Surface tension

Surface tension is the energy required to increase a liquid’s surface area, arising from cohesive forces between its molecules. It also acts as a force per unit length along a liquid interface.

Cohesion: Cohesion is attraction between molecules of the same substance. Cohesive attraction leaves surface molecules with fewer neighbors and raises the surface’s energy.
Capillary action: Capillary action is the movement of a liquid through narrow spaces caused by interfacial forces and adhesion. Surface tension helps pull liquid through narrow tubes and porous materials.
Surface viscosity: Surface viscosity measures resistance to flow within an interface or interfacial layer. It resists interfacial motion, whereas surface tension sets the energetic cost of area.
Water strider: A water strider is an insect adapted to move across the surface of water. Its legs distribute weight without breaking the water surface, demonstrating surface tension’s support.
Intermolecular force: An intermolecular force is an attraction or repulsion between molecules. These molecular interactions produce the cohesive forces behind surface tension.
Wetting: Wetting describes how a liquid spreads across or adheres to a solid surface. Surface tension helps set whether a liquid beads up or spreads across a surface.
Bulk viscosity: Bulk viscosity measures a fluid’s resistance to deformation and flow within its volume. It describes resistance inside a liquid, unlike surface tension at its boundary.
Soap bubble: A soap bubble is a thin liquid film enclosing air, stabilized by surfactant molecules. Its curved film and pressure illustrate surface tension acting on a minimal-area enclosure.
Contact angle: A contact angle is the angle at which a liquid interface meets a solid surface. Surface tension and solid–liquid interactions together determine how a droplet meets a surface.
Pendant drop method: The pendant drop method measures liquid surface tension from the shape of a suspended droplet. A droplet’s balance of gravity and surface tension provides a practical measurement.

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