immersion for a 1.5-mm-radius SS304 waveguide in a fluid with viscosity η≈600 cP. Viscosity sensitivity increases as waveguide radius decreases, but reducing the radius also increases energy loss and limits usable immersion length. Selective tapering confines viscous dissipation to the reduced-radius section while maintaining low-loss wave propagation through the thicker section. Finite-element analysis and experiments show that taper geometry governs wave focusing and acts as a controlled reflection source, thereby localizing viscous dissipation within the uniform reduced-radius section downstream of the taper. Experiments using Newtonian fluids spanning viscosities from 632 to 38 552 cP confirm that attenuation is governed by localized surface shear stress rather than total wetted length. Consequently, viscosity sensitivity is localized over a wavelength-scale length (∼30 mm at 100 kHz), with sensitivity tunable by a factor of ∼2 for representative diameter contrasts (e.g., 10 vs 3 mm). More generally, taper geometry (length and radius ratio) governs the magnitude of the localized sensing, while the reduced-radius section provides localized viscosity sensing, and the thicker section maintains efficient, low-loss wave propagation. Sensitivity is further influenced by excitation frequency, waveguide radius, and immersion length.