Material property measurement using the quasi-Scholte mode—A waveguide sensor

Abstract

In the food industry and other industries, rheological measurements and determination of particle sizes in suspensions and emulsions is of great importance for process and quality control. Current test cell based ultrasonic methods exist but are often inconvenient. An attractive alternative could be to insert a simple measurement “dipstick” into the fluid; this paper presents an initial study of the feasibility of using measurements of the velocity and attenuation of the quasi-Scholte mode on a plate to obtain the longitudinal velocity and attenuation of an embedding medium. The attenuation of the quasi-Scholte mode is caused by two mechanisms: shear leakage and attenuation due to the bulk longitudinal attenuation of the embedding material. In a calibration test the bulk longitudinal velocity and viscosity of glycerol were determined experimentally. Measurements agreed well with results from conventional methods and literature data. Quantitative results and an independent validation for honey, a very viscous fluid, are also presented. For Newtonian liquids like glycerol and honey, the shear leakage and longitudinal bulk attenuation are both related to viscosity. To demonstrate the sensitivity to nonviscous attenuation mechanisms, qualitative measurement results on fluids that mainly exhibit attenuation due to scattering are presented.

Dispersion and Sensitivity Analysis of Quasi-Scholte Wave Liquid Sensing by Analytical Methods

“…The liquid layers are also handled with 4-row column vectors, although they do not support shear motion or stress. As presented in [3], defining a very small shear velocity (~1 m/s) for the liquid layer is sufficient to define the system appropriately. Also, semi-infinite half space definition is utilized by defining very large the top and bottom layer thicknesses (~1000 m).…”

“…Scholte wave velocity is always lower than the longitudinal velocity of the fluid and the transverse velocity of the interfacing solids [21]. Sensing applications with quasi-Scholte waves have been reported for liquid characterization [3,11,[22][23][24] with their most important advantages being single transducer compatibility, high sensitivity to perturbations in liquids, and lossless transmission through the waveguide enabling separation of transducer and the target. Dipstick sensing system can be employed, which briefly consists of a thin plate solid waveguide, to which a shear wave transducer is attached to one end, and the other side is immersed in the target liquid.…”

“…They are nondispersive for acoustically thick interfacing media; and dispersive propagation can be observed, for example, when an acoustically thin (i.e., for small thickness-frequency products) solid waveguide is immersed in a liquid. Dispersive Scholte waves, named as quasi-Scholte mode [3], have varying phase and group velocity as a function of frequency. The energy content of the waves is concentrated in a few wavelengths around the interface in each medium with its distribution depending on the material properties, thickness, and frequency at quasi-Scholte mode.…”

“…The received signal is postprocessed to extract Scholte wave velocity that is used to characterize the fluid. Interested readers may refer to [2,3,22] for further details on the experimental setup and postprocessing.…”

“…Ultrasonic sensors have become popular in the recent decades in a wide range of applications for research purposes and as commercial products such as displacement sensors [1], in liquid property measurements [2][3][4][5][6][7][8][9][10][11], cure monitoring [12], material property determination [13,14], filling level sensor [15], temperature sensing [16][17][18][19], and structural health monitoring [20]. Ultrasonic sensing relies on monitoring quantitative changes in ultrasonic wave propagation, usually in terms of ultrasonic wave velocity or attenuation resulting from perturbations in target sensing media/structure.…”