CNRS, Solvay, LOF, UMR 5258, Université Bordeaux , F-33600, Pessac, France.
Laboratoire FAST, Université Paris-Sud, CNRS, Université Paris-Saclay , F-91405, Orsay, France.
Langmuir. 2017 Dec 12;33(49):14078-14086. doi: 10.1021/acs.langmuir.7b03297. Epub 2017 Dec 4.
In flow-coating processes at low substrate velocity, solvent evaporation occurs during the film withdrawal and the coating process directly yields a dry deposit. In this regime, often referred to as the evaporative regime, several works performed on blade-coating-like configurations have reported a deposit thickness h proportional to the inverse of the substrate velocity V. Such a scaling can be easily derived from simple mass conservation laws, assuming that evaporation occurs on a constant distance, referred to as the evaporation length, noted L in the present paper and of the order of the meniscus size. However, the case of colloidal dispersions deserves further attention. Indeed, the coating flow leads to a wet film of densely packed colloids before the formation of the dry deposit. This specific feature is related to the porous nature of the dry deposit, which can thus remain wet when capillary forces are strong enough to prevent the receding of the solvent through the pores of the film (the so-called pore-emptying). The length of this wet film may possibly be much larger than the meniscus size, therefore modifying the solvent evaporation rate, as well as the scaling h ∼ 1/V. This result was suggested recently by different groups using basic modeling and assuming for simplicity a uniform evaporation rate over the wet film. In this article, we go a step further and investigate the effect of multidimensional vapor mass transfer in the gas phase on L and h in the specific case of colloidal dispersions. Using simplified models, we first provide analytical expressions in asymptotic cases corresponding to 1D or 2D diffusive vapor transport. These theoretical investigations then led us to show that L is independent of the evaporation rate amplitude, and roughly independent of its spatial distribution. Conversely, h strongly depends on the characteristics of vapor mass transfer in the gas phase, and different scaling laws are obtained for the 1D or the 2D case. These theoretical findings are finally tested by comparison with experimental results supporting our theoretical simplified approach.
在低基板速度的流涂工艺中，溶剂在薄膜抽出过程中蒸发，涂层工艺直接产生干燥沉积物。在这个通常称为蒸发区的区域中，许多在类似于刮刀涂布的配置上进行的工作报告说，沉积物厚度 h 与基板速度 V 的倒数成正比。这种比例关系可以很容易地从简单的质量守恒定律中推导出来，假设蒸发发生在一个恒定的距离上，称为蒸发长度，在本文中记为 L，大约与弯月面的大小相当。然而，胶体分散体的情况需要进一步关注。事实上，在形成干燥沉积物之前，涂层流动会导致密集堆积的胶体形成湿膜。这种特殊的特征与干燥沉积物的多孔性质有关，当毛细力足够强以防止溶剂通过薄膜的孔（所谓的孔排空）回流时，干燥沉积物可以保持湿润。这个湿膜的长度可能比弯月面的大小长得多，因此会改变溶剂蒸发速率，以及 h∼1/V 的比例关系。最近，不同的研究小组使用基本模型并假设简单地在湿膜上存在均匀的蒸发速率，提出了这个结果。在本文中，我们更进一步，研究了在胶体分散体的特殊情况下，气相中多维蒸汽质量传递对 L 和 h 的影响。使用简化模型，我们首先在对应于 1D 或 2D 扩散蒸汽传输的渐近情况下提供了分析表达式。这些理论研究随后使我们得出结论，L 与蒸发速率幅度无关，并且大致与其空间分布无关。相反，h 强烈依赖于气相中蒸汽质量传递的特性，并且对于 1D 或 2D 情况，得到了不同的比例关系。最后，通过与支持我们的理论简化方法的实验结果进行比较，验证了这些理论发现。
