Pattern formation and interactions of like charged colloidal particles at the air/water interface

In the last decade, there have been experimental reports on the formation of colloidal mesostructures at the air/water interface. These patterns, range from the formation of transient colloidal chains and soap-froth structures that evolve to more energetically stable colloidal clusters. If the surface colloidal density is high, a crystalline-like structure can also be formed. This kind of mesostructures has been observed in particles that range in size from a few nanometers to a few microns. In the case of micron-size charged colloidal particles, the charge asymmetries on the particle's surface at the air/water interface produce the formation of dipole-dipole interaction, which should be repulsive. The formation of these mesostructures, where the equilibrium distance among particles is in the micrometer range, has been interpreted as the result of a competition between long-range repulsive and attractive interactions. Measurements of the pair interaction potential in these systems show clear evidence of micron-range attractive interactions between the colloidal particles, whose pattern formation behavior has been reproduced by computer simulations that use such micrometer range attractive interaction. However, a good theoretical understanding on the origin of the attractive component is still missing. Here, we review our main findings on these systems and we discussed them in view of recent results obtained by other groups.