prewetting

Prewetting is a surface phase transition observed in fluids in contact with a solid substrate, where the thickness of the adsorbed liquid film changes discontinuously as a control parameter is varied away from bulk liquid–vapor coexistence. This phenomenon occurs at temperatures below the wetting temperature and is driven by the competition between fluid–fluid and solid–fluid interactions, notably long-range van der Waals forces. In a temperature–chemical potential (or pressure) phase diagram, there exists a line of first-order transitions, the prewetting line, marking a jump between a thin and a thick adsorbed film. The line extends from the vicinity of the bulk coexistence line into a metastable region and usually terminates at a prewetting critical point. As the temperature increases toward the wetting temperature, the two film branches merge and complete wetting sets in at coexistence.

Theoretical descriptions employ interfacial Hamiltonians and density functional theory, which capture the balance of interfacial energies and disjoining pressure. Experimental evidence comes from adsorption isotherms showing a sudden step in film thickness, along with reflectivity, ellipsometry, or force measurements between surfaces that reveal abrupt changes in film structure.

Prewetting is distinguished from complete wetting and partial wetting by the existence of a finite-thickness film transition below the wetting temperature and is an important concept in surface science, capillary condensation, and micro/nano-scale coating processes.