Xavier Noblin

Superoleophobic behavior of fluorinated conductive polymer films combining electropolymerization and lithography

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.

Adaptive pumping for spectral control of random lasers

Random lasers generate the optical feedback required for stimulated emission by scattering light from disordered particles. Their inherent randomness, however, makes controlling the emission wavelength difficult. It is now shown that this problem can be remedied by carefully matching the pump laser to the specific random medium. The concept is applied to a one-dimensional optofluidic device, but could also be applicable to other random lasers.

The Fern Sporangium: A Unique Catapult

High-speed observations reveal how rapid changes in cell shape powerfully eject fern spores. Various plants and fungi have evolved ingenious devices to disperse their spores. One such mechanism is the cavitation-triggered catapult of fern sporangia. The spherical sporangia enclosing the spores are equipped with a row of 12 to 13 specialized cells, the annulus. When dehydrating, these cells induce a dramatic change of curvature in the sporangium, which is released abruptly after the cavitation of the annulus cells. The entire ejection process is reminiscent of human-made catapults with one notable exception: The sporangia lack the crossbar that arrests the catapult arm in its returning motion. We show that much of the sophistication and efficiency of the ejection mechanism lies in the two very different time scales associated with the annulus closure.

Optofluidic random laser

Random lasing is reported in a dye-circulated structured polymeric microfluidic channel. The role of disorder, which results from limited accuracy of photolithographic process, is demonstrated by the variation of the emission spectrum with local-pump position and by the extreme sensitivity to a local perturbation of the structure. Thresholds comparable to those of conventional microfluidic lasers are achieved, without the hurdle of state-of-the-art cavity fabrication. Potential applications of optofluidic random lasers for on-chip sensors are discussed. Introduction of random lasers in the field of optofluidics is a promising alternative to on-chip laser integration with light and fluidic functionalities.

Water jet rebounds on hydrophobic surfaces: a first step to jet micro-fluidics

When a water jet impinges upon a solid surface it produces a so called hydraulic jump that everyone can observe in the sink of their kitchen. It is characterized by a thin liquid sheet bounded by a circular rise of the surface due to capillary and gravitational forces. In this phenomenon, the impact induces a geometrical transition, from the cylindrical one of the jet to the bi-dimensional one of the film. A true jet rebound on a solid surface, for which the cylindrical geometry is preserved, has never been yet observed. Here we experimentally demonstrate that a water jet can impact a solid surface without being destabilized. Depending on the incident angle of the impinging jet, its velocity and the degree of hydrophobicity of the substrate, the jet can (i) bounce on the surface with a fixed reflected angle, (ii) land on it and give rise to a supported jet or (iii) be destabilized, emitting drops. Capillary forces are predominant at the sub-millimetric jet scale considered in this work, along with the hydrophobic nature of the substrate. The results presented in this communication raise the fundamental question of knowing why such capillary hydraulic jump gives rise to this unexpected jet rebound phenomenon. This study furthermore offers new and promising possibilities to handle a small quantity of water through “jet micro-fluidics”

The fern cavitation catapult: mechanism and design principles.

Leptosporangiate ferns have evolved an ingenious cavitation catapult to disperse their spores. The mechanism relies almost entirely on the annulus, a row of 12–25 cells, which successively: (i) stores energy by evaporation of the cells’ content, (ii) triggers the catapult by internal cavitation, and (iii) controls the time scales of energy release to ensure efficient spore ejection. The confluence of these three biomechanical functions within the confines of a single structure suggests a level of sophistication that goes beyond most man-made devices where specific structures or parts rarely serve more than one function. Here, we study in detail the three phases of spore ejection in the sporangia of the fern Polypodium aureum. For each of these phases, we have written the governing equations and measured the key parameters. For the opening of the sporangium, we show that the structural design of the annulus is particularly well suited to inducing bending deformations in response to osmotic volume changes. Moreover, the measured parameters for the osmoelastic design lead to a near-optimal speed of spore ejection (approx. 10 m s–1). Our analysis of the trigger mechanism by cavitation points to a critical cavitation pressure of approximately −100 ± 14 bar, a value that matches the most negative pressures recorded in the xylem of plants. Finally, using high-speed imaging, we elucidated the physics leading to the sharp separation of time scales (30 versus 5000 µs) in the closing dynamics. Our results highlight the importance of the precise tuning of the parameters without which the function of the leptosporangium as a catapult would be severely compromised.

Electrowetting control of bouncing jets

As recently studied, a sub-millimetric liquid jet can bounce on sufficiently hydrophobic surfaces [F.Celestini et al., Soft Matter 6(23), 5872-5876 (2010); A. Kibar et al., Exp. Fluids 49(5), 1135-1145 (2010)]. As the hydrophobicity is reduced, the reflection angle (θr) increases and the jet rebound deviates more and more from specular reflection. In the present study, we vary the wetting properties of the substrate using the electrowetting effect to induce a change in the reflection angle. A liquid jet is sent toward a metallic surface coated by an insulating, hydrophobic layer. Applying an ac voltage between the metallic nozzle and the electrode below the insulating layer, we can precisely control the reflection angle of the jet. The effects of the amplitude and the frequency of the applied voltage are analyzed. This study can find applications for the control of jet dynamics.

Active control of the emission of an optofluidic random laser

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.

Photoelastic study of acoustic wave propagation in grain packings

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.