Capillary action

Capillary action is the rise or fall of a liquid in a narrow space, driven by surface tension and interactions between the liquid and surrounding surfaces.

Surface tension: The tendency of a liquid’s surface to minimize its area because molecules at the surface experience unbalanced cohesive forces. It creates the curved meniscus and pressure difference that drive liquid movement.
Adhesion: The attraction between unlike substances, such as a liquid and a solid surface. Liquid–wall attraction helps determine the meniscus shape and direction of motion.
Xylem: The vascular tissue in plants that transports water and dissolved minerals from roots toward leaves. Narrow xylem conduits support capillary effects alongside transpiration and water cohesion.
Darcy's law: A relation describing fluid flow through a porous medium as proportional to pressure gradient and permeability. It describes pressure-driven porous flow, unlike capillary action driven by interfacial forces.
Contact angle: The angle where a liquid–vapor interface meets a solid surface, measured through the liquid. It indicates whether the liquid wets the capillary wall and whether the liquid rises or falls.
Cohesion: The attraction between molecules of the same substance. Cohesion within the liquid contributes to surface tension and holds the rising column together.
Paper chromatography: A separation technique in which a solvent moves through paper and carries mixture components at different rates. Capillary flow carries the solvent and dissolved substances through the paper.
Poiseuille's law: A relation giving the steady flow rate of a viscous fluid through a cylindrical tube under a pressure difference. It predicts externally pressure-driven tube flow rather than spontaneous meniscus-driven rise.
Meniscus: The curved surface of a liquid at a boundary with another fluid or a solid. Its curvature sets the pressure difference across the liquid surface in a narrow space.
Interfacial tension: The energy per unit area associated with an interface between two phases. Surface tension is the special case at a liquid–gas interface.

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