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In surface science, a double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object.
Double layer forces occur between charged objects across liquids, typically water. This force acts over distances that are comparable to the Debye length, which is on the order of one to a few tenths of nanometers. The strength of these forces increases with the magnitude of the surface charge density (or the electrical surface potential). For ...
where U is the velocity vector, ρ is the density of the fluid, / is the material derivative, μ is the viscosity of the fluid, ρ e is the electric charge density, ϕ is the applied electric field, ψ is the electric field due to the zeta potential at the walls and p is the fluid pressure. Laplace’s equation can describe the external ...
The total electrical double layer due to the formation of the counterion layers results in electrostatic screening of the wall charge and minimizes the Gibbs free energy of EDL formation. The thickness of the diffuse electric double layer is known as the Debye screening length /. At a distance of two Debye screening lengths the electrical ...
A similar theory was created ten years later by O'Brien and Hunter. [25] Assuming a thin double layer, these theories would yield results that are very close to the numerical solution provided by O'Brien and White. [26] There are also general electroacoustic theories that are valid for any values of Debye length and Dukhin number. [5] [11]
The equation above is usually referred to as the Helmholtz–Smoluchowski equation. The above equations assume that: the double layer is not too large compared to the pores or capillaries (i.e., κ a ≫ 1 {\displaystyle \kappa a\gg 1} ), where κ is the reciprocal of the Debye length
[1] [2] [3] There is a common source of all these effects—the so-called interfacial 'double layer' of charges. Influence of an external force on the diffuse layer generates tangential motion of a fluid with respect to an adjacent charged surface. This force might be electric, pressure gradient, concentration gradient, or gravity.