Nestlé Research Center, Vers-Chez-Les-Blanc, Lausanne, CH-1000 26, Switzerland.
J Colloid Interface Sci. 2010 Mar 1;343(1):79-86. doi: 10.1016/j.jcis.2009.11.035. Epub 2009 Nov 23.
From two-drop collision experiments, it is known that local extensional flow favors coalescence. Recently, Bremond et al. used microfluidic methods to evidence this point. Similarly, we used specific microfluidic geometries to impose sudden extensional flow, following drop collision under controlled conditions, and coalescence events were recorded with a high-speed camera. In this study we focus on the effect of surfactant on the coalescence, or stabilisation against it, between drops flowing apart due to either imposed external flow or capillary forces related to drop shape relaxation. Coalescence can be induced even when drops are initially separated by an intersticial lubricating film by far thicker than the critical thickness for rupturing under the action of Van der Waals forces. This is particularly relevant to avalanches of coalescence events, in flowing or even quiescent emulsions or foams. When non-ionic surfactant was used, it was observed that small concentrations apparently enhance coalescence in extension. But at higher concentrations it provides stabilisation through a specific mechanism of thread formation and rupture; the stabilisation mechanism can be complex.
从两滴碰撞实验可知，局部拉伸流有利于聚结。最近，Bremond 等人使用微流控方法证明了这一点。同样，我们使用特定的微流道几何结构来施加突然的拉伸流，在受控条件下跟随液滴碰撞，并使用高速摄像机记录聚结事件。在这项研究中，我们关注的是表面活性剂对拉伸流中聚结的影响，或者对聚结的稳定作用，拉伸流是由外部强制流动或与液滴形状松弛相关的毛细力引起的。即使在初始时液滴被远远超过范德华力作用下破裂的临界厚度的间隙润滑膜隔开，也可以诱导聚结。这对于流动或甚至静止的乳液或泡沫中的聚结事件的雪崩尤其重要。当使用非离子表面活性剂时，观察到小浓度显然会增强拉伸中的聚结。但在更高的浓度下，它通过形成和破裂的特定机制提供稳定作用；稳定机制可能很复杂。
