Thoughts on some outstanding issues in the physics of equilibrium wetting and conceptual understanding of contact lines
School of Engineering, The University of Edinburgh, Kings Buildings, Mayfield road, Edinburgh, Scotland, UK
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Revised: 15 June 2011
Published online: 30 August 2011
Equilibrium wetting is a fundamental phenomenon, relevant to many scientific areas. Since the pioneering work on equilibrium wetting of Thomas Young (1805) , researchers strived to advance our understanding of this fundamental problem. Despite its apparent simplicity, equilibrium wetting phenomenon still holds many unanswered questions and represents a challenge to modern physicists and engineers. The relationship between quantities amenable to measurements, like macroscopic wetting contact angle, and other surface ener- gies and physical properties remains to be fully elucidated. Wetting is a physical problem which spans over two length scales, inner region (“microscopic”) length scale and outer region (“macroscopic”). The three-phase contact line, where the macroscopic region meets the micro- scopic one, and underlying surface forces, represents a challenge to fully understand and model. In this paper, a brief review of the basics of wetting and existing concepts is first presented. Then two important questions are discussed in the light of the latest experimental findings: first the relevance of the continuum concept when describing interfaces near the three-phase contact line, and second the effect of adsorption on interfacial energies and its use to explain some interesting observations like the dependence of equilibrium contact angle on pressure and size of droplets. These recent observations raise some fundamental questions about how the three-phase contact line is conceptualised.
© EDP Sciences, Springer-Verlag, 2011