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Comment s’hydrate une couche à l’avant de la ligne de contact avançante d’une goutte de solvant volatile

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.

A. Tay, C. Monteux, D. Bendejacq, and F. Lequeux, Eur. Phys. J. E (2010)

Schematic diagram of the solvent uptake by the coating ahead of a moving contact line. In the dry regime (iii), the velocity is so fast that the coating is not hydrated by the water. In the thin coating regime (i), the solvent equilibrates across the coating. In the thick coating regime (ii), the solvent has not enough time to equilibrate across the coating. Typical values at the crossover are 1 nm for ec and 1m/s for Uc.