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International audience; We investigate the nucleation of cavitation bubbles in a conned Lennard-Jones uid submitted to negative pressures in a cubic enclosure. We perform molecular dynamics (MD) simulations with tunable interatomic potentials that enable to control the wettability of the solid walls by the liquid, i.e its contact angle. For a given temperature and pressure, as the solid is taken more hydrophobic, we put in evidence an increase of the nucleation probability. A Voronoi tessellation method is used to accurately detect the bubble appearance and its nucleation rate as a function of the contact angle. We adapt the Classical Nucleation Theory (CNT) proposed for the heterogeneous case on a at surface to our situation where bubbles may appear at walls, edges or corner of the conned box. We nally calculate a theoretical mean expectation time in these three cases. The ratio of these calculated values over the homogeneous case is computed and compared successfully against MD simulations. Beyond the innite liquid case, this work explore the heterogeneous nucleation of cavitation bubble, not only in the at surface case, but for more complex conning geometries.

In the current work, we developed a novel method to fabricate hybrid surfaces consisting of mixed hydrophilic/superhydrophobic properties. These surfaces specifically consist of a regular array of hydrophilic pillars (displaying a receding contact angle lower than 90°) surrounded by a superhydrophobic thinner layer made via the electropolymerization of a fluorinated monomer. Then, we determined the wetting properties of various forms of this complex surface, i.e., displaying different surface properties, by specifically determining their advancing (θa) and receding (θr) contact angles. Two main parameters were varied: the pillar density (from 21.2% to 6.5% based on using a spacing d between pillars varying from 25 to 45 micrometers) and the polymer charge density (from 0 to 100 mC cm-2). We observed that, for low charge density values, only the ground surface was covered by the hydrophobic polymers; while for higher charge density values, polymerization reached higher levels on the lateral surfaces of the nonconductive cylindrical pillars, eventually up to their top surfaces and covering them for the highest charge densities. This feature gave us an additional parameter that we could use to control the surface wettability. We also found that contact angles (advancing and receding) increased markedly with increasing polymer charge density above a critical value (which was higher for receding angles). And we measured advancing and receding contact angles to, respectively, increase and decrease with increasing pillar density. We interpreted qualitatively these behaviors, the main point being the importance of the impalement (null, partial or total).

We present here the use of the capillary bridge technique to study the wetting properties (advancing and receding contact angles) of transparent, textured and superhydrophobic surfaces over large wetted area. Apparent contact angles on such surfaces are classically measured using a goniometer in combination with video camera side visualization and a drop shape analysis. Recent experiments of Schellenberger et al. [F. Schellenberger, N. Encinas, D. Vollmer and H. J. Butt, Phys. Rev. Lett., 2016, 116(9), 096101] show that this method can significantly underestimate the apparent advancing contact angle. We use for the first time the capillary bridge setup for such textured surfaces, leading to a large (up to several cm2) wetted area, instead of having a reduced contact zone as in the drop case (mm2 or less). (1) We show here how to use the method and its characteristics to explore the wetting properties of superhydrophobic surfaces. We have developed a new analysis method in order to obtain the value of the contact angle for any position of the substrate. (2) We compare with the classical drop side view method, showing that advancing contact angles are systematically higher. (3) We compare to a few existing models, concluding a good agreement for receding values but not for advancing angles, for which models must be refined.

Numerous scenarios exist for a cavitation bubble growing in a liquid. We focus here on cavitation phenomena within water under static tension in a confined environment. Drawing inspiration from the natural materials in plants, we design a novel experimental setup where a micrometric volume of water is confined by a hydrogel-based material. We show that, submerging the sample in a hypertonic solution, the water within the cavity is placed under tension and the acoustic emission produced by the resulting bubble nucleation is precisely detected. This new experimental procedure is able to strongly reduce the acoustic reflections occurring at the hydrogel/air interface with more classical techniques. We also propose a mathematical model to characterise the pressure wave emitted in order to correctly take into account the dissipation effect induced by the visco-elastic behaviour of the confining hydrogel. Both bubble resonant frequency and damping are captured by the model and quantitatively match the values found in the experiments.

International audience; Poly(tetrafluoroethylene) (PTFE) materials were exposed to low and high-energy oxygen plasma, and the stability of the materials' surface was evaluated using contact angle, surface roughness, and surface chemistry characterizations. Lower-energy oxygen plasma treatments exhibited hydrophilic behavior with contact angles as low as 87°, and the higher-energy oxygen plasma treatments exhibited superhydrophobic behavior with contact angles as high as 151°. The wettability of all the treated samples as stored in air and in water was found to be stable in time as evidenced by the statistically insignificant differences in the advancing, receding, and hysteresis contact angles. Low contact angle hysteresis (θH < 5°) and low sliding angle (α ≈ 4°) were exhibited by the superhydrophobic surface. The surface morphology was found to be responsible for the changes in the wettability of the PTFE samples since (1) there was an increase in the surface rms roughness as the plasma discharge energy was increased, and (2) there were no significant changes in the observed group frequencies of the FT-IR spectra of the treated PTFE from the untreated PTFE.

International audience; We report an experimental study of bubble dynamics in a non-Newtonian fluid subjected to a pressure decrease. The fluid is a hydrogel, composed of water and a synthetic clay, prepared and sandwiched between two glass plates in a Hele-Shaw geometry. The rheological properties of the material can be tuned by the clay concentration. As the imposed pressure decreases, the gas initially dissolved in the hydrogel triggers bubble formation. Different stages of the process are observed : bubble nucleation, growth, interaction and creation of domains by bubbles contact or coalescence. Initially bubble behave independently. They are trapped and advected by the mean deformation of the hydrogel, and the bubble growth is mainly driven by the diffusion of the dissolved gas through the hydrogel and its outgassing at the reactive-advected hydrogel-bubble interface. In this regime, the rheology of the fluid does not play a significant role on the bubble growth. A model is proposed and gives a simple scaling that relates the bubble growth rate and the imposed pressure. Carbon dioxide is shown to be the gas at play and the hydrogel is degassing at the millimeter scale as a water solution does at a smaller scale. Later on, bubbles are not independent anymore. The growth rate decreases and the morphology becomes more anisotropic as bubbles interact because they are separated by a distance smaller than the individual stress field extension. Our measurements show that the interaction distance scales as the bubbles size.

The surface construction to reach super oil non-wetting properties is very complex because of the necessary force for impeding the natural spreading of low surface tension oils. Here, a polymer, which is able to reach the superoleophobicity when it is electrodeposited on smooth surfaces, has been deposited on micro-patterned substrates made of cylindrical arrays (∅: 13 µm, H: 25 µm, distance between cylinders: 40 µm) in order to determine the effect of the pattern on the super oil-repellency properties. The surface analysis using various oils has shown that the pattern used highly decreases the time of deposition and, as a consequence, the required amount of polymer to obtain anti-oil surfaces. This work is the first step in the short term prospects for the elaboration of superoleophobic surfaces combining electropolymerization with lithography.

This study focuses on the effects of vertical vibrations on sessile drops deposited on hydrophobic substrates. At low amplitudes the contact line remains pinned because of contact angle hysteresis and only drop surface modes areobserved. Above a first threshold the contact line starts to move and exhibits a stick-slip behavior that presents some analogies with the solid friction on amechanical oscillator. At larger amplitudes, non-axisymmetric contour modes show up (modes m=2, 3...). They can be interpreted as a coupling between surface modes and contact line motion. These subharmonic modes are welldescribed within the framework of parametric oscillators. We also discuss here why vibrations can help to measure equilibrium contact angle.

The long evolution of vascular plants has resulted in a tremendous variety of natural networks responsible for the evaporatively driven transport of water. Nevertheless, little is known about the physical principles that constrain vascular architecture. Inspired by plant leaves, we used microfluidic devices consisting of simple parallel channel networks in a polymeric material layer, permeable to water, to study the mechanisms of and the limits to evaporation-driven flow. We show that the flow rate through our biomimetic leaves increases linearly with channel density (1/ d ) until the distance between channels ( d ) is comparable with the thickness of the polymer layer (δ), above which the flow rate saturates. A comparison with the plant vascular networks shows that the same optimization criterion can be used to describe the placement of veins in leaves. These scaling relations for evaporatively driven flow through simple networks reveal basic design principles for the engineering of evaporation–permeation-driven devices, and highlight the role of physical constraints on the biological design of leaves.

Microfluidic separation of magnetic particles is based on their capture by magnetized microcollectors while the suspending fluid flows past the microcollectors inside a microchannel. Separation of nanoparticles is often challenging because of strong Brownian motion. Low capture efficiency of nanoparticles limits their applications in bioanalysis. However, at some conditions, magnetic nanoparticles may undergo field-induced aggregation that amplifies the magnetic attractive force proportionally to the aggregate volume and considerably increases nanoparticle capture efficiency. In this paper, we have demonstrated the role of such aggregation on an efficient capture of magnetic nanoparticles (about 80 nm in diameter) in a microfluidic channel equipped with a nickel micropillar array. This array was magnetized by an external uniform magnetic field, of intensity as low as 6-10 kA/m, and experiments were carried out at flow rates ranging between 0.3 and 30 μL/min. Nanoparticle capture is shown to be mostly governed by the Mason number Ma, while the dipolar coupling parameter α does not exhibit a clear effect in the studied range, 1.4 < α < 4.5. The capture efficiency Λ shows a strongly decreasing Mason number behavior, Λ?Ma-1.78 within the range 32 ≤ Ma ≤ 3250. We have proposed a simple theoretical model which considers destructible nanoparticle chains and gives the scaling behavior, Λ?Ma-1.7, close to the experimental findings. © 2016 American Physical Society.

We present an innovative mirrorless optofluidic random laser where the optical cavity has been replaced by a random scattering structure. We achieve emission control at any desired wavelength by iteratively shaping the optical pump profile.

We report random laser emission in an innovative structure PDMS microfluidic channel filled with a dye solution. A systematic study on the origin of random lasing and its depedency on pump laser parameters is presented.

We have designed a promising optofluidic laser device based on a microfluidic structured channel filled with an ethanolic dye solution. We report random laser emission attributed to the inherent randomness of the structure. The systematic study of the emission dependence on various pump laser parameters reveals fundamental aspects as well as potential applications of this system.

By means of photoelasticity, we success in visualizing in real time the propagation of acoustic waves in a granular packing of cylinders. As previously mentioned by Nesterenko [1] for the case of spherical grains, the nonlinearity of the contact law between grains induces a dependence of the wave velocity both on its amplitude and on the confinement force. Our experimental procedure allows an access to the local state of stress of individual grains as a function of time with a good accuracy. Our results concerning the wave velocity as a function of the amplitude, and of the confinement force, are compared to theoretical predic tions and to the spherical beads case.