Georges Nassar

Monitoring of milk gelation using a low-frequency ultrasonic technique

Abstract A low-frequency ultrasonic device (50–100 kHz) in highly sharpened end sensors that behave as point sources was applied to explore the relations between the physical properties measured through the variation of the wave time-of-flight (transit time of wave) and structural changes during gel formation. This is related to two factors: the ambient temperature and the mechanical resistance of the medium. The network evolution was interpreted by an approach based on the Flory model. The physical significance of this model was shown through a series of experiments using a low-frequency ultrasonic technique. Response curves demonstrate the different stages during gel formation.

High Frequency Ultrasound, a Tool for Elastic Properties Measurement of Thin Films Fabricated on Silicon

The main goal of this work is to develop an ultrasonic high frequency method for characterization of thin layers. The development of high frequency acoustic transducers for longitudinal waves and shear waves on silicon has enabeled the characterization of thin films deposited on this substrate. Three types of transducers have been achieved : (i) single crystal LiNbOSubscript text3 Y+163° for shear waves generation, and (ii) Y+36° for longitudinal waves, bonded and thinned on silicon substrate to achieve ultrasonic transducers in the frequency range 300-600 MHz ; (iii) thin films ZnO transducers were realized due to sputtering technologies working in the frequency range 1 GHz- 2.5 GHz. Using an inversion method and a network analyser which provide the scattering S11 parameter of the transducer versus the frequency we deduce the elastic properties of films deposited on the wafer surface. Thanks to these transducers the acoustic properties of thin films such as SU-8 based nanocomposites (doped with TiO2 , SrTiO3 or W nanoparticles) will be presented. In order to achieve mechanical impedance matching between silicon and water we control the mass of the embedded particles which provide a way to adjust the elastic properties of the characterized material. In another application an Indium metallic layer have been characterized in the high frequency range. We also use this method to characterize dielectric permittivity of the ZnO transducers.

Inline high frequency ultrasonic particle sizer

This paper reports the development of a new method of particle sizing in a liquid. This method uses high frequency focused ultrasounds to detect particles crossing the focal zone of an ultrasonic sensor and to determine their size distribution by processing the reflected echoes. The major advantage of this technique compared to optical sizing methods is its ability to measure the size of particles suspended in an opaque liquid without any dedicated sample preparation. Validations of ultrasonic measurements were achieved on suspensions of polymethyl methacrylate beads in a size range extending from a few micrometer to several hundred micrometer with a temporal resolution of 1 s. The inline detection of aggregate formation was also demonstrated.

Optimization of a low-frequency ultrasonic technique to monitor the change in physical states in viscoelastic media: Gelation process

A new low-frequency ultrasonic device (50-100 kHz) in highly sharpened end sensors that behave as point sources were examined. The application of this new ultrasonic technique with two sensors coupled in the near field is to explore the relations between the physical properties measured through the evolution of the wave time of flight and structural changes during gel formation which is related to two factors: the ambient temperature and the mechanical resistance of the medium. The network evolution was interpreted by an approach based on the Flory model. The physical significance of this model was shown through a series of experiments using a low-frequency ultrasonic technique. Response curves demonstrate the different stages during gel formation.

Characterization of the state of a droplet at a micro-textured silicon wafer using a finite difference time-domain (FDTD) modeling method

In this study, we introduce a finite difference time domain method to study the propagation and reflection of an acoustic wave on smooth and micro-textured silicon surfaces in interaction with droplets in different states. This will enable numerical investigations of interfaces composed of periodically distributed well-defined pillars. One type of transducer was modeled generating longitudinal wave. Three configurations were studied: the Cassie state, the Wenzel state and a composite state for which the droplet collapsed into the middle height of the pillars. After analysis of the displacement along y direction in the silicon wafer, we were able to show that a longitudinal wave is sensitive to the detection of the state of the droplet. The first experimental results made it possible to show a good agreement between modeling and experiments.

Low frequency composite acoustic sensor for highly absorbing media characterization

Ultrasonic techniques are widely used in nondestructive testing and evaluation of media. But when media are highly absorbing, it becomes often impossible to operate at classical ultrasonic frequencies. In this work, we propose a low frequency acoustic device to characterize such highly absorbing media. This device is composed of a piezoelectric disc embedded in a metallic ring. The technique consists in bringing into resonance the entire structure. Firstly, we will study analytically and numerically the main resonance modes of the sensor, which are the flexion modes and the radial modes. The results of the modelization are compared to measurements obtained through an impedance analyzer. In a second part, we will use this model of sensor to characterize two different media. The first type consists of a series of polymer disks with different viscoelastic properties. Using radial modes of vibration, we will characterize these polymer disks through measurement of the acoustic velocity. After that, we will cha...

Ultrasonic and acoustic method for viscoelastic complex media characterization

In this paper, the potentialities of a low frequency ultrasonic/acoustic technique devoted to the study and characterization of the viscoelastic complex media is investigated. This work shows the limit of the use of ultrasound in a viscoelastic media with a complex matrix. In this context the cheese was indicated as a model of propagation medium, such a product having a very complex matrix in term of texture, openings, crystallization, moisture,... . Theoretical basis of sound attenuation in cheese is recalled, especially the effects of the matrix viscoelasticity and the scattering of ultrasonic energy by holes and cracks. Depending on the degree of openness, ultrasonic velocity or attenuation is chosen to represent the evolution of the cheese. For very high degree of openness, ultrasounds are no longer usable and a tap‐test acoustic technique is employed and allows a quality indicator to be constructed. Experimental validations were done with optical images of cut cheeses and rheological measurements. Th...

Study and development of a low‐frequency acoustic sensor dedicated to the vibratory analysis and the mechanical characterization of the plates

A low‐frequency acoustic method was implemented for the purpose of nondestructive control and evaluation of metal plates coating. In this method, a mechanical pulse (acoustic impact) is used to generate acoustic vibrations on a frequency band between 100 Hz to 20 kHz, and a compact acoustic sensor, constructed with by composite materials with an embedded ferroelectric disk, is used to receive the acoustic vibrations. The technique consists in setting in resonance the integrality of a reduced size mechanical structure. The modal frequencies of plates are calculated by two methods: analytically and by finite element method. Then, a numerical modeling of the acoustic behaviour of the sensor is presented. Finally, experimental trials are described and results showing the sensitivity of the method to evaluate coating of metal plates are analyzed.