Critical Role of the Atmosphere in Dip-Coating Process

Dip-coating is a common liquid deposition technique employed in research, and also for industrial production, to obtain polymer, hybrid, and inorganic thin layers of controlled thickness. During liquid deposition, the substrate withdrawal speed allows, in principle, an easy tuning of deposited film thickness (first modeled by Landau and Levich). Yet, experimentally, unexplained thickness irreproducibility or strong fluctuations of the sol gel films are often observed when coating large substrates, which is a critical issue for optical coatings such as anti-reflective/reflective coatings. In this study, we point out for the first time that uncontrolled solvent relative pressure gradients (coming from solvent evaporation) are responsible for these thickness fluctuation issues. We investigated and quantified their impact for various solutions (of sol gel or polymer) and pointed out that the solvent evaporation rate is not constant but strongly depends on the geometric configuration of the dip-coating experiment. From this understanding, we demonstrated how an accurate tuning of processing atmosphere can provide a very good control on layer thickness in the practical case of the deposition of anti-reflective water repellent coating. In a second example, we used this phenomenon for developing a very easy synthesis strategy leading to giant and controlled thickness gradient profiles.