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The incidents taking place in our day to day lives are explained by different phenomena. The two main phenomena that can be observed in our daily lives are cohesion and adhesion. The terms cohesion and adhesion may seem similar in pronunciation, but they are very different concepts showing different characteristics. These two phenomena can be more easily explained with the help of surface tension seen in water molecules. The forces of attraction existing in cohesion are Van der Waal force and hydrogen bonding, whereas, in the adhesion, electrostatic force of attraction is observed. The cohesive force is responsible for the formation of water droplets on the surface.
Adhesion
Adhesion refers to the ability of more than two different molecules to link together.
Electrostatic forces are exerted on various substances, and adhesion forces might be one of the outcomes. Cohesive forces are linked to Van der Waals forces and hydrogen bonding, which allows liquids, such as water molecules, to remain separate. When water is spilt over a glass surface, both adhesive and cohesive forces come into action on the water’s surface. The adhesive force helps the liquid spread across the surface, whereas the cohesive force helps form water droplets on the surface.
There is a difference in adhesion and cohesion forces. If the cohesion forces acting between water molecules are larger than the adhesion forces acting between them, the water gets scattered. For example, the individual molecules will attract each other, resulting in settling.
In adhesive forces, mechanical forces and electrostatic forces are seen. The adhesive force can be observed in wetting agents. Due to adhesion, the liquid remains stuck to the surface. Another example is when water is added to an empty glass, it forms a uniform layer on the glass surface. This happens because the adhesion force acting between the water and the glass surface is so strong that the water molecule changes from its spherical shape and forms a layer on the glass surface.
Mechanisms
Some adhesion mechanisms have been proposed to explain why one material adheres to another:
Mechanical
The mechanical property of adhesive materials is that it packs the voids present in the surface and holds them together by joining. Different joining phenomena occur at various length scales, and two materials form a mechanical bond.
Chemical
Two or more materials join together during the chemical mechanism of adhesive material to create a new compound. The strongest bonding or joint is commonly referred to as covalent bonding and ionic bonding, where the electrons present in the atom are mutually shared among each other. Whereas a weaker bond, commonly referred to as hydrogen bonding, is formed between a hydrogen atom and an electronegative atom such as fluorine, oxygen etc.
The chemical adhesion is seen between the two different atom surfaces, which tend to form either a covalent bond, a hydrogen bond or an ionic bond.
- Dispersive
Another term used for dispersive adhesion is physisorption. In physisorption, two molecules are bonded by a force of attraction referred to as Van der Waals force. Due to the attraction, the molecules acquire a slightly positive charge and slightly negative charge, forming a polar molecule. The larger molecules show a higher positive charge and negative charge, which may be permanent in some molecules or may be temporarily occurring in some molecules due to the electron movement.
Differences between adhesion and cohesion
Adhesion | Cohesion |
Adhesion occurs when two molecules or substances are different. | The cohesion force is the force that attracts two comparable substances or molecules. |
The force that binds water molecules with the walls of xylem vessels is known as adhesion. | Water molecules have a high degree of cohesion. |
Adhesion has two effects: capillary action and meniscus (the curved surface generated by any liquid in a cylinder) | The meniscus, capillary action, and also surface tension are the consequences of cohesion. |
Electrostatic or mechanical forces exist between two types of dissimilar substances, which create adhesion. | Hydrogen bonding along with Van der Waals forces help in cohesion. |
The liquid gets distributed all over the surface due to a significant adhesive force. | A strong cohesive force causes water droplets to form on any surface. |
Conclusion
Measurement of adhesion forces can be done while two surfaces are attracted to each other or when the attraction force between the two surfaces is removed. Traditional measurement techniques are used to measure adhesion forces during the removal of attraction between surfaces by the process of detaching surfaces that are in contact with each other, whereas improved techniques are used to measure adhesion forces during attraction. Due to the imprecise estimates obtained by using traditional detachment techniques to measure adhesion forces, accurate adhesion force measurements were required, which were achieved by using defined external forces to aid in the detachment of particles from surfaces.
