Figure 6.

Two possibilities for the contact between AFM tip and air–water interface: a) the interface is pinned to the tip while it is pressed towards the base; b) the tip penetrates the interface. To find the model that best describes our system we consulted force–distance curve measurements like those schematically shown on the right. The individual points in the curves correspond to the images. 1 and 5: The tip is pressed towards the surface. The normal force is positive. 2: No resulting force applied by the tip. 3: The tip is withdrawn from the surface. The restoring force of the air–water interface pulls the tip downwards resulting in a negative force. 4: After the tip loses contact with the interface, the acting force is zero. Subsequently, the force–distance curves should differ in cases a) and b). As shown on the right, we expect the curve to show a change in slope at point 2 in case b).