Enhanced sensitivity of resonant liquid sensors by phononic crystals

Abstract

Mechanical resonators have a long tradition. We concentrate on new results with a sensor for liquid analytes, the phononic crystal (PnC) sensor. Here, the liquid analyte becomes the integral part of a phononic crystal. The liquid-filled cavity acts as a defect in an otherwise regular structure. The sensor probes the entire liquid volume. The primary sensor input value is the speed of sound in the liquid; the primary output parameter is a shift in the resonance frequency. We theoretically analyze 1D- and 2D-PnC sensors. An optimal relation of frequency shift and bandwidth of the resonance is the key to an enhanced sensitivity of the sensor to liquid analyte properties. We introduce a new 2D PnC sensor design concept: The sensor-specific feature is an analyte-filled point defect. This defect becomes the analyte-filled capillary in the real sensor. This is the step toward the integration of PnC and microfluidic components. Electromechanical transducers excite and detect longitudinal acoustic waves along the channel, not at the front ends of the capillary. The sensor-specific task of the 2D-PnC is the conversion of this longitudinal wave into the axisymmetric mode in the liquid-filled cavity. In contrast to other modes, this mode avoids shear displacement at the solid-liquid interface and thereby absorption of acoustic energy due to liquid shear viscosity. Experiments prove the correctness of our approach.