Nanomechanical investigation of ice interfaces via atomic force microscopy

We have indented the surface of ice at temperatures between -1°C and -17°C with atomic force microscope tips. The ease of plastic deformation of the ice and the rapid flattening of surface features via vapor transport make it difficult to obtain interesting images of ice at temperatures near the melting point. In spite of this, we have obtained images of an ice "crystallite" evolving in time at -17°C. When imaging is not possible, force curves provide an alternate method to study the indentation process.; Force curve measurements show that the maximum hardness of the ice approaches the hardness required for pressure melting, even at the lowest temperatures studied. Excluding this data, the variation of hardness with indentation speed is much greater than that expected for plastic flow. Another mechanism for indentation that has a much stronger dependence on indentation speed is the flow of the interfacial quasi-liquid layer that often exists at the interface between ice and solids below the melting point. For a thick viscous layer, a Newtonian treatment of the flow of quasi-liquid between the tip and the ice suggests that indentations at different indentation velocities should have the same force/velocity ratio for a given pit depth. This is observed for silicon tips with and without a hydrophobic coating at temperatures between -1°C and -10°C implying the presence of a quasi-liquid layer at the interface between tip and ice.; A simple model for viscous flow that incorporates the approximate shape of our tip is used to obtain an estimate of the layer thickness, assuming the layer has the viscosity of supercooled water. The largest layer thicknesses inferred from this model are too thin to be described by continuum mechanics, but the model fits the data well. This suggests that the viscosity of the confined quasi-liquid is much greater than that of bulk supercooled water. The hydrophobically coated tip has a significantly thinner layer than the uncoated tip, but the dependence of thickness on temperature is similar. The estimated viscous layer thickness increases with increasing temperature as expected for a quasi-liquid premelt layer.