INSTABILITY AND RUPTURE OF THIN LIQUID FILMS ON SOLID SUBSTRATES

Abstract

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.