Premelting at ice-solid interfaces studied via velocity-dependent indentation with force microscope tips - art. no 134102

We have indented the surface of ice at temperatures between -1 degreesC and - 17 degreesC with sharp atomic force microscope tips. For a thick viscous interfacial melt layer, a Newtonian treatment of the flow of quasiliquid b t tween the tip and the ice suggests that indentations at different indenta tion velocities should have the same force/velocity ratio for a given pit d epth. This is observed for silicon tips with and without a hydrophobic coat ing at temperatures between -1 degreesC and -10 degreesC implying the prese nce of a liquid-like layer at the interface between tip and ice. At tempera tures below about -10 degreesC the dependence of force on velocity is weake r, suggesting that plastic flow of the ice dominates. A simple model for vi scous flow that incorporates the approximate shape of our tip is used to ob tain an estimate of the layer thickness, assuming the layer has the viscosi ty of supercooled water. The largest layer thicknesses inferred from this m odel are too thin to be described by continuum mechanics, but the model fit s the data well. This suggests that the viscosity of the confined quasiliqu id is much greater than that of bulk supercooled water. The hydrophobically coated tip has a significantly thinner layer than the uncoated tip, but th e dependence of thickness on temperature is similar. The estimated viscous layer thickness increases with increasing temperature as expected for a qua siliquid premelt layer.