Analysis of composite resonators for ultrasonic micromanipulation and separation

Piezoelectric composite resonators for the separation or manipulation of particles dispersed in a liquid by means of acoustic forces are investigated theoretically and experimentally. Such devices typically comprise at least four material layers: a piezoelectric transducer, a carrier (e.g., glass), the liquid (suspension), and an acoustic reflector. A mathematical matrix model for multilayer structures [M. Gröschl, Acust. Acta Acust. 84, 432–447 (1998)] is used to calculate the electric admittance spectrum of the resonator and results are compared to measurements obtained with a specialized computer-controlled measuring system. Additionally, the model allows the calculation of the spatial course of electric and acoustic field quantities (e.g., displacement amplitude, energy flow, energy density) along the multilayer structure. To characterize resonators for particle separation, the resonator performance number is introduced, which is a measure for the stored acoustic energy in the liquid (suspension) with respect to the total electric energy supplied by the driving source. It is shown that, for a given setup, the performance number depends strongly on frequency. The frequency range of maximum performance numbers defines the optimal operating range of the particle separator. [Work supported in part by the European Commission, Project ♯ FMRX-CT97-0156.]