Eur. Phys. J. Special Topics 197, 3–9 (2011) c  EDP Sciences, Springer-Verlag 2011 DOI: 10.1140/epjst/e2011-01430-3 THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS Regular Article A few preliminary remarks, quotations and some references M.G. Velarde a Instituto Pluridisciplinar, Universidad Complutense, Paseo Juan XXIII, 1, 28040 Madrid, Spain Received 02 June 2011 / Received in final form 15 June 2011 Published online 30 August 2011 Abstract. A few basic concepts are recalled regarding equilibrium wetting features, in the context of this discussion and – debate issue [59]. After reading all manuscripts here included in this special issue I found pertinent entering a few preliminary remarks of rather elementary type, highlighting a few quotations and also some references (far from a complete set) of possible “historical” interest in the context of our “Discussion and Debate”. For instance, when quite many publications, papers or books, start by saying “Let us consider an equilibrium liquid drop (to be) deposited on a solid substrate”, Fig. 1(a), we take for granted that equilibrium refers to thermodynamic equilibrium and it implies mechanical equilibrium (the converse is not true). Consider the case of partial wetting, Fig. 1(b). For thermodynamic equilibrium the chemical potential across the system must be constant and hence, simultaneously, there ought to be liquid-vapor equilibrium, vapor-solid equilibrium, and liquid-solid equilibrium. Con- sequently, for a drop to be at thermodynamic equilibrium over a partially wetted solid the conditions must be of vapor oversaturation. Otherwise either vapor flow and/or evaporation/condensation take over trying to bring the system to equilibrium. I leave aside the question of drop size. Oversaturation implies the appearance of a liquid film over the initially bare solid. Accordingly, no thermodynamic equilibrium is pos- sible for a liquid drop placed over a bare solid under partial wetting conditions. This elementary albeit basic conceptual fact has been simply overlooked by most writers though it has been highlighted by few eminent colleagues. Harkins on p. 278 of his (posthumous) 1952 book clearly said: “If a liquid is in contact with a clean surface of a solid, there is a solid-liquid interface between the solid and the liquid, while any bare surface of the solid adsorbs the vapor of the liquid until the fugacity of the adsorbed material is equal to that of the vapor and of the liquid”. Adamson on p. 338 of his 1982 book wrote: “It is important to keep in mind that the phases are mutually in equilibrium. In particular the solid surface must be in equilibrium with the saturated vapor pressure and therefore be covered by an adsorbed film” (more on this further below). a e-mail: mgvelarde@pluri.ucm.es; http://www.ucm.es/info/fluidos