Pierre Campistron

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.

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.

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.

Ultrasonic Adhesion Measurement of Whey Protein Fouling

The knowledge of milk deposit adhesion is a prerequisite for setting innovative operating conditions of processing and strategies of cleaning. Unfortunately, these data are lacking because few methods of characterization of the adhesion quality exist. In this work, an ultrasonic method toward the quantification of the adhesion of a fouling dairy deposit onto a stainless-steel surface is proposed. The method is based on the investigation of the shear waves reflection at the interface between deposit and substrates. First, the associated theory for quantifying adhesion by a reflection coefficient is introduced and the ultrasonic method proposed is briefly presented. Then, experiments led with glucose syrups of various viscosities and different substrates (glass and stainless steel) for validating the adhesion parameter values are described and results are compared to classical tests for measuring adhesion strength. Finally, some preliminary trials for measuring the adhesion of a whey protein fouling onto coated stainless steel were carried out and discussed.

A continuous wave method for ultrasonic characterization of liquid materials

This paper describes a method for measuring the velocity and attenuation of high-frequency sound waves traveling through liquid materials. This method is based on the measurement of the forward scattering parameter (S21) of the acoustic system composed of two transducers and the liquid between them. The acoustic parameters are obtained from the analysis of S21 in the time domain. Various experiments using distilled water as the liquid sample have been carried out. They show the influence on the measurement sensitivity of receiver detection bandwidth, liquid thickness, and difference of temperature between the room and the test cell. The method was used for the measurement of the attenuation and velocity of mixtures of alcohol and water.

SU-8 photoresist and SU-8 based nanocomposites for broadband acoustical matching at 1 GHz

So as to integrate acoustic functions in BioMEMS using 1 GHz ZnO transducers deposited on silicon substrates, acoustic waves propagation through the silicon substrate and its transmission in water needs to be maximized (the insertion losses at the Si / water interface are about 6dB). In the context of integration, it is interesting for mechanical impedance matching to use photosensitive materials such as SU-8 so that patterns may be obtained. Nanocomposite materials based on SU-8 mixed with nanoparticles having adequate impedances were fabricated. These new materials are characterized in terms of their acoustic velocity, impedance and attenuation. For this, the nanocomposite layers are deposited on the substrate by spin coating to obtain a thickness of about 10 μm, in order to separate acoustic echoes from the material (even if λ/4 layer thickness is lower than 1 μm). The insertion losses of the device immersed in water can be simulated as a function of frequency for a given reflection coefficient between the silicon substrate and the photoresist. The characteristics of some nanocomposites made with SU-8 and various concentrations of nanoparticles like Ti02, SrTiO3 or W have been determined.

Contribution of the shear wave ultrasonic reflectometry to the stickiness measurements

HIGHLIGHTSShear wave reflection coefficient is used for stickiness evaluation.This non‐invasive testing method is fully described from sensor to signal processing.Evaluation of the adhesion of fluids is tested for 6 decades of viscosity range.Influence of the normal force on the adhesion of two solids is studied.Measurements are correlated with more traditional indicators. ABSTRACT Today, non‐invasive quantification of the adhesion of a deposit to a surface is always a challenge and, unfortunately, few tools are available in this area. This is an obstacle, in several industrial processes, to the identification of conditions limiting the fouling and to the establishment of eco‐efficient cleaning strategies. In this paper, a non‐invasive ultrasonic technique was developed in the aim of characterizing the adhesion of viscoelastic fluids or solid deposited on a substrate. We adopted the idea that the more a deposit is difficult to clean the more adherent it is. From this point of view the value of the reflection coefficient of an ultrasonic shear wave informs us about the adhesion of the deposit on a surface. A large bibliography on the adhesion measurement is given. Then the principle of ultrasonic test is presented and cares required for the measurement of the reflection coefficient are widely discussed. The ultrasonic reflection coefficients obtained with different controlled samples covering a wide range of interfaces (liquid/substrate, solid/substrate) are presented and compared with other indicators of adhesion. All the data on various samples showed that the ultrasonic test is a tool to discriminate non‐destructively a large range of interface quality, allowing ranking according to the adhesive strength.

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.

Ultrasonic characterization of fluid saturated absorbing polymethylacrylate porous plates.

Experiments are led on five manufactured porous polymethylmethacrylate (PMMA) samples, numbered 1–5. Their constituting materials are packed beads whose diameters run from 200–300 μm for plate 1 up to 600–700 μm for plate 5. The plates are normally insonified by wide band transducers acting within the frequency range 100–800 kHz. The samples obey Biot’s theory taking into account the viscoelasticity of the solid frame. The reflection and transmission coefficients are measured and then added or subtracted to give the two transition terms, issued from S‐matrix theory. They are connected to the symmetrical or asymmetrical motions of the faces of the plates. Those terms allow us a characterization of the porous media by means of a computer program that seeks the values of the Biot parameters giving the best agreement between theory and experiments.