Mechanical properties and structure of particle coated interfaces: Influence of particle size and bidisperse 2D suspensions

We report surface pressure-area (Pi-A) isotherms of bidisperse mixtures of anionic polystyrene latex particles at a water/n-decane interface as well as optical photographs of the interface for various compressions and mixture ratios. In the case of mixtures of 3 and 5 mu m particles, we observe crystalline layers at high or low concentration ratios, where the impurity particles concentrate at the grain boundaries of the crystalline structure. At intermediate ratios, the layers become highly disordered. However, in both cases, we show that the shape of the isotherms remains unchanged. In the case of the mixtures of 9 mu m particles with either 3 or 5 mu m particles, the smaller particles aggregate around the larger particles through capillary interaction resulting in the formation of large fractal aggregates. At high compression, these layers contain holes that seem very compressible. As a result, the surface pressure isotherms show a smaller surface pressure jump than for other mixtures.