Monitoring saltwater corrosion of steel using ultrasonic coda wave interferometry with temperature control.

Assessing corrosion is crucial in the petrochemical and marine industries. Usual ultrasonic methods based on pulse-echo and guided waves to detect corrosion lack of precision and struggle in structures with a complex shape. In this paper, a complementary and sensitive ultrasonic method based on coda wave interferometry is presented to detect and quantify thickness loss caused by saltwater corrosion of a steel sample. The method consists in exciting the sample and measuring periodically the scattered coda signal. Correlation of the coda signal with a reference taken for the sample initial state permits the monitoring of corrosion spread with a high accuracy. A laboratory experiment is conducted with two steel samples immersed in saltwater with coda and temperature measured simultaneously. One of the samples is protected from corrosion and is used as a control sample to determine the influence of temperature on the coda signals. It is shown that the coda signals on the corroded sample can be temperature-corrected using the temperature measurement only. A control sample is not needed. A good correlation is found between a parameter quantifying the stretching of the coda over time and the corrosion surface, which is monitored with a camera. Finally, a simple theoretical model of coda signal is proposed to quantify the real-time average corrosion rate during the experiment with a sub-micrometric precision. The estimated final average corrosion depth is validated by independent depth profile measurements. The uncertainties and sensitivity of the presented method are investigated.

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.

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.

Controlling the transmission of ultrahigh frequency bulk acoustic waves in silicon by 45° mirrors.

In this paper, we present a feasible microsystem in which the direction of localized ultrahigh frequency (∼1GHz) bulk acoustic wave can be controlled in a silicon wafer. Deep etching technology on the silicon wafer makes it possible to achieve high aspect ratio etching patterns which can be used to control bulk acoustic wave to transmit in the directions parallel to the surface of the silicon wafer. Passive 45° mirror planes obtained by wet chemical etching were employed to reflect the bulk acoustic wave. Zinc oxide (ZnO) thin film transducers were deposited by radio frequency sputtering with a thickness of about 1µm on the other side of the wafer, which act as emitter/receptor after aligned with the mirrors. Two opponent vertical mirrors were inserted between the 45° mirrors to guide the transmission of the acoustic waves. The propagation of the bulk acoustic wave was studied with simulations and the characterization of S(21) scattering parameters, indicating that the mirrors were efficient to guide bulk acoustic waves in the silicon wafer.

SU-8-based nanocomposites for acoustical matching layer.

SU-8, an epoxy-based photoresist, was introduced as the acoustical matching layer between silicon and water for lab-on-chip applications integrating acoustic characterization. Acoustical performances, including the acoustic longitudinal wave velocity and attenuation of the SU-8-based matching layer, were characterized at a frequency of 1 GHz at room temperature. The gain in echo characterization with a SU-8/SiO2 bilayer and with different nanocomposite monolayers made of SU-8 and TiO2 nanoparticles (size around 35 nm) between silicon and water was characterized as being above 10 dB in each case. With the increase of concentration of TiO2 in SU-8 based composites from 0 to 30 wt%, acoustical impedance of the nanocomposites increased from about 3 to 6 MRayls, respectively. The acoustical attenuation in the nanocomposites is between 0.5 and 0.6 dB/microm. The most efficient matching was obtained with the nanocomposite integrating 30 wt% TiO2 nanoparticles, with which the enhanced loss is about 0.34 dB as the attenuation is about 0.5 dB/microm. This type of matching layer has potential applications in lab-on-chip technology for high frequency transducers or in the fabrication of high frequency piezocomposites.

On the use of ultrasounds to quantify the longitudinal threshold force to detach osteoblastic cells from a conditioned glass substrate.

Cell adhesion on a biomaterial is an important phase of the cell-material interactions and the quality of this phase governs the success of the biomaterial integration. Understanding of the phenomena of cell adhesion and in particular understanding of cell adhesion on biomaterials is of crucial importance for the development of new biomaterials with excellent biocompatibility. One of the physical quantitative indexes to evaluate the quality of cell-material adhesion is its strength. Determining the strength of adhesive bonds requires applying external forces to the cells. Thus, a few methods have been developed to evaluate the strength of cell-material adhesion (micropipette, microplates, microcantilever, ...). These methods apply shear forces on adherent cells. The aim of our work is the development of a new ultrasonic characterization method of cellular adhesion on substrates. With our method, longitudinal acoustic waves are applied on cell culture to impose a longitudinal strain on cells. Only the cells subjected to a sufficient level of strain will be detached from the substrate. The idea is to correlate cell detachment rate to the longitudinal strain threshold supported by cells. From this result, we can deduce the critical force just sufficient to detach the cell. This global method can be adapted for different cell types and for different substrates. This method can provide an evaluation of the effect of functionalization on substrates. The technique is investigated for the 200 kHz ultrasound frequency. An insonificator adapted to the use of cell culture boxes was developed and calibrated. Tests were carried out on a glass substrate with or without biological conditioning. We used the MC3T3-E1 osteoblastic cell line. Our results to date provide the value of the necessary force to detach with reproducibility osteoblastic cells from glass.

Electrical characterization of plate piezoelectric transducers bonded to a finite substrate.

This article presents a new technique for characterizing piezoelectric transducers attached to a finite substrate. It consists of determining the impedance of the transducer cleared of the effects caused by finite dimensions of the substrate. This technique is validated by comparison with measurements on a transducer mounted on an effectively half-infinite substrate. It is applied for the electrical matching of a lithium niobate plate transducer bonded to a fused quartz rod.