  The "free" water surface is generally prone to contamination with surface
impurities be they surfactants, particles or other surface active agents. The
presence of such impurities can modify flow boundary near such interfaces in a
drastic manner. Here we show that vibrating a small sphere mounted on an AFM
cantilever near a gas bubble immersed in water, is an excellent probe of
surface contamination. Both viscous and elastic forces are exerted by an
air-water interface on the vibrating sphere even when very low doses of
contaminants are present. The viscous drag forces show a cross-over from
no-slip to slip boundary conditions while the elastic forces show a nontrivial
variation as the vibration frequency changes. We provide a simple model to
rationalize these results and propose a simple way of evaluating the
concentration of such surface impurities.
