electroosmosisbulk

Bulk electroosmosis, sometimes referred to as electroosmotic flow in porous media, is the macroscopic movement of liquid through a charged solid under an applied electric field. It occurs in a wide range of porous materials, including soils, clays, ceramics, membranes, and microfluidic networks, where a solid–liquid interface forms an electrical double layer.

The mechanism rests on the electric double layer that develops at the solid surface. The diffuse layer contains mobile ions that respond to an external electric field. Their motion drags the adjacent bulk fluid via viscous forces, generating a net flow through the pore network. In the thin-double-layer limit, the flow velocity is described by the Helmholtz–Smoluchowski relation: u_EO = - (ε ζ / η) E, where ε is the fluid permittivity, ζ is the zeta potential (the potential in the slipping plane), η is the dynamic viscosity, and E is the applied electric field. In a porous medium, the bulk velocity is approximately μ_EO E, where μ_EO = -ε ζ / η is an electroosmotic mobility modified by porosity, tortuosity, and pore structure.

Key factors influencing bulk electroosmosis include the magnitude and sign of the zeta potential, electrolyte composition and ionic strength (which affect the thickness of the double layer), pH, temperature, and the geometric properties of the pore network. Higher zeta potential and lower viscosity enhance the flow, while increasing ionic strength can suppress it by compressing the double layer.

Applications span geotechnical remediation (electrokinetic cleanup of contaminated soils), groundwater treatment, and microfluidics, where electroosmosis provides a non-mechanical pumping mechanism. Limitations arise when the double layer is not thin relative to pore size, in highly heterogeneous media, or when electrode reactions induce pH gradients and gas formation.