Aggregation due to capillary forces during drying of particle submonolayers

When colloids deposited on a macroscopic substrate are imaged after drying, two-dimensional aggregates are commonly observed. When the colloids are initially stable and well dispersed in solution, the final presence (or not) of these surface aggregates is linked to the balance between the mutual colloids–surface interactions and the capillary forces. In this work, we control the adhesion of α-Al2O3 colloids onto mica using a surface molecular ligand (citric acid) and measure the associated structure of the layer both in situ and after drying. The dynamic property of speckle pattern obtained from light scattering of the layer is used to probe in situ the dynamics of the layer. It is therefore concluded that even the weakest adhesion of the particles does no allow in situ surface mobility nor aggregation. The adhering particles aggregate only during the drying stage, due to the influence of lateral capillary forces. A model for the aggregation mechanism is introduced taking into account the balance of forces acting on the particles during the drying stage.