Michel Voué

Drop Impact on Porous Superhydrophobic Polymer Surfaces

Water drop impacts are performed on porous-like superhydrophobic surfaces. We investigate the influence of the drop size and of the impact velocity on the event. The Cassie-Baxter/Wenzel transition is observed to be a function of the drop size, as well as the outcomes of the impact or deposition process, which can be deposition, rebound, sticking, or fragmentation. A quantitative analysis on the experimental conditions required to observe rebound is provided. Our analysis shows that the wettability hysteresis controls the limit between deposition and rebound events. This limit corresponds to a constant Weber number. A survey of literature results on impact over patterned superhydrophobic surfaces is provided as a comparison.

Dynamics of wetting

We review the mechanisms controlling the dynamics of wetting in partial and complete wetting regimes. It is shown that the behaviour in several timescales may characterize the dynamics since different channels of energy dissipation have to be considered within spreading.

Effect of temperature on the dynamic contact angle

Abstract The temperature influence on the contact angle relaxation of partially wetting drops is measured for squalane on poly(ethyleneterephthalate). The results are analyzed with the molecular kinetic model, a hydrodynamic model and a model that combines the two previous ones. For each temperature all three approaches can fit our experiments well, but only the molecular kinetic approach leads to physically meaningful parameters. The associated free energy of wetting, introduced by that approach, increases weakly with temperature.

Surface functionalization of germanium ATR devices for use in FTIR-biosensors

Biosensors based on intrinsic detection methods have attracted growing interest. The use of Fourier transform infra-red (FTIR) spectroscopy with the attenuated internal total reflection (ATR) mode, in the biodetection context, requires appropriate surface functionalization of the ATR optical element. Here, we report the direct grafting of a thin organic layer (about 20 A depth) on the surface of a germanium crystal. This covering, constructed with novel amphiphilic molecules 2b (namely, 2,5,8,11,14,17,20-heptaoxadocosan-22-yl-3-(triethoxysilyl) propylcarbamate), is stable for several hours under phosphate buffered saline (PBS) flux and features protein-repulsive properties. Photografting of molecule 5 (namely, O-succinimidyl 4-(p-azidophenyl)butanoate) affords the activated ATR element, ready for the covalent fixation of receptors, penicillin recognizing proteins BlaR-CTD for instance. The different steps of the previous construction have been monitored by water contact angle (theta(w)) measurements, spectroscopic ellipsometry (covering depth), X-ray photoelectron spectroscopy (XPS) by using a fluorinated tag for the control of surface reactivity, and FTIR-ATR spectroscopy for the structural analysis of grafted molecules. Indeed, contrarily to silicon device, germanium device offers a broad spectral window (1000-4000 cm(-1)) and thus amide I and II absorption bands can be recorded. This work lays the foundations for the construction of novel FTIR biosensors.

Spreading and wetting at the microscopic scale: recent developments and perspectives

In both the experimental and numerical approaches of wettability, the spreading phenomenon is now understood as the consequence of the competition between a driving term (i.e. the affinity of the liquid for the solid) and the friction of the liquid molecules on the substrate. The aim of this article is to review how the combination of molecular dynamics (MD) simulations and of microscopic scale experiments leads to a better understanding of the physico-chemical properties of the solid/liquid interface and to present the most recent developments achieved in the complete wetting regime.

Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers

Superhydrophobic surfaces are generated by chemisorption on aluminum substrates of fluorinated block copolymers synthesized by reversible addition-fragmentation chain transfer in supercritical carbon dioxide. In an appropriate solvent, those block copolymers can form micelles with a fluorinated corona, which are grafted on the aluminum substrate thanks to the presence of carboxylic acid groups in the corona. Water contact angle and drop impact analysis were used to characterize the wettability of the films at the macroscale, and atomic force microscopy measurements provided morphological information at the micro- and nanoscale. The simple solvent casting of the polymer solution on a hydroxylated aluminum surface results in a coating with multiscale roughness, which is fully superhydrophobic over areas up to 4 cm(2).

Superhydrophobic surfaces from various polypropylenes

Superhydrophobic surfaces were prepared from solutions of isotactic polypropylenes of various molecular weights using soft chemistry. Varying the conditions of the experiments (polymer concentration and initial amount of the coated solution) allowed us to optimize the superhydrophobic behavior of the polymer film. Results show that decreasing the concentration and/or film thicknesses decreases the probability to get superhydrophobicity for all polypropylenes tested. Measurement and analysis of advancing and receding contact angles as well as estimation of surface homogeneity were performed. Similar results were obtained with syndio- as well as atactic polypropylenes.

Droplet spreading: a microscopic approach

Abstract It is shown that the technique of molecular dynamics allows a complete study of the dynamics of droplet spreading at the microscopic scale, for both complete and partial wetting regimes. In particular, we are able to study, in great detail, the net fluxes inside the drop during spreading. We confirm that at the molecular level, the mechanism of spreading can be viewed as a competition between a surface tension driving force and friction between the liquid and solid atoms. This result supports the molecular kinetic theory of wetting as well as the de Gennes–Cazabat model of spreading.

Drop Impact on Soft Surfaces: Beyond the Static Contact Angles

The wettability of cross-linked poly(dimethylsiloxane) elastomer films and of octadecyltrichlorosilane self-assembled monolayers with water has been measured and compared using various methods. Contact angle hysteresis values were compared with values reported in the literature. A new method to characterize advancing, receding contact angles, and hysteresis using drop impact have been tested and compared with usual methods. It has been found that for the rigid surfaces the drop impact method is comparable with other methods but that for elastomer surfaces the hysteresis is function of the drop impact velocity which influences the extent of the deformation of the soft surface at the triple line.

Ligand-receptor interactions in complex media: A new type of biosensors for the detection of coagulation factor VIII

Detection of receptor-ligand interaction in complex media remains a challenging issue. We report experimental results demonstrating the specific detection of the coagulation factor VIII in the presence of a large excess of other proteins using the new BIA-ATR technology based on attenuated total reflection Fourier transform infrared spectroscopy. The principle of the detection is related to the ability of factor VIII molecules to bind to lipid membranes containing at least 8% phosphatidylserine. Several therapeutic concentrates of factor VIII were analyzed and the binding of the coagulation factor was monitored as a function of time. We show that a non-specific adsorption of stabilizing agents (typically, von Willebrand factor and human serum albumin) may be avoided by controlling the geometry of the ATR element. A linear response of the sensors as a function of the factor VIII concentration is described for different lipid membrane compositions.