Tuning super-lubricity via molecular adsorption

The state of vanishing friction known as super-lubricity is highly desirable to reduce parasitic energy loss. However, without efficient means to turn on and off the super-lubricity, it would be difficult to realize its full potential in engineering applications. Here we report that the nanoscale super-lubricity between amorphous silica tip and graphite basal plane can be tuned by physisorbing various molecules from the environment. In particular, the super-low fiction can be further reduced by about 54% in n-pentanol vapor, but increases by around 25 and 45 times in water and phenol vapor respectively. The vapor molecules influence the friction by adsorbing onto the silica surface and being entrained into the sliding interface. Based on the structures of adsorbed molecules, it could be deduced that adsorbates with high conformational entropy can suppress the interfacial commensuration and enhance the super-lubricity, while the ones that can facilitate commensurate interactions with the graphite surface increase friction. This finding provides the basic principle enabling on-demand control of friction into and out of the super-lubricious state of solid interface.