Why does water wet some surfaces and bead up on others, while mercury beads up on almost all surfaces?

Verified step by step guidance

Understand the concept of surface tension and how it relates to the cohesive and adhesive forces between molecules.

Recognize that water has strong hydrogen bonds, which contribute to its high surface tension, allowing it to wet surfaces where adhesive forces are stronger than cohesive forces.

Identify that mercury has metallic bonds, which are much stronger than the adhesive forces with most surfaces, leading to high cohesion and causing it to bead up.

Consider the role of surface energy: surfaces with high surface energy (like glass) are more likely to be wetted by water, while low surface energy surfaces (like wax) cause water to bead up.

Apply the concept of contact angle: a low contact angle indicates wetting (water spreads out), while a high contact angle indicates beading (water forms droplets).

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Surface Tension

Surface tension is a physical property of liquids that describes the elastic-like force at the surface of a liquid. It arises from the cohesive forces between liquid molecules, which are stronger at the surface due to the lack of neighboring molecules above. This phenomenon explains why water forms droplets on certain surfaces and why it can 'wet' others, depending on the balance between cohesive and adhesive forces.

Adhesion and Cohesion

Adhesion refers to the attraction between different substances, while cohesion is the attraction between like molecules. In the case of water, if the adhesive forces between water molecules and a surface are stronger than the cohesive forces among the water molecules, the water will spread out and wet the surface. Conversely, if cohesion dominates, water will bead up, as seen on hydrophobic surfaces.

Hydrophobic and Hydrophilic Surfaces

Hydrophobic surfaces repel water and do not allow it to spread, leading to beading, while hydrophilic surfaces attract water, promoting spreading and wetting. The chemical composition and structure of a surface determine its hydrophobicity or hydrophilicity. Mercury, being a dense liquid with strong cohesive forces, beads up on most surfaces due to its high surface tension and lack of adhesive interaction with them.

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