No abstract

The wettability of coatings is very sensitive to the amount of solvent they may contain. When a droplet of volatile solvent, such as water, is deposited on a substrate, its vapor may quickly condensate just ahead of the contact line. We give an estimation of the extent of solvent uptake by a coating of variable thickness e , in front of an advancing contact line of given velocity U . Depending on the values of e and U , we observe three regimes: at low velocity and for a thin coating, the coating adsorbs a fraction of solvent that can quickly equilibrate across its entire thickness, so that it mainly appears solvophilic, while this is not the case for a thick coating. For high velocities, regardless the coating thickness, the coating ahead of the contact line does not have enough time to adsorb a significant amount of solvent, so that it mainly appears solvophobic. All these phenomena appear to be controlled by a molecular cut-off length.

Thermal motion gives rise to fluctuations in free surfaces; measurement of the thermally excited waves on such surfaces provides information on the mechanical properties of the medium. We have developed an optical tool to probe the thermally excited waves on free surfaces: surface fluctuation specular reflection (SFSR) spectroscopy. It consists in measuring the fluctuations in the position of a laser beam that is specularly reflected onto the free surface of a medium. The position of the reflected beam is sensitive to the roughness of the probed surface; the thermal waves are detected by subtracting the light intensities collected on the two quadrants of a photodiode, on which the beam is centered. We show how the measured signal is related to the medium properties. We also present measurements performed on Newtonian liquids as well as on a viscoelastic solid; we show that in all cases, there is a very good agreement between experimental and computed spectra. SFSR thus applies to a broad range of materials. It moreover offers a very good temporal resolution and should provide a useful tool for dynamical measurements on complex fluids.