We have developed a vibrating microsensor allowing simultaneous measurement of density and viscosity in liquids. A small-sized spherical oscillating probe is immersed in the liquid and is connected to a microcantilever driven in flexural vibration by a piezoelectric transducer. Cantilever resonance parameters ͑frequency and damping͒ are used to measure the probe-fluid interaction. The developed analytical model enables us to compute fluid characteristics from measured data by solving the inverse problem. Measurements performed for various fluids are compared to literature data. Investigation depth of the sensor is determined by studying the effect of the presence of an immersed wall in the vicinity of the probe.
The proposed microsensor is derived from the electromechanical resonator of our ac force microscope: the scanning microdeformation microscope (SMM). A submillimetric spherical probe immersed in the fluid sample replaces the tip usually used in SMM. This sphere is connected to a cantilever, which is excited at the resonance frequency. The measurement of the resonance frequency and of the damping enables the quantitative characterization of the investigated fluid. A theoretical approach allows to model the system behavior. The model takes into account two aspects: the mechanical vibration of the cantilever and the fluid mechanics. The values predicted by the model are in good agreement with the experimental measurements performed for various viscous media.
Local measurement of the elastic or viscoelastic properties of solids and fluids has been widely investigated with far-field acoustics e.g., (determination of resonance frequencies and damping, interference between different waves or modes). However, actual techniques need high operating frequencies to shorten the wavelength. Conversely, the proposed method is based on near-field acoustics: a subwavelength acoustic source interacts with the investigated medium within a short distance related to the radius of the source. The resonance frequency of a cantilever is used to measure the interaction between this small vibrating source and the investigated medium. For the characterization of solids, this device is a part of the scanning microdeformation microscope (SMM) that is a kind of ac contact force microscope. For the characterization of fluids, the vibrating tip is replaced by a small sphere. Standard operating frequency of the resonator is 20–30 kHz. The paper presents both theoretical and experimental aspects of the electromechanical resonator, the vibrating element (the tip with radius in the 0.5 to 10-mm range for the investigation of solid samples and the sphere having a 100- to 500-mm diameter for the investigation of fluids), and the interaction. Main features, advantages, and limitations of the presented method will be discussed.
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