Deciphering the atomistic mechanism governing humidity-dependent interfacial friction remains a fundamental challenge across physics, chemistry, medicine, and materials science. Here, we employ first-principles calculations to systematically investigate the frictional behaviors of bilayer graphene (BLG) with varying interfacial water (${\text{H}}_2\text{O}$) coverage, with a particular focus on the contrasting responses of commensurate and incommensurate contacts. Our findings reveal a striking dichotomy: As ${\text{H}}_2\text{O}$ coverage increases to approximately 0.4 monolayer, commensurate BLG interfaces experience a dramatic reduction in friction, reaching a minimum, whereas incommensurate contacts exhibit a moderate increase in friction, peaking near 0.6 monolayer. Beyond these critical humidity cites, friction in commensurate systems gradually increases toward a saturation regime, while incommensurate systems display a declining trend before plateauing. To validate these distinct behaviors, we establish a generalized registry index framework that extends the conventional registry index [N. Marom $et al$., Phys. Rev. Lett. 105, 046801 (2010)] by explicitly incorporating (i) interlayer-distance-dependent vdW interactions modulated by water intercalation, (ii) humidity-induced commensurate-incommensurate phase transitions, and (iii) molecular pinning sites imparted by confined ${\text{H}}_2\text{O}$ molecules within the BLG heterostructure. Collectively, this work not only reveals the atomistic mechanism of humidity as an effective tuning parameter for frictional control in two-dimensional materials but also provides a robust descriptor with predictive power for the rational design of adaptive, environment-responsive nanolubrication systems.

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• Electrical properties
• Friction
• Intercalation
• Lubrication
• Stress
• Twistronics
• Wetting