Figure 7.

Force–distance curve measured at the air–water interface of a submerged air-retaining sample. Positive height values: the tip is above the air–water interface. The curve shows the characteristics proposed in Figure 5a exhibiting no change in slope between attractive (positive force) and repulsive (negative force) regimes (dashed line, see point 2 in Figure 6). We conclude that the air–water interface is not penetrated by the AFM tip but rather pinned to its apex. We calculated the force constant of the interface to be 0.07 N/m from the gradient of the curve in the repulsive force regime.