Contactless rheology of finite-size air-water interfaces

We present contactless atomic force microscopy measurements of the hydrodynamic interactions between an oscillating sphere and a bubble in water at the microscale. 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 and with the volume of liquid down to tens of microliters, paving the way toward robust contactless tensiometry of polluted air-water interfaces.