REACTIVE WETTING

The wetting dynamics of a drop on a substrate depends the flow and dynamics occuring at the scale of a few nanometers at the contact line between the solid and the drop. We study situations, inspired from industrial applications, where a mass transfer, caused by evaporation or substrate dissolution, occur very close to the contact line, which induces local variations of the viscosity or interfacial tensions, and modifies the wetting dynamics.

Wetting of soluble substrate

When a droplet spreads on a hydrophilic polymer film, the contact angle is not zero, as a dry polymer always present hydrocarbon moities at the air-solid interface. However, as the water drop spreads, water evaporating from the drop, diffuses into the film and hydrates it. The content of water in the film depends on the contact line velocity and thickness of the polymer film and controls the dynamic contact angle. In the case where the film is composed of a charged polymer, a gradient in osmotic pressure is created at the contact line, which can lead to the pinning of the contact line.

Publications

• Tay, A., Bendejacq, D., Monteux, C. & Lequeux, F. How does water wet a hydrosoluble substrate? Soft Matter 7, 6953 (2011).
• Tay, A., Monteux, C., Bendejacq, D. & Lequeux, F. How a coating is hydrated ahead of the advancing contact line of a volatile solvent droplet. Eur. Phys. J. E (2010) doi:10.1140/epje/i2010-10662-7.
• Monteux, C., Tay, A., Narita, T., De Wilde, Y. & Lequeux, F. The role of hydration in the wetting of a soluble polymer. Soft Matter 5, 3713 (2009).
• Tay, A., Lequeux, F., Bendejacq, D. & Monteux, C. Wetting properties of charged and uncharged polymeric coatings—effect of the osmotic pressure at the contact line. Soft Matter 7, 4715 (2011).

Wetting and evaporation

When a droplet containing colloidal particles is evaporating, the high rate of evaporation at the contact line and pinning of the contact line by the particles causes the accumulation of the solute at the contact line and to the formation of rings of colloids at the periphery of the drop. We have investigated the coupling between the pinning of the contact line and the evaporation flux in diverse situations. In the case of bidisperse colloidal dispersions, the small particles tend to segregate at the contact line. In the case of polymers solutions, the accumulation of polymer at the contact line induces a viscosity increase in the few nanometers close to the contact line, which controls the dynamic contact angle. As a consequence the dynamic contact angle depends on the relative humidity. We are now investigating the case of evaporating thin films of surfactant solutions, where additional Marangoni flows can either destabilize or stabilize the films, depending on the adsorption dynamics of the surfactants.

• Noirjean, C., Marcellini, M., Deville, S., Kodger, T. E. & Monteux, C. Dynamics and ordering of weakly Brownian particles in directional drying. Phys. Rev. Mater. 1, (2017).
• Liu, Y., Lee, D. Y., Monteux, C. & Crosby, A. J. Hyperbranched polymer structures via flexible blade flow coating. J. Polym. Sci. Part B Polym. Phys. 54, 32–37 (2016).
• Monteux, C. & Lequeux, F. Packing and Sorting Colloids at the Contact Line of a Drying Drop. Langmuir 27, 2917–2922 (2011)
• Monteux, C. & Lequeux, F. Packing and Sorting Colloids at the Contact Line of a Drying Drop. Langmuir 27, 2917–2922 (2011)
• Kajiya, T., Monteux, C., Narita, T., Lequeux, F. & Doi, M. Contact-Line Recession Leaving a Macroscopic Polymer Film in the Drying Droplets of Water−Poly( N , N -dimethylacrylamide) (PDMA) Solution. Langmuir25, 6934–6939 (2009)
• Monteux, C., Elmaallem, Y., Narita, T. & Lequeux, F. Advancing-drying droplets of polymer solutions: Local increase of the viscosity at the contact line. EPL Europhys. Lett.83, 34005 (2008)