  The ice surface is known for presenting a very small kinetic friction
coefficient, but the origin of this property remains highly controversial to
date. In this work, we revisit recent computer simulations of ice sliding on
atomically smooth substrates, using newly calculated bulk viscosities for the
TIP4P/Ice water model. The results show that spontaneously formed premelting
films in static conditions exhibit an effective viscosity which is about twice
the bulk viscosity. However, upon approaching sliding speeds in the order of
m/s, the shear rate becomes very large, and the viscosities decrease by several
orders of magnitude. This shows that premelting films can act as an efficient
lubrication layer despite their small thickness, and illustrates an interesting
interplay between confinement enhanced viscosities, and shear thinning. Our
results suggest that the strongly thinned viscosities that operate under the
high speed skating regime could largely reduce the amount of frictional
heating.
