Thin films on surfaces can be obtained by spreading a liquid film containing a solute or a dispersed phase and subsequently evaporating the solvent. This process provides a versatile way of deposition of various solutes or dispersed particles or droplets, of organic, mineral or hybrid natures. This deposition process is particularly of interest for coating glass surfaces. However, during drying, different effects can result in unwanted lateral gradient of properties – thickness in general, but also composition in the case of complex mixtures. As a result, the coating process is very limited industrially for coatings requiring optical quality. The understanding of the lateral motions of fluid occurring between the spreading and the solvent evaporation stages is therefore a crucial problem.
Marangoni effects during the drying of thin liquid films
During the drying process of thin liquid films, any surface tension gradients arising in the films may induce Marangoni flows. These flows can result in film dewetting or in hydrodynamic instabilities that may eventually lead to film opening. As shown below through the thickness map of an evaporating film of a surfactant solution, both film opening and front receding are taking place.
We experimentally investigate and monitor the instabilities occurring in evaporating films made of binary liquids or surfactant solutions. Besides, we model the latter phenomena using a lubrication approximation and also numerically simulate them using the Navier-Stokes solver Basilisk.
Drying of polymer solutions
We investigate the effect of heterogeneities of the evaporation flux on the drying of supported thin films of polymer solutions. The figure below shows the thickness map of the dry deposit obtained by limiting the evaporation flux in a given circular region: a depression has formed under that region, and the polymer has accumulated into a ridge.