Nat Mater. 2011 Dec 4;11(2):138-42. doi: 10.1038/nmat3190.
Young's law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface.
杨定律预测，与液体界面处于平衡状态的胶体球将跨越两种流体，其在界面上方的高度由平衡接触角定义。这一原理被用于解释胶体为何经常与液体界面结合，并被应用于乳化、水净化、矿物回收、封装和制造纳米结构材料。然而，对于结合的动力学过程却知之甚少。在这里，我们发现，聚苯乙烯微球吸附到水/油界面的特征是突然破裂和出乎意料的缓慢松弛。松弛过程在时间上呈对数关系，表明完全平衡可能需要数月时间。令人惊讶的是，粘性耗散似乎作用不大。相反，观察到的动力学行为与玻璃状系统中的老化非常相似，与描述接触线在纳米级表面不均匀性上的激活跳跃的模型吻合较好。这些结果可能为胶体在界面上的相互作用的一些长期存在的问题提供线索。
