Moving contact lines and dynamic wetting

5 problems

• Threshold speed for air entrainment in forced wetting

  Partially resolved

  When a plate or tape is pushed quickly into a liquid, above a critical speed the liquid can no longer wet the solid and a thin air film is dragged in, which ruins coatings. What sets that critical speed?

• Is there a predictive law for dynamic contact angle versus speed?

  Contested

  The angle a moving liquid edge makes with a solid changes with its speed, and two rival theories (bending of the liquid surface by viscous flow, or molecules hopping between surface sites) both fit data once their parameters are adjusted. Which one governs, and under what conditions?

• Predicting contact-angle hysteresis from measured surface defect statistics

  Open

  Raindrops stick to a tilted window because the drop edge catches on tiny surface defects, so the angle needed to advance is larger than the angle at which it recedes. Can this gap be computed from a map of the defects alone?

• Why measured contact-line roughness exceeds the elastic-line model prediction

  Contested

  A liquid edge creeping over a dirty surface catches on tiny defects and becomes ragged, and the theory of a springy line pinned by randomly placed defects predicts how ragged. Experiments find a rougher edge than the theory, and nobody knows what the theory leaves out.

• Which microscopic physics removes the moving contact-line stress singularity?

  Open

  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?