B. Nongaillard

Acoustic measurement of compressibility and thermal expansion coefficient of erythrocytes

Mechanical properties of human erythrocytes, namely adiabatic compressibility and thermal expansion coefficient, have been determined using a classical ultrasound velocity and attenuation burst transmission technique. The theoretical model concerns the corpuscular part of the elastic wave propagating in a suspension of viscous particles of small size compared with the wavelength. The thermal wave contribution was taken into account. Normal and stiffened red blood cells were suspended in saline of different NaCl concentration.

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.

Acoustic Tracking of Cassie to Wenzel Wetting Transitions

Many applications involving superhydrophobic materials require accurate control and monitoring of wetting states and wetting transitions. Such monitoring is usually done by optical methods, which are neither versatile nor integrable. This letter presents an alternative approach based on acoustic measurements. An acoustic transducer is integrated on the back side of a superhydrophobic silicon surface on which water droplets are deposited. By analyzing the reflection of longitudinal acoustic waves at the composite liquid-solid-vapor interface, we show that it is possible to track the local evolution of the Cassie-to-Wenzel wetting transition efficiently, as induced by evaporation or the electrowetting actuation of droplets.

Ultrasonic monitoring of sol–gel transition of natural hydrocolloids

Abstract The sol–gel transition of iota carrageenan and pectin is followed by measuring the evolution of ultrasonic velocity of a compressional wave at a frequency of 500 kHz. Measurements are performed as a function of temperature, from about 90 to 20 °C. Results are compared to those obtained in rheology in oscillatory conditions. It is shown that the ultrasonic velocity is sensitive to the sol–gel transition when going from a viscous to a “solid-like” state with a large elastic component. However, the method fails when the transition occurs gradually to give a weak elastic gel.

Ultrasonic spectroscopy of composite materials

Abstract Composite materials are more and more used in high technology industries owing to their unique mechanical properties. For such heterogeneous and quasi-periodic materials, classical non-destructive evaluation (NDE) methods prove very unsatisfactory, and even often inapplicable. Our aim here is to investigate the particular case of ultrasonic NDE of fibre-reinforced composite materials. When using the classical A-scan echography method, it is difficult to interpret time diagrams. This is mainly due to the heterogeneous nature of the materials, the complexity of the diagrams being especially increased by the occurrence of distributed defects (like porosities). Using spectral analysis techniques (ultrasonic spectroscopy), information may be extracted from these intricate time diagrams, In particular, the occurrence of a selective absorption frequency in both the transmitted and reflected energy spectra is evident. This absorption phenomenon is clearly related to the material structure and periodicity and advantage may then be taken of its characteristics (centre frequency, bandwidth and relative depth of the absorption dip in the spectrum) to characterize the material and identify the presence of defects of porosities.

Measurement of the thickness of thin layers by ultrasonic interferometry

An ultrasonic interferometer working in a pulsed mode is described in this paper. It allows for the measurement of coating thicknesses as thin as 5 μm with a 5% precision over different substrates at a very high rate (up to 1000 times per second). The optimal conditions for this interferometric measurement are defined theoretically and the probe characteristics have been optimized technologically. This, together with the design of a very large frequency bandwidth (from 90 to 510 MHz) electronic setup, leads to interesting performances. The advantages of the system for achieving thickness measurements are discussed and comparisons are made with other systems.

Characterization of the state of a droplet on a micro-textured silicon wafer using ultrasound

In this work, we propose acoustic characterization as a new method to probe wetting states on a superhydrophobic surface. The analysis of the multiple reflections of a longitudinal acoustic wave from solid-liquid and solid-vapor interfaces enables to distinguish between the two well known Cassie-Baxter and Wenzel wetting configurations. The phenomenon is investigated experimentally on silicon micro-pillars superhydrophobic surfaces and numerically using a finite difference time domain method. Numerical calculations of reflection coefficients show a good agreement with experimental measurements, and the method appears as a promising alternative to optical measurement methods.

High-frequency acoustic for nanostructure wetting characterization.

Nanostructure wetting is a key problem when developing superhydrophobic surfaces. Conventional methods do not allow us to draw conclusions about the partial or complete wetting of structures on the nanoscale. Moreover, advanced techniques are not always compatible with an in situ, real time, multiscale (from macro to nanoscale) characterization. A high-frequency (1 GHz) acoustic method is used for the first time to characterize locally partial wetting and the wetting transition between nanostructures according to the surface tension of liquids (the variation is obtained by ethanol concentration modification). We can see that this method is extremely sensitive both to the level of liquid imbibition and to the impalement dynamic. We thus demonstrate the possibility to evaluate the critical surface tension of a liquid for which total wetting occurs according to the aspect ratio of the nanostructures. We also manage to identify intermediate states according to the height of the nanotexturation. Finally, our measurements revealed that the drop impalement depending on the surface tension of the liquid also depends on the aspect ratio of the nanostructures. We do believe that our method may lead to new insights into nanoscale wetting characterization by accessing the dynamic mapping of the liquid imbibition under the droplet.

Visualization of thick specimens using a reflection acoustic microscope

A reflection acoustic microscope may be used to image structural details under the apparent surface of a thick sample. To design such an apparatus, the geometrical parameters of the acoustic lenses must be carefully defined. The acoustic field distribution has been computed in the previous structure for this purpose. Some preliminary experimental results are also reported here.

Evaporation of Binary Sessile Drops: Infrared and Acoustic Methods To Track Alcohol Concentration at the Interface and on the Surface

Evaporation of droplets of three pure liquids (water, 1-butanol, and ethanol) and four binary solutions (5 wt % 1-butanol-water-based solution and 5, 25, and 50 wt % ethanol-water-based solutions) deposited on hydrophobic silicon was investigated. A drop shape analyzer was used to measure the contact angle, diameter, and volume of the droplets. An infrared camera was used for infrared thermal mapping of the droplets surface. An acoustic high-frequency echography technique was, for the first time, applied to track the alcohol concentration in a binary-solution droplet. Evaporation of pure alcohol droplets was executed at different values of relative humidity (RH), among which the behavior of pure ethanol evaporation was notably influenced by the ambient humidity as a result of high hygrometry. Evaporation of droplets of water and binary solutions was performed at a temperature of 22 °C and a mean humidity of approximately 50%. The exhaustion times of alcohol in the droplets estimated by the acoustic method and the visual method were similar for the water-1-butanol mixture; however, the time estimated by the acoustic method was longer when compared with that estimated by the visual method for the water-ethanol mixture due to the residual ethanol at the bottom of the droplet.