Interferometric phononic crystal liquid sensors

In the thesis study, liquid density sensors based on Mach-Zehnder interferometer using two-dimensional phononic crystals consisting of periodic solid scatterers in water were designed with numerical calculation methods and their properties of determining the ratio of methyl alcohol (methanol) in ethyl alcohol (ethanol) were investigated. Phononic crystal band structure, linear defect band structures, acoustic properties of the Mach-Zehnder interferometer and the change in the frequency response of the interferometer depending on the methanol by volume ratio in ethanol were investigated with the Finite Element Method simulations. A wide band gap between 119.673-263.901 kHz was observed in the phononic crystal, which consists of 2.0 mm radius steel bars placed in a square lattice arrangement with a lattice constant of 4.2 mm in water. In the empty line defect waveguide obtained by removing a row of steel bars, the linear defect band providing transmission between 187.460-238.964 kHz was observed. The basic dispersion properties of this band do not change significantly when a low density polyethylene hose with inner and outer radii of 1.0 mm and 1.4 mm, respectively, filled with ethanol is placed in the guide center, and the transmission band covers the operating frequency of 200 kHz. Transmission with little loss was observed at 45°, 90° and 135° bends of the linear guides. Additionally, it was determined that 50-50% of the T and Y branching of the guides act as bundle separators. At the output of the interferometer, which is formed based on 90° bending and T branching, 3 high quality factor transmission peaks occur around 200 kHz. It was observed that the peak frequency of these peaks shifted linearly to lower values (red) at 180 Hz, 160 Hz, 140 Hz by increasing the methanol ratio in ethanol in the range of 0-20%. Also, a fourth peak appeared around 202 kHz for non-zero methanol ratios. It was observed that the intensity of this peak, whose quality factor is around one hundred thousand, increases with the methanol ratio and the peak frequency shifts to red at 180 Hz. These results showed that the proposed Mach-Zehnder interferometer can detect methanol ratio in ethanol with high sensitivity. This sensor also provides linear response, is label-free and does not require warm-up or recovery time. The sensor can also be calibrated and used for other binary liquid mixtures.