  We present contactless atomic-force microscopy measurements of the
hydrodynamic interactions between a rigid sphere and an air bubble in water at
the micro-scale. The size of the bubble is found to have a significant effect
on the response due to the long-range capillary deformation of the air-water
interface. To rationalize the experimental data, we develop a viscocapillary
lubrication model accounting for the finite-size effect. The comparison between
experiments and theory allows us to measure the air-water surface tension,
without contact, paving the way towards robust contactless tensiometry of
polluted air-water interfaces.
