  It is shown that when the nanobubble contact line is pinned to a penetrating
tip the interface behaves like a Hookean spring with spring constant
proportional to the nanobubble surface tension. Atomic force microscope (AFM)
data for several nanobubbles and solutions are analysed and yield surface
tensions in the range 0.04--0.05 N/m (compared to 0.072 N/m for saturated
water), and supersaturation ratios in the range 2--5. These are the first
direct measurements of the surface tension of a supersaturated air-water
interface. The results are consistent with recent theories of nanobubble size
and stability, and with computer simulations of the surface tension of a
supersaturated solution.
