Adhesion is defined as the interfacial force of attraction between two materials. These physical attractions are mainly due to van der Waals forces and electrostatic forces.

Mechanism of Liquid–solid Adhesion

In liquid penetrant testing, there are usually three surface and interfaces involved, the solid-gas interface, the liquid–gas interface, and the solid–liquid interface. When a liquid spreads over a solid surface, two conditions must be met. Firstly, the surface energy of the solid-gas interface must be greater than the combined surface energies of the liquid–gas and the solid–liquid interfaces. Secondly, the surface energy of the solid-gas interface must exceed the surface energy of the solid–liquid interface. The adhesion mechanism between liquid and solid substrate has been explained on the basis of mechanical interlocking, adsorption, diffusion, and electrostatic theory. These theories can be used, solely or in combination with each other, to describe almost any kind of adhesion phenomenon (Nihlstrand 1996).
The mechanical interlocking theory of adhesion states that good adhesion occurs only when a liquid penetrates into the pores, holes and crevices, and other irregularities of the adhered surface of a substrate and locks mechanically with the texture of the substrate (Allen 2005). Pretreatment methods applied on surfaces enhance adhesion (Clearfield et al. 1991) as they result in micro-roughness on the adherend surface, which can improve adhesion strength by providing mechanical interlocking. Beyond mechanical interlocking, the enhancement of adhesion strength is due to the roughing of the adherend substrate. Further, surface roughness increases surface area, improves kinetics of wetting, and increases plastic deformation for better adhesion (Evans et al. 1979; Jennings et al. 1972). The size and shape of the special features created on the surface have an influence on the adhesion, providing a tortuous path which prevents the separation of the adhesive from the adherend (Fisher 2005). However, this theory is not able to explain the good adhesion strength attained in some cases between smooth surfaces.
The diffusion theory attributes the adhesion of polymeric materials to the interpenetration of chains at the interface. The major driving force for polymer autohesion and heterohesion is due to mutual diffusion of polymer molecules across the interface (Voyutski 1963). To describe self-diffusion phenomenon of polymers, several theories have been proposed: entanglement coupling (Klein 1979), cooperativity (Edwards and Grant 1973), and reptation (Brochard and De Gennes 1980). The reptation model (Wu et al. 1986) has been applied to study tack, green strength, healing, and welding of polymers. Some evidence may have demonstrated that the interdiffusion phenomenon exists in mobile and compatible polymers and may promote the intrinsic adhesion. The diffusion theory, however, has found limited application where the polymer and adherend are not soluble (Skeist 1990) or the chain movement of the polymer is constrained by its highly cross-linked, crystalline structure or when it is below its glass transition temperature.
According to the electrostatic theory of adhesion, the force of attraction between the liquid and substrate is attributed to the transfer of electrons across the interface creating positive and negative charges that attract one another. The result is the creation of an electrical double layer (EDL) at the interface (Allen 2003). These electrostatic forces at the interface account for resistance to separation of the liquid and the substrate. But, this theory also could not be accepted due to the fact that the EDL could not be identified without separating the adhesive bond. Also, as argued by many researchers (Roberts 1977; Possart 1988), the effect of the electrical double layer on the adhesive bond strength was exaggerated.
The adsorption theory states that adhesion results from intimate intermolecular contact between two materials and involves surface forces that develop between the atoms in the two surfaces (Ramarathnam et al. 1992). These forces may be due to physical adsorption which is mainly van der Waals forces or secondary forces. These adhesion forces may also be due to acid–base interaction or hydrogen bond (Allara et al. 1986; Fowkes 1987).
To obtain good adsorption, intimate contact must be reached such that van der Waals interaction or the acid–base interaction or both take place; hence, good wetting is essential.

Wetting

Wetting ability of a liquid is a function of the surface energies of the solid-gas interface, the liquid–gas interface, and the solid–liquid interface. The intermolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. The adhesion forces between the liquid and the second substance will compete against the cohesive forces of the liquid. Liquids with weak cohesive bonds and a strong attraction to another material (or the desire to create adhesive bonds) will tend to spread over the material. Liquids with strong cohesive bonds and weaker adhesion forces will tend to bead up or form a droplet when in contact with another material (Park and Seo 2011). One way to quantify a liquid’s surface wetting characteristics is to measure the contact angle of a drop of liquid placed on the surface of an object.
According to Young’s equation, the surface tensions (liquid/vapor: γLV, solid/liquid: γSL, and solid/vapor: γSV) at the three phase contacts are related to the equilibrium contact angle θ through

The contact angle (θc), as seen in the Fig. 1, is the angle at which the liquid–vapor interface meets the solid–liquid interface. The contact angle is determined by the resultant between adhesive and cohesive forces.
The one important factor that influences adhesion strength is the ability of the liquid to spread uniformly on the substrate (Al-Zahrani et al. 2009).
For spontaneous wetting to occur:

However, this is an ideal case, and it is not possible with any polymer.
The tendency of a drop to spread over a flat solid surface increases as the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability (Sharfrin et al. 1960) as shown in Table 1.