The Last Dance of Lubrication: Capturing Ultralow Friction before Full Evaporation

Unsteady friction phenomena driven by lubricant evaporation are pervasive across a wide range of applications, from precision machinery to extreme-environment systems. Contrary to the assumption that the friction coefficient (COF) monotonically increases with lubricant evaporation, we identified a remarkable ultralow friction state preceding dry friction. Using a PDMS-ceramic tribological system lubricated by ethanol, we observed that the COF dropped to 0.01-0.03 and persisted for ∼10 s before the full evaporation of lubricant, governed by a nanoscale boundary film rather than a liquid film. This state exhibits 1/10 the lubricated and 1/100 the dry friction COF values. Introducing water vapor prolonged this state to over 4000 s, while liquid water abruptly raised the COF above 1.7. Dynamic modulation between dry and vapor lubrication enabled rapid COF switching (2-3 to 0.02) within seconds. These findings redefine friction dynamics and provide insights into advanced friction control and system optimization.