A novel approach for differentiation of liquid samples with surface acoustic wave transducers and embedded microcavities

We discuss a novel method for sensing and differentiation of analytes trapped in a microcavity with an emphasis on liquids. The proposed sensing mechanism relies on capturing the analyte of interest in a microcavity etched on the delay line in contrast to the conventional mass loading method. The structure mainly consists of input and output interdigitated transducer (IDT) electrodes in an otherwise standard delay line configuration operated in Rayleigh mode along with a microcavity etched between the IDTs to trap minute amounts of liquids. Firstly, the responses of the system with the microcavity are explored using finite element method (FEM) analysis. Then, experimental results from delay lines on two different substrates, namely, Y-Z lithium niobate and ST-X quartz are analyzed. The system can distinguish between liquids with glycerin concentrations ranging from 60% to 90% in water and less than 5 pL in volume in the high frequency range of 197 MHz and 213 MHz based on frequency and phase shift readings. Lithium niobate samples with 1.2 μm deep microcavities provide an overall frequency sensitivity of −7.7 kHz/(% glycerin). Quartz samples with 8.5 μm deep microcavities have a sensitivity of −0.13°/(% glycerin). The minimum density–viscosity product experimentally differentiated using embedded microcavities is 1.9 kg/m2 √s. It is concluded that this method can be used to trap and interrogate minute amounts of liquids with different properties. Experimental results demonstrate that our approach can possibly be extended to certain solids, and to more complex structures like single biological cells.