Definition of Contact Angle
The contact angle is the angle, conventionally measured through the liquid, where a liquid-vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid, indicating how strongly the liquid molecules are attracted to the solid surface relative to themselves.
Key Principles and Measurements
A small contact angle (less than 90°) indicates high wettability (hydrophilic surface), meaning the liquid spreads widely. A large contact angle (greater than 90°) indicates low wettability (hydrophobic surface), meaning the liquid beads up. It is determined by the balance of intermolecular forces: adhesive forces between the liquid and solid, and cohesive forces within the liquid itself.
A Practical Example
When water is dropped onto a clean glass surface, it typically forms a small contact angle and spreads out, indicating good wetting. Conversely, when water is dropped onto a waxy leaf or a specially treated waterproof fabric, it forms spherical beads with a large contact angle, indicating poor wetting.
Importance and Applications
Understanding contact angle is critical in fields such as manufacturing (e.g., coating, painting, printing), biology (e.g., cell adhesion, self-cleaning surfaces), and engineering (e.g., microfluidics, oil recovery, anti-fogging coatings). It impacts how liquids interact with materials, from medical devices to consumer electronics.