Micro-Electro-Mechanical-Systems (MEMS), which combine electronics and mechanical components, are used in the determination of rheological properties of fluids. The frequency response of a microcantilever immersed in a fluid is very much influenced by the density and viscosity of the fluid. The changes in the dynamic properties of the microcantilevers when immersed in a liquid medium, namely a glycerol- water mixture, have been studied comprehensively. The microcantilevers of lengths varying from 150µm to 350µm were used; whose width and thickness were 20 µm and 4µm respectively. A Doppler vibrometer, with suitable modifications, was used to detect the resonance frequency of the cantilever beams. For data collection and processing, computer interface with NI instruments has been adopted with LabVIEW programming.

The micro cantilever beam used here is vibrated by an AC supply with terminals between the beam and the substrate. The electrostatic force between the terminals causes the beam to vibrate. The frequency of the supplied AC source is varied between a range to find the resonant frequency. The resonant frequency variation measurement for finding the factors affecting it is of much importance. Due to the miniature size of the cantilever beam Laser Doppler Vibrometer (LDV) is best suited for this purpose. Laser Doppler Vibrometer, is a non-contact technique and ensures high measurement accuracy with reduced testing time. If an acoustic, radio or light wave of a specific frequency is beamed at a moving object, the frequency of the wave reflected from the moving object differs in proportion to the velocity of the object. This phenomenon is known as Doppler shift or the Doppler Effect.

The present work has been applied to the measurement of low concentrations of Triglycerides (of 5 uM concentrations).

 
  Imaging using waves are limited by the wavelength used and is well reported to be l/2. However, using nearfield imaging methods, the achievable resolution becomes determined by the aperture size of the device. The aperture size can be reduced to 10-100 nm using tips that are etched. Alternatively, commercial tips such as once used in AFM can also be used.

A set-up for preparing probe tips on the nano-scale has been put together. A few probe tips have been prepared and their SEM pictures taken. The set-up is currently being optimized for Tungsten as the probe tip material. These Tungsten tips are to be used for scanning with ultrasound and microwaves near field imaging techniques.

Results of some experiments in imaging with a probe-tip in the through transmission mode with 5 MHz ultrasonic transducers are presented below. For purposes of comparison, images obtained by (i) focused transducer acting as a receiver, and (ii) focused transducers acting as senders and receivers are also presented. These experiments have been carried out with facilities developed at the CNDE. Though these facilities have been designed for scanning and imaging at the macro-level, we have used it as a “scaled-up” version of what is expected at the micro-level. In the process of carrying out these experiments, several issues relating to the sample thickness, axial resolution of the pulse to be employed and the SNR have come up. Additional trials need to be conducted at the macro-level before these issues can be sorted out at the micro-level.  The resolution achievable at 5 MHz with the tip can be seen to be comparable if not higher than what has been obtained with a 25 MHz focused transducer (differences between through transmission and pulse-echo mode of image formation needs to be taken into account for a more quantitative comparison).