In this article, we report on studies aimed at sensing of the stiffness of membranes, particularly in the case of vesicles. A
local approach could be done with AFM techniques but the local information is not pertinent for non homogeneous
membranes. To solve this problem, we have developed and checked a specific sensor based on a vibrating sphere. The
near-field acoustic wave enables to characterize biological particles which change the apparent viscosity and density of
the surrounding fluid. The microsphere is well suited for very small volumes of liquid (typically about one microliter).
Globally, the microsensor is based on a silicon cantilever which is glued on a piezoelectric transducer at its clamped end.
The sphere is connected to the cantilever with a small glass rod (the core of an optical fiber). When operating, the sphere
is immersed inside the investigated liquid and the piezoelectric actuator is excited with a low frequency generator. The
vibration of the cantilever is sensed with a heterodyne laser probe. The plot of the response of the sensor (Bode plot)
allows computing the properties of the liquid. When biological cells or vesicles are in the fluid, the effect of the
biological elements is detectable and can be discriminated. We will present the microsensor and the optical probe which
have allowed doing the described measurements. The theoretical study will show the influence of the different
parameters and the crucial role of the optical probe to detect the low amplitude vibrations of the cantilever. The
experimental results demonstrate the high sensitivity of the sensor to small variation of the composition of the fluid
(water), particularly in the case of small vesicles of different kinds. Practically, this is a simulation of cells sensing.
The micromechanical cantilevers have become a powerful tool for the study of forces on nanoscale and serves as the heart of the Atomic Force Microscope (AFM) and of all the Scanning Force Microscopes designs on this basic idea. These micromechanical cantilevers are forced to not negligible thermomechanical oscillations at room temperature induced by the thermal noise, which is a Brownian motion. These oscillations impose a fundamental limit to the accuracy of force detection setups in AFM.
However these thermomechanical oscillations can be analyzed in order to obtain information about the tip-sample interaction. Several publications have been presented in the last years describing the evolution of the resonant frequency of the microcantilevers through the spectral power density in the case of non-contact behavior or by studying liquids or gas samples, with the standard optical lever technique. They have observed that it is impossible to detect the resonant frequency by this way in the case of contact with hard samples.
Here we report on the investigation of mechanical sample properties by analysis of the thermomechanical noise of the first symmetric eigenmodes of a rectangular microcantilever. The presented work is the first study demonstrating the possible detection of the first flexural vibration modes of the microcantilever in contact with hard samples, by optical probing of the thermomechanical noise. By Analyzing the spectral density of the thermomechanical fluctuations attributed to the first symmetric flexural vibrational modes of the surface-coupled cantilever, the longitudinal stiffness of the tested sample can be obtained.
Optical probing is a non invasive tool useful to characterize the vibrations of the small moving components of microsystems (MEMS/MOEMS). This paper presents two complementary methods that can sense in-plane components of the vibration. The first one is a heterodyne interferometer, commonly used for out-of-plane component detection. The edge of the sample partially occults the laser beam, and, consequently, the intensity is amplitude modulated when the sample vibrates. The electronics has been modified so that both phase and amplitude of the output signal are extracted. Actual sensitivity is about 10-11 m/√Hz. In the second gradient method, a parallel acquisition of synchronous images is performed with a camera and a microcomputer, which stores the successive images for subsequent processing. Before digital lock-in processing, the images sequence is inter-correlated and interpolated to increase the accuracy of the method. This simple processing technique allows nanometer sensitivity. Both techniques are presented, analyzed and compared from theoretical and experimental point of view.
Optical measuring techniques in MEMS are attractive ways of detection and reproducible methods. Applications of appropriate optical measuring techniques can be found in many situations: local study of materials constants, characterization of micromechanical systems, vibration analysis, and environmental behavior.
In two-arms interferometers, spurious beams are often cause of instabilities. Resulting multiple-beam interference limit the probe sensitivity. Optical components are usually added to spatially shift or to suppress these `spurious beams' returning inside the interferometer. Experimentally, we have shown that the so-called `spurious beams' returning towards the laser can be successfully used to obtain stable interference. So, we have developed new structures of interferometers based on the reflection of a returning beam on the high-quality front mirror of He-Ne laser. The new structures present several advantages compared with classical interferometers (Michelson or Mach-Zehnder, e.g.). They are more simple and, at least, the same sensitivity has been measured. Alignment is simplified and power balancing is very easy.
Heterodyne laser probes are very sensitive noncontacting instruments which provide wide bandwidth and high resolution. Most interferometers are based on a symmetrical two-arms structure, but in all cases, bench scientists know many undesirable beams (due to various reflection) often reduce expected signal to noise ratio. So one major problem of laser probe designers consists in spurious beams elimination. In practice, the so called `spurious beams' can successfully be used for new concepts of heterodyne laser probe. We have verified most assemblies could be simplified (the second arm often becomes useless) and alignment is reduced to minimum. Moreover, optical stability is generally better when using He-Ne laser. The only apparent disadvantage seems to be a small decreasing of the signal to noise ratio (typically about 3 dB, but that value depends on the structure).
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