Capillarytryk

Capillarytryk, often referred to as capillary pressure in English, is the pressure difference across the interface between two immiscible fluids confined in a capillary or pore space. It arises from interfacial tension and the curvature of the fluid meniscus and is a key factor in multiphase flow in porous media, soils, and microfluidic systems.

The theoretical basis is given by the Young-Laplace relation, which links interfacial tension, curvature, and pressure difference. In a simple cylindrical capillary of radius r, with interfacial tension γ and contact angle θ, the capillary pressure is ΔP = 2 γ cos θ / r, where ΔP is the pressure of the non-wetting phase minus that of the wetting phase. In porous media, capillary pressure is expressed as Pc = P_nw − P_w and typically varies with the wetting phase saturation, pore size distribution, and wettability, described by capillary pressure–saturation (Pc–S) curves. Hysteresis is common between drainage and imbibition due to changes in pore geometry and contact angle.

Capillarytryk depends on several factors, including interfacial tension, surface wettability, pore size, temperature, and the fluids involved. It governs phenomena such as capillary rise in narrow tubes, Pc-S behavior during fluid displacement, and capillary trapping. In practical terms, Pc affects how fluids distribute and move in soils, aquifers, and petroleum reservoirs, influencing saturation, breakthrough timing, and recovery efficiency.

Measurement and characterization methods include mercury intrusion capillary pressure (MICP), capillary pressure–saturation tests, centrifuge methods, and microfluidic or imaging-based approaches. Units are typically pascals or kilopascals, though practical measurements may be expressed as height equivalents (e.g., cm of water).