Phononic-fluidic cavity sensors for high-resolution measurement of concentration and speed of sound in liquid solutions and mixtures

A phononic-fluidic cavity sensor is a new type of acoustic fluid sensor to measure volumetric liquid properties. In our work, it consists of solid-air 3D phononic crystal (PnC) layers confining a fluidic cavity resonator to generate a strong, well separated cavity resonance within the phononic band gap. This allows for the measurement of changes in speed of sound of a liquid analyte with very high, linear sensitivity. In this work, we present theoretical and experimental results for very sensitive determination of sodium chloride and glucose concentrations in aqueous solutions. The 3D-printed measurement cell consists of a rectangular liquid chamber surrounded by an optimized PnC with a simple cubic ball and beam design acting as a metamaterial combining Bragg and local resonance scattering to create optimal boundary conditions for the liquid cavity resonator. Analytical transmission line modeling is used to illustrate the working principle of the sensor. Numerical finite element models describe the phononic band structure and transmission behavior, as well as the frequency response of the sensor element at different mass fractions of the sample solutions as validation for our experiments. A high sensitivity of concentration and subsequently speed of sound is demonstrated over a very large concentration range of 0%–30%.