Glassy skin formation
in drying liquid films

One process for depositing thin films on glass involves drying a liquid layer previously spread on the surface. During drying, vertical gradients in solute concentration can appear. This phenomenon is controlled by a Péclet number, which compares a characteristic evaporation time with a diffusion time for solute molecules. When the Péclet number is greater than unity, diffusion can no longer homogenise the solute concentration, and the solute accumulates near the interface with the air[1]. When the solute is an amorphous polymer, a glass transition driven by the solvent concentration can occur near this interface, leading to the formation of a skin. The presence of a solid skin can have important consequences for drying-related phenomena. In particular, recent observations show that the effect of defects on the final morphology of the film is probably modified by such a skin (Figure 1). However, it is difficult to predict the appearance of such a skin, as variations in the mutual polymer/solvent diffusion coefficient are generally poorly understood because they are difficult to measure. In addition, the thickness of the skin may be small (less than a micron), which makes it difficult to demonstrate experimentally.

We propose to study the formation of vitreous skin during the drying of polymer solutions as a subject of Ph. D thesis. Different polymer/solvent couples with different mutual diffusion coefficients will be selected.
Experiments will be developed to demonstrate the appearance of a very thin skin. These experiments will be based, for example, on measurements of the response to mechanical stress. The results will be compared with recent theoretical predictions on the formation of glassy skins in polymer solutions. The consequences for lateral flow in the film resulting from defects or non-uniform evaporation rates will be studied. The final objective of the thesis is to be able to predict skin formation and to determine the key parameters for obtaining uniform dry deposits after drying of polymer solutions.
[1] A. F Routh, “Drying of thin colloidal films”, Reports on Progress in Physics, vol 76, 2013.
[2] L. Talini and F. Lequeux, “Formation of glassy skins in drying polymer solutions: approximate analytical solutions”, Soft Matter, vol 19, 2023,

Date and localisation :
The thesis will start in October 2024. It will take place mainly in the Surfaces du Verre et Interfaces (SVI) laboratory, located at Saint-Gobain Research Paris, 39 quai Lucien Lefranc, 90303 Aubervilliers. http://svi.cnrs.fr/. The thesis will thus benefit from an industrial environment allowing exchanges with research engineers from other departments at Saint-Gobain Research Paris. It will be carried out in collaboration with the SIMM laboratory (ESPCI).

Preferred profile :
 Soft matter physicist;
 Taste for experiments ;

Contact :
Laurence Talini (laurence.talini@cnrs.fr) and Martin Coux (martin.coux@saint-gobain.com)

These 2024 – Talini-Coux – SVI Glassy skin formation_EN_FR