Physical Adhesion: What It Consists Of And Examples

The physical adhesion is the binding 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 by 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 the instantaneous dipoles of the electrons around the 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 force of attraction between the two materials, by exchanging electrons, called the Coulomb force.

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

What is physical adherence?

Physical adherence 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 ) for the solid-liquid adhesion case.

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

Surface free energy is the energy required to generate a unit of surface area of ​​the material. From the surface free energy of two materials, the work of adhesion (adherence) can be calculated.

Adhesion work is defined as the amount of energy that is 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. Adhesion work measures the force 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 the free energy after separation ( final γ ) and the free energy before separation ( initial γ ).

ΔE = W 12 = final γ – initial γ = 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  W 12  is the adhesion work that measures the adhesion strength of the materials.

γ 12 = interfacial free energy

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

W SL = γ S + γ LV – γ SL          [2]

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

γ LV = surface free energy of the liquid in equilibrium with vapor

W SL work of adhesion between solid material and liquid

γ 12 = interfacial free energy

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

π equil = γ S – γ SV          [3]

γ SV = surface free energy of the solid in equilibrium with the vapor

W SL = π equil + γ SV + γ LV – γ SL          [4]

Substituting γ SV  – γ SL =    γ LV cos θ C in equation [4], we obtain

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

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

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

Examples

Tire grip

Physical grip is an important characteristic for evaluating the efficiency and safety of tires. Without good grip, the tires cannot accelerate, brake the vehicle, or be steered from one place to another, and the driver’s safety can be compromised.

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

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

Adhesion of polished glass plates

When two polished and moistened glass plates come into contact, they experience a physical adhesion that 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 upper plate and likewise adhere to the lower plate preventing both plates from separating.

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

Dental adhesion

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

Efficiency in the placement of enamels and dentins in dental tissues, and in the incorporation of artificial structures such as ceramics and polymers that replace the dental structure, will depend on the degree of adherence of the materials used.

Adhesion of cement to structures

Good physical adhesion of cement to brick, masonry, stone or steel structures is manifested in a high capacity to absorb the energy that comes from normal and tangential stresses to the surface that joins the cement with the structures, that is, in a high capacity to bear loads.

In order to obtain good adhesion, when the cement meets the structure, it is necessary that the surface on which the cement is to be placed has sufficient absorption and that the surface be sufficiently rough. Lack of adherence results in cracks and detachment of the adhered material.

References

  1. Lee, L H. Fundamentals of Adhesion. New York: Plenium Press, 1991, pp. 1-150.
  2. Pocius, A V. Adhesives, Chapter27. [aut. book] JE Mark. Physical Properties of Polymers Handbook. New York: Springer, 2007, pp. 479-486.
  3. Israelachvili, J N. Intermolecular and surface forces. San Diego, CA: Academic Press, 1992.
  4. Relationship between adhesion and friction forces. Israelachvili, JN, Chen, You-Lung and Yoshizawa, H. 11, 1994, Journal of Adhesion Science and Technology, Vol. 8, pp. 1231-1249.
  5. Principles of Colloid and Surface Chemistry. Hiemenz, PC and Rajagopalan, R. New York: Marcel Dekker, Inc., 1997.

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