Finite Element Analysis of Lateral Field excited thickness shear sensors

Lateral field electroded (LFE) sensors have been recently introduced that can study the permittivity and conductivity (electrical properties) of liquids in contact with the surface opposite the electroded side. The unique feature of these sensors is that the response depends in part on changes in the electrical field distribution in the quartz blank due to the electrical properties in the liquid. This work uses finite element analysis (FEA) to model the past plano-plano devices and a new plano-convex design as the electrical boundary conditions on the side opposite the electrodes change from free of surface charge to constant potential, either grounded or floating. Results are presented for the mode shapes, frequencies, and motional capacitance (C_m) of several modes present in the blanks. The C_m of the different modes changes dramatically between these electrical boundary condition extremes. The plano-convex design has better-defined mode shapes at the expense of sensing dynamic range.