Quantitative investigation of convective flows in drying polymer films using 3D micro particle tracking velocimetry

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Thin, liquid-processed polymer films are an important component of many innovative products. Prominent examples are varnishes, optical films, organic light-emitting diodes or printed biosensors. The drying of such films often leads to unwanted layer thickness inhomogeneity and a deformation of the free surface, which has a considerable influence on the quality of the products. These effects can be attributed to surface-tension driven convective flow, so-called Marangoni convection, which is induced by locally inhomogeneous solvent evaporation. To increase the surface quality, it is necessary to avoid or at least reduce these flows. In order to investigate this phenomenon, not only an analysis of the deformations of the free surface, but also the knowledge of the flows in the film during the process is essential. In several very good research projects, analytical solutions and numerical simulations for the description of thermally induced convection have already been developed. In the context of film drying, however, other phenomena occur which have not been subject of research efforts so far. In addition to temperature effects, the solvent content decreases continuously during drying, which additionally influences the formation of surface tension gradients and convective flows and leads to a solidification of the already deformed film surface. These coupled phenomena have to be considered simultaneously during drying. To the best of our knowledge, no relevant work has been published on this subject in the literature.With the proposed research project, we aim to close this gap by experimental, quantitative investigations of solutal Marangoni flows induced by concentration gradients using 3D micro particle tracking velocimetry. In addition, the determination of corresponding stability limits will contribute to a significant gain of knowledge in this field of research.