Evaporation effect on the contact angle and contact line dynamics
Résumé
This chapter shows how evaporation and condensation can modify the wetting conditions. First, we review the microscopic-scale phenomena acting near the contact line: Kelvin effect, hydrodynamic slip, vapor recoil, surface forces, and interfacial resistance. Then we address the theory of liquid flow in the wedge under evaporation at partial wetting conditions, more common in practice than complete wetting. The importance of the correct formulation of the boundary conditions at the contact line is shown. Two main evaporation regimes are addressed next. First, the evaporation into a pure vapor atmosphere is considered (like in bubble growth in boiling). It is controlled by the flow in the liquid. In the presence of contact line receding, this problem is solved by asymptotic matching of the three liquid regions: (i) the microregion near the contact line controlled by the phenomena described above, (ii) the intermediate region where the surface tension competes with the viscous effects, and (iii) the macroregion controlled by the liquid bulk effects. The asymptotic matching results in an expression for the apparent contact angle that depends both on the evaporation rate and on the contact line velocity. From such an analysis, the contact line receding dynamics caused by evaporation can be found. The theory is then compared to the available experimental data. Finally, we consider another regime of wedge evaporation, that in the atmosphere of the neutral gas, controlled by the vapor diffusion in the diffusion boundary layer. As the evaporation is weaker in this case, its effect on the apparent contact angle is smaller. However, we show how it depends on the key parameters of evaporation, e.g., the boundary layer thickness.
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