Periodic elastic composite structures attract great attention. They offer the ability to design artificial properties to advance the control over the propagation of elastic/acoustic waves. In previous work, we drew attention to composite periodic structures comprising liquids. It was shown that the transmission spectrum of the structure, specifically a well-isolated peak, follows the material properties of liquid constituent in a distinct manner. This idea was realized in several liquid sensor concepts that launched the field of phononic crystal liquid sensors. In this work we introduce a novel concept—narrow band solid-liquid composite arrangements. We demonstrate two different concepts to design narrow band structures, and show the results of theoretical studies and results of experimental investigations that confirm the theoretical predictions. This work extends prior studies in the field of phononic crystal liquid sensors with novel concepts and results that have a high potential in a field of volumetric liquid properties evaluation.

A two-dimensional triangular lattice solid/fluid phononic crystal (PnC) is proposed as a sensitive biosensor to detect the temperature of the Methyl Nonafluorobutyl Ether (MNE) in the range of 10–40 °C. Temperature changes in MNE represent an important issue for its various applications. Indeed, the MNE is widely used in cosmetic and beauty products for its non-toxic, non-flammable, and colorless chemical properties. For this purpose, a sensitive biosensor for temperature and physical properties of MNE appears necessary. The PnC biosensor consists of a triangular lattice of tungsten cylinders embedded in an epoxy background inside of which a line of hollow cylinders filled with MNE is introduced as a waveguide for guiding resonant modes with low group velocity. We show that the PnC biosensor provides sharp guided modes in the bandgap with high quality factors and frequencies depending on the MNE temperature with high sensitivity. The introduction of damping inside the liquid shows that the shear viscosity, compared to longitudinal, affects drastically the amplitude of the resonant peaks. However, the length of the waveguide can be adapted so as to recover the resonant peak in the presence of viscosities.

Resonant mechanical sensors are often considered as mass balance, which responds to an analyte adsorbed on or absorbed in a thin sensitive (and selective) layer deposited on the surface of the resonant device. In a more general sense, the sensor measures properties at the interface of the mechanical resonator to the medium under inspection. A phononic crystal (PnC) sensor employs mechanical resonance as well; however, the working principle is fundamentally different. The liquid medium under inspection becomes an integral part of the PnC sensor. The liquid-filled compartment acts as a mechanical resonator. Therefore, the sensor probes the entire liquid volume within this compartment. In both sensor concepts, the primary sensor value is a resonant frequency. To become an attractive new sensing concept, specifically as a bio and chemical sensor, the PnC sensor must reach an extraordinary sensitivity. We pay attention to the liquid viscosity, which is an important factor limiting sensitivity. The main part of our analysis has been performed on 1D PnC sensors, since they underlie the same material-related acoustic dissipation mechanisms as 2D and 3D PnC sensors. We show that an optimal relation of frequency shift to bandwidth and amplitude of resonance is the key to an enhanced sensitivity of the sensor-to-liquid analyte properties. We finally address additional challenges of 2D and 3D PnC sensor design concept. We conclude that the sensor should seek for a frequency resolution close to 10⁻⁶ the probing frequency, or a resolution with speed of sound approaching 1 mm s⁻¹, taking water-based analytes as an example.

Phononic crystals are resonant structures with great potential to be implemented in applications as liquid sensors. The use of the symmetry reduction technique allows introducing relevant transmission features inside bandgaps by creating defect modes in a periodic regular structure. These features can be used as measures to quantify changes in the speed of sound of liquid samples that could be related to the concentration of analytes or the presence of pathogens among other interesting applications. In order to be able to implement this new technology in more challenging applications, such as biomedical applications, it is necessary to have a very precise and accurate measurement. Changes in temperature greatly affect the speed of sound of the liquid samples, causing errors in the measurements. This article presents a phononic crystal sensor that, by introducing additional defect modes, can carry out differential measurements as a temperature compensation mechanism. Theoretical studies using the transmission line model and analytes at various temperatures show that the proposed temperature compensation mechanism enhances the performance of the sensor in a significant way. This temperature compensation strategy could also be implemented in crystals with different topologies.

A phononic crystal device is investigated as sensor platform utilizing resonant transmission. Here, we use a phononic crystal plate consisting of a solid matrix and liquid-filled holes. The device is fully immersed into the liquid in such a way that the incidence direction of sound is normal to the plate. A characteristic transmission peak, calculated using finite-difference time-domain (FDTD) and determined with two different experimental setups, has been found to strongly depend on the liquid sound velocity. The peak maximum frequency serves as measure for liquid composition.