Nature of sensitive high-order resonant modes in piezoelectric excited millimeter sized cantilever (PEMC) sensors

High-order resonant modes of piezoelectric-excited cantilever (PEMC) sensors were previously shown to be very highly sensitive at 0.3-2 fg/Hz for in-liquid applications. The purpose of this work is to show experimentally and with finite element model (FEM) simulations that such sensitive modes are strongly influenced by the sensor width, suggesting that the sensitive modes are torsional or buckling modes. From experimental observations the resonant frequency of high-order modes had a strong dependence on width, where a sensor with a smaller width had resonant modes at a higher frequency. Also the FEM simulations indicate that in this frequency range there are resonant modes with a buckling nature that change for a decrease in width, consistent with experimental observations. In order to establish that the width-dependent modes are mass-sensitive in liquids, resonant frequency change to density changes in flow experiments under fully liquid-immersed conditions were determined. Average frequency shifts of 475 +/- 49 Hz (n = 5), 533 +/- 31 Hz (n=5), 715 +/- 103 Hz (n = 5) and 725 +/- 37 Hz (n = 5) were obtained for the four designs investigated in response to a density change of 0.0118 g/cm(3). The results show that the resonant frequency response to variations in the geometry provides insightful data on the role of width in PEMC sensor design. (C) 2011 Elsevier B.V. All rights reserved.