Surface wetting and contact angle: basics and characterisation

Wettability is of pivotal importance in many areas of science and technology, ranging from the extractive industry to development of advanced functional materials and biomedicine problems. An increasing interest to wetting-related phenomena stimulates impetuous growth of research activity in this field. The presented review is aimed at the cumulative coverage of issues related to wettability and its investigation. It outlines basic concepts of wetting as a physical phenomenon, methods for its characterisation (with the emphasis on sessile drop techniques), and performances of contemporary instrumentation for wettability measurements. In the first section, physics of wettability is considered. The intermolecular interactions related to wetting are classified as dependent on their nature. Thus, discussion of interactions involving polar molecules covers permanent dipole - permanent dipole interactions and freely rotating permanent dipoles. Consideration of interactions resulting from the polarization of molecules includes interactions between ions and uncharged molecules, Debye interactions, and London dispersion interactions. Hydrogen bonds are discussed separately. The second section deals with the issues related to surface tension and its effect on shaping the surface of a liquid brought in contact with a solid body. The relationship between the surface tension and the contact angle as well as equations that quantify this relationship are discussed. The Young–Laplace equation governing the shape of the drop resting on the surface is analysed. The third section is devoted to the experimental characterization of surface wettability and the underlying theoretical analysis. Particular attention is paid to the method known as the Axisymmetric Drop Shape Analysis (ADSA). Principles of automated determination of relevant physical values from experimental data are briefly discussed. Basics of numerical techniques intended for analysing the digitized image of the drop and extracting information on surface tension and contact angle are outlined. In the fourth section, an overview of commercially available instrumentation for studying wettability and the contact angle measurements is presented. The prototype contact angle analyser designed and manufactured at the ISP NASU is introduced.