Role of vapor transfer on flow coating of colloidal dispersions in the evaporative regime
Résumé
Flow coating techniques, such as knife coating, blade coating, or doctor blade, have now become essential processes to coat continuously functional layers on solid substrates starting from dilute inks, and for applications ranging from organic electronics to optical coatings. Figure 1 shows schematically a typical blade-coating setup for the specific case of a colloidal dispersion. A liquid film is drawn out of a liquid reservoir confined between a fixed blade and a moving substrate at a velocity V. The fine description of such flow-coating processes, and particularly the prediction of the final deposit thickness hd as a function of the process parameter and physico-chemical features of the ink, is still a major issue. Many works previously reported in the literature have clearly identified two regimes for such flow-coatinglike processes depending of the coating speed V [4-6]. At high V, a liquid film is drawn out of the reservoir and dries later on. In this regime, often referred to as the Landau-Levich regime, drying and coating are separated in time, and the height of the liquid film results from a balance between surface tension and friction induced by the substrate motion [7, 8]. At low V, solvent evaporation cannot be neglected during the film withdrawal, and the coating process yields directly a dry deposit: this is the evaporative regime.
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