Surface Tension and Capillarity

Also called

surface tension · capillarity · capillary rise · capillary action · meniscus · angle of contact · contact angle · water strider · insect walking on water · floating needle · mercury depression · detergent lowers surface tension · excess pressure in a drop · excess pressure in a bubble · soap bubble · cohesive forces · wetting · पृष्ठ तनाव · केशिकत्व · संपर्क कोण

Topics

• Surface tension S is the force per unit length acting along the surface of a liquid, measured in N/m.
• Molecules at a surface are pulled inward by their neighbours, so the surface behaves like a stretched elastic skin.
• The surface skin can hold up a light insect or a needle laid flat: the surface dimples but does not break.
• Capillary rise h = 2S cos θ / (r ρ g): water climbs higher in a thinner tube.
• Water wets glass (contact angle below 90°) and rises in a tube; mercury does not wet glass and is depressed.
• Soap and detergents lower the surface tension of water, so specks on the surface rush away from a soap drop.
• The excess pressure inside a liquid drop is 2S/R: a smaller drop has a higher pressure inside it.
• A soap bubble has two surfaces, so its excess pressure is 4S/R, twice that of a drop of the same radius.
• Surface tension pulls free drops into spheres: a sphere has the least surface area for a given volume.

About this model

A GLB file of about 369 KB, shown in Lably's 3D viewer and, on a phone, placed in your room with AR. Credit: Model, science and animation: Lably (built procedurally with three.js; no downloaded meshes). | CC BY 4.0. Original geometry and animation made for Lably.

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