Kapillarlast

Kapillarlast, or capillary load, is the mechanical load that capillary forces exert at a liquid–solid interface on a structure or body. It arises from the surface tension of a liquid forming a curved meniscus and from the resulting pressure difference and line forces acting on the solid. The concept is relevant wherever small-scale wetting, capillary action, or drying-induced suction occurs, including porous media, thin films, textiles, and microstructures.

The magnitude of capillary forces can be estimated from the geometry of the liquid–solid contact and the liquid’s surface tension gamma. A common approximation is that the force is proportional to the length of the liquid–solid contact line L and the cosine of the contact angle theta: F ≈ gamma · L · cos theta. For a circular capillary tube of radius r, the capillary force on the wall is F = 2πr γ cos theta, while the capillary pressure inside the tube is P_cap = 2 γ cos theta / r. The capillary pressure acts as a suction (negative pressure relative to the outside) in wetting scenarios (theta < 90°).

Contexts and applications. In geotechnical engineering, capillary suction contributes to the matric suction in unsaturated soils, affecting effective stress, shear strength, and deformation. In textiles and porous materials, capillary forces drive wicking, bonding, and drying dynamics. In microfluidics and MEMS, capillary load can power motion or alter stability of small components, necessitating careful design to avoid undesired sticking or leakage. In materials processing, capillary effects influence drying, sintering, and capillary bridging between particles.

Measurement and relevance. Capillary load depends strongly on liquid properties, geometry, and wetting conditions; it is a key consideration in any application involving small scales or rapid moisture changes.