Capillarydriven

Capillary-driven refers to processes and devices in which liquid transport is propelled primarily by capillary forces arising from surface tension at a liquid–solid–air interface. These forces create a capillary pressure that can draw fluid into narrow channels, pores, or tubes due to wetting by the liquid on the walls. The magnitude of capillary pressure depends on the liquid’s surface tension γ, the contact angle θ between the liquid and the solid, and a characteristic dimension such as a capillary radius or pore size. Gravity and viscous resistance can oppose capillary rise, leading to equilibrium or dynamic competition during flow.

In a cylindrical capillary, the capillary pressure difference across a meniscus is ΔP = 2 γ cos θ / r. If θ < 90°, cos θ > 0 and the liquid tends to rise; for θ > 90°, it tends to retreat. For dynamic capillary-driven flow in tubes or porous media, models such as Washburn’s law describe capillary imbibition, with L^2 = (r γ cos θ / (2 η)) t under neglect of gravity. More complete treatments account for gravity, inertia, and porous media permeability via Darcy-like relations. The interaction of capillary, gravitational, and viscous forces sets the speed and extent of transport.

In practice, capillary-driven systems exploit narrow channels, porous substrates, or paper-based circuits where capillary pressure moves liquids passively. Smaller dimensions generally increase capillary pressure and actuation strength, while the contact angle and surface treatment determine wettability. Materials commonly used include glass, silicon, polymers (such as PDMS), and porous papers or membranes.

Applications include lab-on-a-chip devices, point-of-care diagnostics, and paper-based microfluidics, where capillary-driven transport enables reagent delivery, sample loading, and sequential reactions without external pumps. The approach offers simplicity and low power demand but is sensitive to environmental conditions, surface aging, and liquid properties that affect wettability.