No.76: Generation of adhesive strength

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Key Note

The surface of the ice that sticks to the fingertips temporarily melts into water (liquid) and is then refrozen by the surrounding cold ice. This series of phenomena is crucial for explaining the mechanism by which paint adheres to objects. In this case, the fingertips represent the surface condition of the object being painted, demonstrating how paint sticks. 

The Mysterious Relationship Between Ice and Fingers

Ice that has been chilled below freezing point will instantly stick to your fingertips upon contact. This phenomenon is particularly pronounced when the fingers are wet. Experiencing it firsthand reveals the wonder of this phenomenon. 

1. The Phenomenon of Sticking and Adhesion

Intermolecular Forces: Unlike the chemical bonds that bind atoms together, there exists a force that causes molecules to attract each other, known as “intermolecular forces.” For example, when the intermolecular forces among water molecules are strong, water exhibits the properties of ice (solid). As this force weakens, water transitions into a liquid and then into vapor (gas).

Cohesion and Adhesion: The force that causes molecules of the same kind to attract each other is called “cohesion.” For instance, two glass plates cleaned and tightly fitted together with water in between them are difficult to separate. This phenomenon occurs due to the attractive force between different types of substances, termed “adhesion.” This force is the principle behind the adhesion of paint to objects.

2. The Mechanism of Intermolecular Forces

The Role of Electric Charge: All substances are made of atoms and are electrically neutral. However, the balance of electric charge can be disturbed by the emission or absorption of electrons. This imbalance in electric charge causes different substances to attract each other.

Van der Waals Forces and Hydrogen Bonding: Among the forces that cause molecules to attract each other are the Van der Waals forces and hydrogen bonding. These forces act between molecules, and the Van der Waals forces, in particular, become more significant as the molecular weight of resins increases. The essence of adhesion lies in the Van der Waals forces.