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Abstract:At the microscale, cantilever vibrations depend not only on the microstructure's properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever's vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, and improve sensitivity in sensing applications, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include inplane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. However, such systems require sampling (no insitu measurements) and a relatively large sample volume (a few milliliters). Moreover, the frequency range is limited to low frequencies (less than 200Hz). To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range.

Silicon microcantilevers can be used to measure the rheological properties of complex fluids. In this paper, two different methods will be presented. In the first method, the microcantilever is used to measure the hydrodynamic force exerted by a confined fluid on a sphere that is attached to the microcantilever. In the second method, the measurement of the microcantilever's dynamic spectrum is used to extract the hydrodynamic force exerted by the surrounding fluid on the microcantilever. The originality of the proposed methods lies in the fact that not only may the viscosity of the fluid be measured, but also the fluid's viscoelasticity, that is, both viscous and elastic properties, which are key parameters in the case of complex fluids. In both methods, the use of analytical equations permits the fluid's complex shear modulus to be extracted and expressed as a function of shear stress and/or frequency.

Due to the need for a microrheometer monitoring the high-frequency viscoelasticity of soft matter in situ, we describe a cantilever-based microrheometer to achieve this purpose. The elastic and viscous moduli of complex fluids can be measured with an acceptable accuracy over a high frequency bandwidth of 1-100 kHz. Some preliminary data on small samples (~10-100 μL) of simple Newtonian and viscoelastic polymer and surfactant solutions showed the ability to measure the dynamic moduli in the range of 0.01-10 kPa. This approach will provide a new way to characterize in situ, dynamic microrheology of minute and trace materials and will advance the field of biorheology, microfluidics, and polymer processing.

To cite this version:Sébastien Tétin, Benjamin Caillard, Francis Ménil, Hélène Debéda, Claude Lucat, et al.. Modeling and performance of uncoated microcantilever-based chemical sensors. Sensors and Actuators B: Chemical, Elsevier, 2010, 143, pp. AbstractChemical sensors based on vibrating silicon microcantilevers without sensitive coating are investigated herein.The sensor signal is the relative variation of the microcantilever resonant frequency which depends on both the viscosity and the density of the fluid surrounding the microcantilever. This principle has been applied to the detection of binary gas mixtures. Experimental data for He/N 2 and CO 2 /N 2 environments are presented and compared to results of theoretical modeling. The advantages of such a gas sensor based on changes of physical properties are discussed (response time, sensitivity, selectivity, stability).

Oscillating microstructures are well-established and find application in many fields. These include, force sensors, e.g. AFM micro-cantilevers or accelerometers based on resonant suspended plates. This contribution presents two vibrating mechanical structures acting as force sensors in liquid media in order to measure hydrodynamic interactions. Rectangular cross section microcantilevers as well as circular cross section wires are investigated. Each structure features specific benefits, which are discussed in detail. Furthermore, their mechanical parameters and their deflection in liquids are characterized. Finally, an inverse analytical model is applied to calculate the complex viscosity near the resonant frequency for both types of structures. With this approach it is possible to determine rheological parameters in the kilo Hertz range in situ within a few seconds. The monitoring of the complex viscosity of yogurt during the fermentation process is used as a proof of concept to qualify at least one of the two sensors into opaque mixtures.

In general, microrheology is carried out using active or passive particle-tracking techniques. In the present paper, a novel technique based on the out-of-plane bending vibrations of a microcantilever beam immersed into a liquid is proposed for microrheological property measurement. We propose to analytically link the damped beam motion with the rheological properties of the fluid in order to establish a dynamic rheogram which spans at least one decade of the kiloHertz frequency domain. The latest improvements in terms of both analytical modeling and experimental setup are detailed, along with a complete explanation of the calculation method. Four rheograms of Newtonian and non-Newtonian liquids obtained from the frequency response of three immersed cantilevers of different geometries are presented.

In the context of building a sustainable future by reducing fossil energy consumption with the objective of minimizing detrimental climate change, particular attention was given to minimizing the complexity, energy consumption and environmental impact of microstructures manufacturing. In this work a new fast-fabrication process for microelectromechanical systems is presented. The name of this new fabrication process is KISSES for Keep It Short, Simple and Environmentally Sustainable. Combining classical deposition techniques (with common metals and polymers and with less common materials such as tree resins, paper and glue), release techniques and a computer numerical control cutting machine, a two-dimensional fabrication process has been developed and the first steps of three-dimensional microfabrication have also been initiated. In order to test this new process, various test structures have been fabricated and tested. These include resonant structures with electronic actuation and electronic measurement, having good quality factors for plastic-based devices, and high-resolution masks (~10 µm) which can be used, for example, for screen-printing techniques. Finally, a temperature sensor and a viscosity sensor have been designed, fabricated with the KISSES process and characterized. These devices exhibit, respectively, a limit of detection of 0.112°C and a viscosity estimation error of less than 10% for viscous silicone oils from 5cP to 50cP. These characterizations of the microdevices show that the proposed process provides a simple method that is capable of fabricating devices that function with high performance. The aim of developing a rapid, simple and environmentally sustainable process has therefore been demonstrated.