Fig. 3: Estimate of the conditions for the activation of the EHD mixed lubrication.
From: Physical state of water controls friction of gabbro-built faults
a Viscosity η vs. height of the asperities H2. Inset shows the friction coefficient (μ) vs. Sommerfeld number (S). By re-arranging equation \(S=\frac{6\eta {VL}}{{H}^{2}{\sigma }_{{{{{{\rm{n}}}}}}}^{{{{{{\rm{eff}}}}}}}}\) for η = f (H2), where L is the characteristic length for pressure changes, V is the slip velocity and σneff is effective normal stress, the range of η for having S ≥ 0.001 according to the measured H values (Fig. 2) spans from 0.023 to 0.092 Pa·s. b The multiaxis plot50 of the Sommerfeld number for S ≥ 0.001. The slip velocity for the activation of EHD mixed lubrication in the presence of water in vapor state is roughly 10 mm/s (red in color box). This slip velocity is consistent with the one below which the high friction coefficient is observed in experiments conducted in the presence of water in vapor state (Fig. 2f). See Fig. 4a for description of the parameters η, H, and L of the Sommerfeld number.
