Mossige E J, Chandran Suja V, Islamov M, Wheeler S F, Fuller Gerald G
Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
Department of Chemical Engineering, Columbia University, New York, NY 10027, USA.
Philos Trans A Math Phys Eng Sci. 2020 Jun 26;378(2174):20190533. doi: 10.1098/rsta.2019.0533. Epub 2020 Jun 8.
Understanding the mechanics of detrimental convective instabilities in drying polymer solutions is crucial in many applications such as the production of film coatings. It is well known that solvent evaporation in polymer solutions can lead to Rayleigh-Bénard or Marangoni-type instabilities. Here, we reveal another mechanism, namely that evaporation can cause the interface to display Rayleigh-Taylor instabilities due to the build-up of a dense layer at the air-liquid interface. We study experimentally the onset time () of the instability as a function of the macroscopic properties of aqueous polymer solutions, which we tune by varying the polymer concentration (), molecular weight and polymer type. In dilute solutions, shows two limiting behaviours depending on the polymer diffusivity. For high diffusivity polymers (low molecular weight), the pluming time scales as [Formula: see text]. This result agrees with previous studies on gravitational instabilities in miscible systems where diffusion stabilizes the system. On the other hand, in low diffusivity polymers the pluming time scales as [Formula: see text]. The stabilizing effect of an effective interfacial tension, similar to those in immiscible systems, explains this strong concentration dependence. Above a critical concentration, [Formula: see text], viscosity delays the growth of the instability, allowing time for diffusion to act as the dominant stabilizing mechanism. This results in scaling as (/). This article is part of the theme issue 'Stokes at 200 (Part 1)'.
了解干燥聚合物溶液中有害对流不稳定性的机制在许多应用中至关重要，例如薄膜涂层的生产。众所周知，聚合物溶液中的溶剂蒸发会导致瑞利 - 贝纳德或马兰戈尼型不稳定性。在这里，我们揭示了另一种机制，即由于气液界面处致密层的形成，蒸发会导致界面呈现瑞利 - 泰勒不稳定性。我们通过实验研究了不稳定性的起始时间（）作为水性聚合物溶液宏观性质的函数，我们通过改变聚合物浓度（）、分子量和聚合物类型来调节这些性质。在稀溶液中，根据聚合物扩散率，呈现出两种极限行为。对于高扩散率聚合物（低分子量），羽流时间尺度为[公式：见正文]。这一结果与先前关于可混溶系统中重力不稳定性的研究一致，在该研究中扩散使系统稳定。另一方面，在低扩散率聚合物中，羽流时间尺度为[公式：见正文]。类似于不混溶系统中的有效界面张力的稳定作用解释了这种强烈的浓度依赖性。高于临界浓度[公式：见正文]时，粘度会延迟不稳定性的增长，使扩散有时间成为主要的稳定机制。这导致尺度为（/）。本文是主题问题“斯托克斯诞辰200周年（第1部分）”的一部分。
