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

薄的液体加工聚合物薄膜是许多创新产品的重要组成部分。突出的例子是清漆、光学薄膜、有机发光二极管或印刷生物传感器。这种薄膜的干燥往往导致不必要的层厚不均匀和自由表面的变形，这对产品的质量有相当大的影响。这些效应可以归因于表面张力驱动的对流，即所谓的马兰戈尼对流，这是由局部不均匀的溶剂蒸发引起的。为了提高表面质量，必须避免或至少减少这些流动。为了研究这一现象，不仅要分析自由表面的变形，而且要了解在此过程中膜内的流动情况。在几个非常好的研究项目中，已经开发了热诱导对流描述的解析解和数值模拟。然而，在薄膜干燥的情况下，还会出现其他尚未研究的现象。除了温度的影响外，干燥过程中溶剂含量不断减少，这也影响了表面张力梯度和对流流的形成，导致已经变形的膜表面凝固。在干燥过程中必须同时考虑这些耦合现象。据我们所知，在文献中没有关于这一主题的相关工作发表。在本研究项目中，我们的目标是通过实验、定量研究浓度梯度引起的溶质马兰戈尼流，利用三维微粒子跟踪测速来缩小这一差距。此外，确定相应的稳定性极限将有助于在这一研究领域获得重要的知识。

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.