  Room Temperature Ionic Liquids (RTILs) are molten salts which exhibit uniques
physical and chemical properties, commonly harnessed for lubrication and energy
applications. The pure ionic nature of RTIL leads to strong electrostatic
interactions among the liquid, furthermore exalted in the presence of
interfaces and confinement. In this work, we use a tuning-fork based dynamic
Surface Force Tribometer (TF-SFT), which allows probing both the rheological
and the tribological properties of RTILs films confined between a millimetric
sphere and a surface, over a wide range of confinements. When the RTIL is
confined between metallic surfaces, we evidence an abrupt change of its
rheological properties below a threshold confinement. This is reminiscent of a
recently reported confinement induced capillary freezing, here observed with a
wide contact area. In parallel, we probe the tribological response of the film
under imposed nanometric shear deformation and unveil a yielding behaviour of
the interfacial solid phase below this threshold confinement. This is
characterized by a transition from an elastic to a plastic regime, exhibiting
striking similarities with the response of glassy materials. This transition to
yielding of the RTIL in metallic confinement leads overall to a reduction in
friction and offers a self-healing protection of the surfaces avoiding direct
contact, with obvious applications in tribology.

Soft Condensed Matter