In this talk, we explore the motion dynamics and the interaction of individual colloidal particles close to an air-water interface in thermal equilibrium.

In order to investigate them without perturbing or altering the experimental system, we designed and built a dual-wave reflection interference microscope working with an air-water interface geometry. Highly resolved 3D particle trajectories close to the interface were obtained, from which information on particle diffusion close to the interface and particle-interface interactions are obtained.

The system shows two different potential energy landscapes resulting to two different equilibrium particle- interface distances. The larger one can be fairly explained by Van der Waals and electrostatic interactions combined with gravity. The shorter one highlights the existence of an unexpected additional attractive interaction. The possible origins of such an interaction are discussed.

Using a novel method of analysis of the particle mean square displacements in a generic potential, we were able to access to particle drag coefficients as a function of the distance from the interface. Peculiarly, the air-water interface acts as a slip boundary for the particle motion parallel to the interface and as a no-slip boundary for the particle motion perpendicular to the interface. This experimental result can be partially rationalized considering recent models based on surface incompressibility. However, some discrepancies between experiments and theories remain. Experimental drag coefficients are larger than the hydrodynamic predictions and depend on the particle electrical charge, pointing therefore to a possible role of electrokinetic phenomena.