Formation of two-dimensional colloidal voids, soap froths, and clusters

We report the observation of new pattern formation by spherical polystyrene particles trapped at the air-water interface; namely, the formation of two-dimensional void, soap-froth, and cluster structures. The formation of the soap-froth structure depends upon the initial surface concentration of particles. The void and soap-froth structures evolve with time. The clusters can be formed after deposition of the sample or as a result of the evolution of the soap-froth structure. The experimental observation can be explained in terms of a balance between electrostatic repulsive and attractive interactions. An optimum cluster size can be obtained from an energy analysis of the system.