Elastic properties of nanoparticle monolayer foams.

Monolayers of nanosized carboxylate-functionalized polystyrene particles at the air/water interface assemble into two coexisting liquid phases of different densities. The rheological properties of the individual phases and of the resulting nanoparticle texture is measured using a magnetic needle microrheometer. We show that, despite the liquid behavior of the individual viscous phases, the entire structure exhibits elastic response to local deformations induced by the needle.