Available online at www.sciencedirect.com Sensors and Actuators A 145–146 (2008) 349–353 A vibrating membrane rheometer utilizing electromagnetic excitation Erwin K. Reichel a,∗ , Christian Riesch b , Bernhard Weiss c , Bernhard Jakoby a a Institute for Microelectronics, Johannes Kepler University, Linz, Austria b Institute of Sensor and Actuator Systems, Vienna University of Technology, Vienna, Austria c Institute for Fluid Mechanics and Heat Transfer, Johannes Kepler University, Linz, Austria Received 30 June 2007; received in revised form 10 October 2007; accepted 21 October 2007 Available online 20 February 2008 Abstract In this contribution the design, fabrication, and characterization of a miniaturized process rheometer in plastic electronics technology is presented. This device type features electromagnetic excitation and an inductive readout mechanism. The resonance behavior of several vibration modes of the clamped–clamped structure is determined. The modeling of the fluid-structure interaction allows the derivation of rheological parameters of the fluid under investigation from the measured frequency response. Vibration amplitudes of several micrometers and low operation frequencies make these devices suitable for the analysis of complex liquids like suspensions and emulsions. Several prototypes were manufactured and characterized in various test liquids. © 2007 Elsevier B.V. All rights reserved. Keywords: Viscosity; Vibrating beam; Acoustic sensor; Electromagnetic excitation 1. Introduction The online measurement of rheological parameters, e.g., viscosity, is crucial in many fields of application such as oil condition monitoring, biomedical analysis, chemical process monitoring, and materials research. In the literature, various methods have been developed to derive viscosity and mass density parameters of liquids from the resonant behavior of immersed vibrating structures, e.g., [1,2]. Both, quartz thickness shear mode resonator (TSM) [3], and shear-polarized surface acoustic wave (SAW) devices are employed for the measure- ment of Newtonian liquids. However, as shown in Ref. [4], due to the small vibration amplitudes, these sensors fail to detect some rheological effects appearing in (non-Newtonian) com- plex liquids like suspensions or emulsions [5]. It is therefore sometimes desirable to use sensors featuring larger vibration amplitudes at lower frequencies. In this contribution, we present a sensor based on a clamped–clamped vibrating membrane actuated by Lorentz forces in a permanent magnetic field, which can be read out ∗ Corresponding author. Tel.: +43 732 2468 9318. E-mail address: erwin.reichel@jku.at (E.K. Reichel). either inductively or by a low-cost reflective infrared-distance sensor. The latter was described in Ref. [6]. Compared to acoustic viscosity sensors with high operat- ing frequencies, our sensor features low operation frequencies (below 10 kHz) and larger mechanical displacement ampli- tudes (tens of micrometers observed at resonances in air). Often the rheological behavior of the liquid under test is determined by the interaction of relatively large structures as bubbles in the case of emulsions or particles in the case of suspensions. These large structures have to be stirred significantly in order to reveal the viscosity measured by laboratory viscometers. For microemulsions it was shown that a micromachined can- tilever with comparable operating frequency and mechanical amplitudes as our sensors is able to reproduce viscosity values measured by laboratory methods while a TSM sensor cannot [4]. 2. Fabrication The schematic of the sensor element is shown in Fig. 1,a photograph in Fig. 2. The base material for the sensor element is a copper plated (1 m copper) polyester foil (50 m thick- ness). The conductive paths, forming two separate coils, were structured by wet-etching following a lithography step similar to 0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.056