Simultaneous liquid viscosity and density determination with piezoelectricunimorph cantilevers

We have examined both experimentally and theoretically a piezoelectric unim orph cantilever as a liquid viscosity-and-density sensor. The fabricated pi ezoelectric unimorph consisted of a PbO . ZrO2. TiO2 (PZT) layer on a thin stainless-steel plate. In addition to a driving electrode, a sensing electr ode was placed on top of the PZT layer, permitting the direct measurement o f the resonance frequency. The cantilever was tested using water-glycerol s olutions of different compositions. In all three of the tested modes, the r esonance frequency decreased while the width of the resonance peak increase d with increasing glycerol content. To account for the liquid effect, we co nsider the cantilever as a sphere of radius R oscillating in a liquid. By i ncluding the high and low frequency terms in the induced mass and the dampi ng coefficient of the liquid, we show that for a given liquid density and v iscosity the oscillating-sphere model predicts a resonance frequency and pe ak width that closely agree with experiment. Furthermore, the viscosity and the density of a liquid have been determined simultaneously using the expe rimentally measured resonance frequency and peak width as inputs to the osc illating-sphere model. The calculated liquid viscosity and density closely agreed with the known values, indicating that our cantilever-based sensor i s effective in determining viscosity and density, simultaneously. We also s how that scaling analysis predicts an increase in the width of the resonanc e peak with decreasing cantilever size, an observation in agreement with th e large peak widths observed for microcantilevers. (C) 2001 American Instit ute of Physics.