INSTABILITY AND RUPTURE OF THIN LIQUID FILMS ON SOLID SUBSTRATES
INSTABILITY AND RUPTURE OF THIN LIQUID FILMS ON SOLID SUBSTRATES
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固体基材上液体薄膜的不稳定性和破裂
DOI：
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发表时间：
2013
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通讯作者：
V. Ajaev
中科院分区：
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作者：
V. Ajaev
固体表面上的薄液膜破裂的研究对于微流体装置、热交换系统、采矿业中的多相流的建模以及生物医学应用（例如眼睛中的泪膜的动力学）是重要的。在本研究中，我们回顾了膜破裂的理论工作，并讨论了它与最近的一些实验的比较。薄液膜的伦敦范德华分散力，静电效应，和热毛细不稳定性的破裂条件进行了讨论。可变形流体界面和固体壁面之间的液膜不稳定性的数学模型对于多相流的许多应用是重要的。本文中的流体界面是液体和蒸汽或气体之间的边界，尽管这里讨论的一些结果也适用于液-液界面。当界面接近壁面时，膜会破裂，从而形成三相接触线。在应用中遇到的许多类型的多相流的整体动力学取决于这些局部现象。在微流体装置中，通过通道输送的液滴和气泡可以由于分离流体界面和壁的膜的破裂而减慢，因为这种膜的存在对于维持微流体输送效率是必不可少的，如所讨论的，例如，见Ajaev和Homsy（2006年）。如Kabov（2000）的实验所示，在基于由液-气界面处的重力或剪切应力驱动的薄膜流的冷却系统中，干点的形成导致来自加热壁的热通量显著减少。液膜在重力作用下流过垂直或倾斜平板上的局部加热器，当加热器产生足够高的热通量时，液膜破裂。这些实验观察对于制定指导方针以避免工业应用中的冷却系统破裂非常重要。气固两相分离薄膜的稳定性对于研究固体颗粒与气泡的相互作用具有重要意义。对于这些研究来说，特别重要的是当颗粒由于分离它们的液膜破裂而附着在气泡上时的条件。这项工作的主要应用是泡沫浮选（Nguyen和Schulze，2004年; Rao，2004年），这是矿物加工中的一个重要步骤，在此过程中，液体罐中的有价值矿物颗粒附着在上升的气泡上，从而与脉石分离，即，保留在罐底部附近的未加工矿石存款的商业上无价值的部分。泡沫浮选中的典型矿物是金属硫化物，然后进行进一步加工，直到产生纯金属，如铜，铅和锌。类似的技术已成功地应用于煤炭工业，最近还用于纸张回收的脱墨过程（Drelich和米勒，2001年）。最近几项关于液膜破裂的研究是由眼睛中泪膜的动力学引起的，并在Braun（2012）中进行了详细讨论。泪膜在每次眨眼后形成，对于在眼睛前部提供高质量的光学表面以及保护免受灰尘和细菌的侵害至关重要。一种被称为干眼综合征（DES）的医学状况发生在眨眼后泪膜破裂过快，通常在几秒钟内。了解这种破裂的原因对于制定DES的治疗策略至关重要。
Studies of rupture of thin liquid films on solid surfaces are important for modeling of multiphase flows in microfluidic devices, heat exchange systems, mining industry, and for biomedical applications such as dynamics of the tear film in the eye. In the present study we review theoretical work on film rupture and discuss its comparison with some recent experiments. Conditions for the break-up of thin liquid films by London-van der Waals dispersion forces, electrostatic effects, and thermocapillary instability are discussed. Mathematical models of instability of a liquid film between a deformable fluid interface and a solid wall are important for a number of applications of multiphase flow. The fluid interface in the present context is a boundary between liquid and vapor or gas, although some of the results discussed here are applicable to liquid-liquid interfaces as well. When the interface approaches the wall, the film can rupture, resulting in the formation of a three-phase contact line. The overall dynamics of many types of multiphase flows encountered in applications depends on these local phenomena. In microfluidic devices, drops and bubbles transported through a channel can slow down as a result of rupture of the film separating the fluid interface and the wall since the presence of such film is essential for maintaining the microfluidic transport efficiency, as discussed, e.g., in Ajaev and Homsy (2006). In cooling systems based on thin-film flows driven by either gravity or shear stresses at the liquid-gas interface, formation of dry spots results in significant reduction of heat flux from the heated wall, as shown experimentally by Kabov (2000). Liquid films flowing under gravity over a localized heater on vertical or inclined flat plates rupture for sufficiently high values of the heat flux generated by the heater. These experimental observations are important for developing guidelines to avoid rupture in cooling systems for industrial applications. Stability of thin films separating solid and gas phases is important for studies of interaction between solid particles and gas bubbles. Of particular importance for these studies are the conditions when particles become attached to air bubbles as a result of rupture of the liquid film separating them. The main application of this work is froth flotation (Nguyen and Schulze, 2004; Rao, 2004), an important step in mineral processing during which particles of valuable minerals in a liquid tank become attached to rising air bubbles and thus separated from the gangue, i.e., commercially worthless part of the raw ore deposit that remains near the bottom of the tank. Typical minerals in froth flotation are metal sulfides, which then undergo further processing until pure metals such as copper, lead, and zinc are produced. Similar techniques are successfully applied in the coal industry and, more recently, for the de-inking process in paper recycling (Drelich and Miller, 2001). Several recent studies of liquid film rupture have been motivated by dynamics of the tear film in the eye and are discussed in detail in Braun (2012). The tear film forms after each blink and is essential for providing a high-quality optical surface at the front of the eye and also for protection against dust and bacteria. A medical condition called the dry-eye syndrome (DES) occurs when the tear film ruptures too quickly after the blink, usually within a few seconds. Understanding the causes of this rupture is essential for developing treatment strategies for DES.