Department of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada.
Department of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada.
J Colloid Interface Sci. 2017 Apr 15;492:199-206. doi: 10.1016/j.jcis.2016.12.055. Epub 2016 Dec 26.
Efforts to stabilize emulsions against coalescence in flow have often focused on modifying properties of the interface between the continuous and dispersed phases, to create a repulsive barrier against coalescence. But prior to experiencing any interaction force, the liquid film between two colliding drops has to drain, and if this drainage process is arrested, coalescence will be suppressed. In this work, scaling analyses and thin-film lubrication simulations are used to study the hydrodynamic drainage properties of thin films of a Bingham fluid (a yield stress fluid, which flows only when a critical stress is exceeded) created between two drops colliding under the action of a constant force. Our study shows that the hydrodynamic drainage process can be arrested completely when the film reaches a critical thickness, before attractive forces result in the rupture of the film, provided that the film shape is in the dimpled configuration. This critical thickness is h=6τR/γ, where τ is the yield stress of the suspending medium, R is the drop radius and γ is the interfacial tension between the fluids. The yield stress can thus serve as an independent tuning parameter that sets an upper bound on the drop size beyond which coalescence is turned off in sheared emulsions.
在流动中稳定乳液以防止聚结的努力通常集中在改变连续相和分散相之间界面的性质上，以形成防止聚结的排斥屏障。但在经历任何相互作用力之前，两个碰撞液滴之间的液膜必须排干，如果这个排水过程被阻止，聚结就会被抑制。在这项工作中，我们使用标度分析和薄膜润滑模拟来研究在恒定力作用下两个碰撞液滴之间形成的宾汉流体（一种屈服应力流体，只有超过临界应力时才会流动）的薄膜的流体动力学排水特性。我们的研究表明，当薄膜达到临界厚度时，排水过程可以完全停止，从而防止在吸引力作用下薄膜破裂，只要薄膜形状处于凹陷配置中。这个临界厚度是 h=6τR/γ，其中 τ 是悬浮介质的屈服应力，R 是液滴半径，γ 是两种流体之间的界面张力。因此，屈服应力可以作为一个独立的调整参数，它设定了一个上限，超过这个上限，剪切乳液中的聚结就会被关闭。
