Modern Physics

# Physical adhesion: what it consists of and examples

The physical adhesion is a connection between two or more surfaces of the same material or different material when contacted. It is produced by the Van der Waals force of attraction and the electrostatic interactions that exist between molecules and atoms of materials.

Van der Waals forces are present in all materials, are attractive and originate from atomic and molecular interactions. Van der Waals forces are due to the induced or permanent dipoles created in the molecules by the electric fields of neighboring molecules; or by instantaneous electron dipoles around atomic nuclei.

Electrostatic interactions are based on the formation of an electrical double layer when two materials come into contact. This interaction produces an electrostatic attraction force between the two materials when exchanging electrons, called the Coulomb force.

Physical adhesion causes the liquid to adhere to the surface on which it rests. For example, when water is placed in a glass, a thin, uniform film is formed on the surface due to the adhesion forces between the water and the glass. These forces act between the molecules in the glass and the molecules in the water and keep the water on the surface of the glass.

Physical adhesion is the surface property of materials that allows them to stay together when in contact. It is directly related to the surface free energy ( ΔE ) in the case of solid-liquid adhesion.

In the case of liquid-liquid or liquid-gas adhesion, the free surface energy is called interfacial or surface tension.

Free surface energy is the energy required to generate a unit of the material’s surface area. Adhesion work (adhesion) can be calculated from the surface free energy of two materials.

Adhesion work is defined as the amount of energy supplied to a system to break the interface and create two new surfaces.

The greater the adhesion work, the greater the resistance to separation of the two surfaces. Bonding work measures the strength of attraction between two different materials when in contact.

### Equations

The free energy of separation of two materials, 1 and 2, is equal to the difference between free energy after separation ( γ final ) and free energy before separation ( γ initial ).

ΔE = W 12 = γ final – γ initial γ 1 + γ 2 – γ 12 [1]

γ 1 = surface free energy of material 1

γ 2 = surface free energy of material 2

The quantity 12 is the adhesion work that measures the adhesion strength of materials.

γ 12 = interfacial free energy

When adhesion is between a solid material and a liquid material, the adhesion job is:

SL = γ S + γ LV – γ SL [2]

γ S = surface free energy of the solid in equilibrium with its own vapor

γ LV = liquid surface free energy in equilibrium with vapor

SL = bonding work between solid material and liquid

γ 12 = interfacial free energy

Equation [2] is written based on the equilibrium pressure (π equil ) which measures the force per unit length of molecules adsorbed on the interface.

π equil = γ S – γ SV [3]

γ SV = solid surface free energy in equilibrium with vapor

SL = π equil + γ SV + γ LV – γ SL [4]

Substituting γ SV – γ SL = γ LV cos θ C in equation [4] gives

SL = π equil + γ SL (1 + cos θ C ) [5]

θ C is the equilibrium contact angle between a solid surface, a drop of liquid and vapor.

Equation [5] measures the adhesion work between a solid surface and a liquid surface due to the adhesion force between the molecules on both surfaces.

## Examples

Physical adhesion is an important feature of the evaluation of tire efficiency and safety. Without good adhesion, tires cannot accelerate, brake the vehicle or be steered from one place to another, and driver safety can be compromised.

Tire grip is due to the friction force between the tire surface and the pavement surface. High safety and efficiency will depend on adhesion to different surfaces, rough and slippery, and in different atmospheric conditions.

For this reason, automotive engineering advances every day in obtaining suitable tire designs that allow good adhesion, even on wet surfaces.

### Adhesion of polished glass plates

When coming into contact with two moistened and polished glass plates, they experience a physical adhesion which is observed in the effort that must be applied to overcome the separation resistance of the plates.

The water molecules bind to the molecules on the top plate and also adhere to the bottom plate, preventing the two plates from separating.

Water molecules have strong cohesion with each other, but they also show strong adhesion to glass molecules due to intermolecular forces.

An example of physical adhesion is a dental plaque attached to a tooth that is usually placed in restorative dental treatments. Adhesion manifests itself at the interface between the adhesive material and tooth structure.

The efficiency of placing enamels and dentins in dental tissues and incorporating artificial structures, such as ceramics and polymers that replace the tooth structure, will depend on the degree of adhesion of the materials used.

### Adhesion of cement with structures

A good physical adhesion of cement to brick, masonry, stone or steel structures manifests itself in a high capacity to absorb energy from normal and tangential tensions to the surface that joins the cement to the structures, that is, A high capacity to support loads .

To obtain a good adhesion, when joining the cement to the structure, it is necessary that the surface on which the cement will be placed has sufficient absorption and that the surface is sufficiently rough. Lack of adhesion translates into cracks and loosening of the adhered material.