Question

Explain why water forms into beads on a waxed car finish.

Short answer

Water forms into beads on a waxed car finish due to the wax creating a hydrophobic layer on the surface, which increases the contact angle between the water and the car finish. The high surface tension of water and the low surface energy of the waxed surface cause the water to minimize its surface area by forming beads. This is a result of the balance between the cohesive forces within the water and the adhesive forces between the water and the waxed surface.

Step-by-step solution

1. Understanding Surface Tension

Surface tension is a property of a liquid that results from the cohesive forces between its molecules. These forces cause the liquid to act as if it has an elastic membrane on its surface. The cohesive forces are stronger at the liquid-air interface because there are no other like molecules above the surface to balance out the attractive forces. This creates a net inward force, causing the surface to contract, and the liquid to take the shape that minimizes its surface area.

2. Introducing Contact Angle

Contact angle is the angle formed by a liquid at the three-phase boundary where the liquid, gas (usually air), and solid meet. The contact angle is a measure of the wetting properties of a liquid on a solid surface. A high contact angle indicates that the liquid does not wet or spread easily on the surface, and it forms droplets. On the other hand, a low contact angle indicates that the liquid wets the surface and spreads easily.

3. Hydrophobic and Hydrophilic Surfaces

Solid surfaces can be classified as hydrophobic or hydrophilic based on their interaction with water. A hydrophobic surface has low affinity for water, resulting in high contact angles and water droplet formation. Conversely, a hydrophilic surface has a high affinity for water, resulting in low contact angles and water spreading on the surface.

4. Role of Wax on a Car Finish

Waxing a car finish creates a hydrophobic layer on the surface, which increases its resistance to water. Wax contains hydrocarbon molecules with nonpolar or weakly polar groups that result in low surface energy. When applied to a car finish, these molecules create a smooth, uniform surface with low surface energy, making it difficult for water to adhere to the surface. The low surface energy causes water to form into beads.

5. Conclusion: Why Water Forms Beads on a Waxed Car Finish

Water forms into beads on a waxed car finish because the wax creates a hydrophobic layer on the surface, increasing the contact angle between the water and the car finish. Due to the high surface tension of water and the low surface energy of the waxed surface, water tends to minimize its surface area by forming beads. This bead formation is a result of the balance between the cohesive forces within the water and the adhesive forces between the water and the waxed surface.