An Electronic "Tongue" Based on Multimode Multidirectional Acoustic Plate Wave Propagation

Abstract

This paper theoretically and experimentally demonstrates the possibility of detecting the five basic tastes (salt, sweet, sour, umami, and bitter) using a variety of higher-order acoustic waves propagating in piezoelectric plates. Aqueous solutions of sodium chloride (NaCl), glucose (C₆H₁₂O₆), citric acid (C₆H₈O₇), monosodium glutamate (C₅H₈NO₄Na), and sagebrush were used as chemicals for the simulation of each taste. These liquids differed from each other in terms of their physical properties such as density, viscosity, electrical conductivity, and permittivity. As a total acoustic response to the simultaneous action of all liquid parameters on all acoustic modes in a given frequency range, a change in the propagation losses (ΔS₁₂) of the specified wave compared with distilled water was used. Based on experimental measurements, the corresponding orientation histograms of the ΔS₁₂ were plotted for different types of acoustic waves. It was found that these histograms for different substances are individual and differ in shape, area, and position of their extremes. Theoretically, it has been shown that the influence of different liquids on different acoustic modes is due to both the electrical and mechanical properties of the liquids themselves and the mechanical polarization of the corresponding modes. Despite the fact that the mechanical properties of the used liquids are close to each other, the attenuation of different modes in their presence is not only due to the difference in their electrical parameters. The proposed approach to creating a multi-parametric multimode acoustic electronic tongue and obtaining a set of histograms for typical liquids will allow for the development of devices for the operational analysis of food, medicines, gasoline, aircraft fuel, and other liquid substances without the need for detailed chemical analysis.

Keywords: acoustic waves of higher order; acoustic “tongue”; attenuation; food quality; liquid properties; piezoelectric plate; taste of liquid.

Figure 1

The basic design used in the study of liquid media using acoustic waves in piezoelectric plates.

Figure 2

A photo of an experimental setup with four acoustic channels on a single piezoelectric 128°Y LiNbO₃ wafer. (1) 128°Y LiNbO₃ wafer, (2) interdigital transducers (IDTs), (3) cell for liquid. The angles between the acoustic channels and the crystallographic X-axis are Θ = 0°, 30°, and 90° и 120°.

Figure 3

A typical view of the frequency dependence of the insertion losses S₁₂ of acoustic waves in 128°Y-X LiNbO₃ with distilled water (black line) and a 0.9% aqueous NaCl (σ = 1.4 S/m) solution (red line) in a cell. The arrows indicate some waves with high total losses.

Figure 4

Geometry of the problem.

Figure 5

Experimental orientational dependencies of ΔS₁₂ for acoustic waves with different operating frequencies: ~34 MHz (black line), ~37 MHz (red line), ~41 MHz (green line), ~44 MHz (blue line), and ~48 MHz (light blue line) propagating in a 128°Y LiNbO₃ plate with a normalized thickness of h/λ = 2.5 in the presence of 0.9% aqueous solutions of (a) NaCl, (b) glucose, (c) citric acid, (d) monosodium glutamate, and (e) sagebrush.

Figure 6

Combined taste histograms of monosodium glutamate (a) and sagebrush (b) aqueous solutions with concentrations of 0.02% (red line) and 0.9% (black line) measured using the entire set of acoustic modes in a given frequency range and all channels of the 128°Y LiNbO₃ plate with a normalized thickness h/λ = 2.5.