  Although the dynamics of colloids in the vicinity of a solid interface has
been widely characterized in the past, experimental studies of Brownian
diffusion close to an air-water interface are rare and limited to
particle-interface gap distances larger than the particle size. At the still
unexplored lower distances, the dynamics is expected to be extremely sensitive
to boundary conditions at the air-water interface. There, ad hoc experiments
would provide a quantitative validation of predictions. Using a specially
designed dual wave interferometric setup, the 3D dynamics of 9 micrometers
diameter particles at a few hundreds of nanometers from an air-water interface
is here measured in thermal equilibrium. Intriguingly, while the measured
dynamics parallel to the interface approaches expected predictions for slip
boundary conditions, the Brownian motion normal to the interface is very close
to the predictions for no-slip boundary conditions. These puzzling results are
rationalized considering current models of incompressible interfacial flow and
deepened developing an ad hoc model which considers the contribution of tiny
concentrations of surface active particles at the interface. We argue that such
condition governs the particle dynamics in a large spectrum of systems ranging
from biofilm formation to flotation process.
