Wetting

The general term wetting can be employed in the following ways: adhesional wetting, spreading wetting and immersional wetting.

Adhesional wetting is a process in which an adhesional joint is formed between two phases.

The work of adhesion per unit area, $w^{{\alpha}{\beta}{\delta}}_A$, is the work done on the system when two condensed phases ${\alpha}$ and ${\beta}$, forming an interface of unit area are separated reversibly to form unit areas of each of the ${\alpha}{\delta}$- and ${\beta}{\delta}$-interfaces.

$$w^{{\alpha}{\beta}{\delta}}_A={\gamma}^{{\alpha}{\delta}}+{\gamma}^{{\beta}{\delta}}-{\gamma}^{{\alpha}{\beta}}.$$

The work of adhesion as defined above, and traditionally used, may be called the work of separation.

The work of cohesion per unit area, $w^{\alpha}_C$, of a single pure liquid or solid phase ${\alpha}$ is the work done on the system when a column of ${\alpha}$ of unit area is split, reversibly, normal to the axis of the column to form two new surfaces each of unit area in contact with the equilibrium gas phase.

$$w^{\alpha}_C=2{\gamma}^{\alpha}.$$

Spreading wetting--a process in which a drop of liquid spreads over a solid or liquid substrate.

A liquid, ${\alpha}$, when placed on the surface of a solid or liquid, ${\beta}$, both previously in contact with a fluid phase ${\delta}$, will tend to spread on the surface if the spreading tension, ${\sigma}^{{\alpha}{\beta}{\delta}}$, defined by

$${\sigma}^{{\alpha}{\beta}{\delta}}={\gamma}^{{\beta}{\delta}}-{\gamma}^{{\alpha}{\delta}}-{\gamma}^{{\alpha}{\beta}},$$

is positive. ${\sigma}^{{\alpha}{\beta}{\delta}}$ is also equal to the work of spreading per unit area ( $w_{\text{spr}}$).

If adsorption equilibrium and mutual saturation of the phases is not achieved instantly, it is possible to distinguish the initial spreading tension, ${\sigma}^{{\alpha}{\beta}{\delta}}_i$, from the final spreading tension, ${\sigma}^{{\alpha}{\beta}{\delta}}_f$, when equilibrium has been reached.

In the case in which ${\sigma}^{{\alpha}{\beta}{\delta}}_i$ is positive, while ${\sigma}^{{\alpha}{\beta}{\delta}}_f$ is negative, the system is said to exhibit autophobicity.

When an area of liquid covered with a spread substance is separated from a clean area of surface by a mechanical barrier, the force acting on unit length of the barrier is called the surface pressure, ${\pi}$ or ${\pi}^s$, and is equal to;

$${\pi}^s={\gamma}^0-{\gamma},$$

where ${\gamma}^0$ is the surface tension of the clean surface and ${\gamma }$ that of the covered surface.

In the case of the spreading of a liquid or an adsorbed film on a solid where the surface pressure cannot be measured directly, the surface pressure may still be defined formally by the above equation.

When a liquid does not spread on a substrate (usually a solid), that is, when ${\sigma}^{{\alpha}{\beta}{\delta}}_f$ is negative, a contact angle (${\theta}$) is formed which is defined as the angle between two of the interfaces at the three-phase line of contact. It must always be stated which interfaces are used to define ${\theta}$. When a liquid spreads spontaneously over an interface the contact angle, between the S/L and L/G surfaces, is zero.

It is of ten necessary to distinguish between the advancing contact angle ( ${\theta}_a$), the receding contact angle ( ${\theta}_r$) and the equilibrium contact angle ( ${\theta}_e$). When ${\theta}_r\ne{\theta}_a$ the system is said to exhibit contact angle hysteresis. When confusion might arise between ${\theta}$ used to denote contact angle, and to denote fraction of surface covered (§1.1.7) it is advisable to attach a subscript to ${\theta}$ for the contact angle.

Immersional wetting--a process in which a solid or liquid, ${\beta}$, is covered with a liquid, ${\alpha}$, both of which were initially in contact with a gas or liquid, ${\delta}$, without changing the area of the ${\alpha}{\delta}$-interface.

The work of immersional wetting per unit area, or wetting tension ( $w^{{\alpha}{\beta}{\delta}}_W$)¹⁶ , is the work done on the system when the process of immersional wetting involving unit area of phase ${\beta}$ is carried out reversibly:

$$w^{{\alpha}{\beta}{\delta}}_W={\gamma}^{{\beta}{\delta}}-{\gamma}^{{\alpha}{\beta}}.$$

Note: in systems in which wetting is accompanied by adsorption the above definitions should, strictly speaking, be expressed in terms of the differential quotients of the work with respect to the relevant change in area.