Wetting and impregnation laws

Wetting, the study of the behavior of liquids on solids, covers a wide range of practical situations.

The first major class of problems concerns film deposition, where the aim is to cover a solid with a liquid. The liquid may be a paint, varnish, glue, ink or lubricant; it may also form the base of a thin film designed to generate specific properties (anti-reflective, anti-icing or anti-fouling coating, for example). The solid may be smooth on a molecular scale (glass), or slightly rough (punched steel, coated wall), or even porous and disordered (fabric, paper, a collection of grains or a pile of sand). In all these examples, deposition may be an end in itself (smoothing a rough surface with a varnish, covering a surface with paint) or, on the contrary, be the prerequisite for subsequent operations (adhesion, for example, in the case of a glue, or dispersion, if we think of a solvent invading a powder).

Another important family of problems involves modifying the surface properties of a solid to alter its wetting properties. The aim may be to make a solid wetting, as in the examples already given, but sometimes, on the contrary, to avoid wetting, particularly with respect to water: think of a bathtub or washbasin liner, the bottom of a non-stick frying pan, or certain particularly impermeable fabrics.

The plan of this article is based on the natural classifications that follow from this first list of examples. First, we introduce the key concept of surface tension, and then analyze the equilibrium situations induced by wetting or impregnation phenomena. We then present the laws of wetting (how a drop behaves on a solid) and discuss how to modulate this behavior, from total wetting (spreading and filmification) to zero wetting. The conditions under which a porous solid or a rough surface becomes impregnated will then be studied. Our aim in these two parts is also to describe how the academic viewpoint, where systems are ideal (perfect solid on which a pure liquid is placed), has been broadened in recent years towards systems closer to reality and practical applications.

The final section shows how these different equilibrium situations can be approached from a dynamic point of view. These kinetic questions are often just as important as those of statics: in general, it's not enough to know the conditions under which an ink is impregnated into a piece of paper; we also need to understand the speed at which impregnation takes place (which will enable us, for example, to distinguish one ink from another). Likewise, the fact that a paint is formulated to spread on a support does not obviate the need to consider the dynamics of its deposition, which we shall see determines the quantity of paint deposited. In the same section,...