Surface properties of ice studied by atomic force microscopy

The atomic force microscope was used to investigate surface properties of ice in a temperature range of -24 to -0.7 degrees C. An upper limit of the thickness of the liquidlike layer on the surface of ice was found to vary between about 12 nm at -24 degrees C and 70 nm at -0.7 degrees C. This was correlated with an increase of the adhesion force. In force-versus-distance measurements the tip penetrated the ice. This behavior can be interpreted in two ways: Either a "soft ice layer" exists between the liquidlike layer and bulk ice or the presence of the solid tip causes the ice surface to change its mechanical properties. Such an interfacial premelting might be relevant for friction. It might add to frictional heating as a cause for the lubricating liquid layer which reduces friction. In addition, friction was measured around -23 degrees C. With the atomic force microscope friction at individual microcontacts with a contact area of a few (10 nm)(2) can be determined. Friction increases with decreasing sliding velocity, which is probably due to the penetration of the tip into the ice at low velocities.