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Title: Which microscopic physics removes the moving contact-line stress singularity?
Plain: If liquid sticks perfectly to the solid wall, the friction at a moving contact line becomes infinite and a drop could never spread. Is it slip of molecules along the wall, an invisible thin film ahead of the drop, a smeared-out liquid-gas boundary, or evaporation that removes this infinity in real liquids?
Precise: The no-slip Stokes solution near a contact line moving at speed $U$ gives viscous stress $\sim \eta U / r$ and a logarithmically divergent dissipation. For specific liquid-solid pairs (for example silicone oil on silanized glass, water on glass, volatile liquids near saturation), determine which cutoff sets the inner length $L_{\mathrm{micro}}$ entering the Cox-Voinov law: Navier slip length $\ell_s$, precursor film, diffuse-interface width with its mobility, evaporation and condensation, or molecular hopping at the line. An answer identifies the dominant mechanism for each class of system and predicts $L_{\mathrm{micro}}$ and its dependence on $U$, viscosity and wettability.
Settled by: Measurements of $L_{\mathrm{micro}}$ at nanometer resolution for several liquid-solid pairs compared against molecular-dynamics simulations of the same pairs, with each candidate mechanism switched on and off.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1971