Physical ageing of the contact line on colloidal particles at liquid interfaces

Young's law(1) 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(2). This has been used to explain why colloids often bind to liquid interfaces(3,4), and has been exploited in emulsification(5), water purification(6), mineral recovery(7), encapsulation(8) and the making of nanostructured materials(9,10). 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(11,12).