Benjamin Caillard

The Microcantilever: A Versatile Tool for Measuring the Rheological Properties of Complex Fluids

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, i.e., 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.

On-chip characterization of the viscoelasticity of complex fluids using microcantilevers

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.

Comparison and experimental validation of two potential resonant viscosity sensors in the kilohertz range

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 kilohertz 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 in opaque mixtures.

Chemical sensing using microcantilever without sensitive coating

Chemical 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/N2 and CO2/N2 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.

Electrical OverStress/ElectroStatic Discharges (EOS/ESD) Specificities in MEMS: Outline of a Protection Strategy

In this paper, increased EOS/ESD concerns related to MEMS structural specificities are indexed and general works about failures and reliability improvement in MEMS are reviewed from an EOS/ESD point of view, as well as existing protection methods. Then a new method is proposed and recommandations for a general scheme for MEMS protection are suggested.

Influence of fluid-structure interaction on microcantilever vibrations: applications to rheological fluid measurement and chemical detection

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, 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 in-plane 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. 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.