Measuring the three-phase contact angle of nanoparticles at fluid interfaces.

We report a generic technique to image and study the wettability of spherical nanoparticles adsorbed at liquid surfaces and demonstrate that nanoparticle monolayers can be imprinted at air-water and oil-water interfaces and their three-phase contact angle at the original liquid interface can be determined by an atomic force microscopy scan on a replica of the interface; the technique is tested using four different types of nanoparticles, the smallest one having a radius of 37 nm.