Pumping of water with ac electric fields applied to asymmetric pairs of microelectrodes

TLDR

Bulk fluid flow can be induced by an AC electric potential below water’s ionization threshold. A planar array of asymmetric electrode pairs is driven by an AC potential; during each half‑cycle the double layer charges nonuniformly, creating a surface‑parallel potential gradient that drives ions in the double layer and drags fluid, producing a net flow. The resulting flow is approximately uniform beyond the electrode periodicity, its potential and frequency dependence are characterized and compared to a simple model, and the method requires only low voltage and simpler electrodes than traveling‑wave techniques.

Abstract

Bulk fluid flow induced by an ac electric potential with a peak voltage below the ionization potential of water is described. The potential is applied to an ionic solution with a planar array of electrodes arranged in pairs so that one edge of a large electrode is close to an opposing narrow electrode. During half the cycle, the double layer on the surface of the electrodes charges as current flows between the electrodes. The electrodes charge in a nonuniform manner producing a gradient in potential parallel to the surface of the electrodes. This gradient drives the ions in the double layer across the surface of the electrode and this in turn drags the fluid across the electrode surface. The anisotropic nature of the pairs of electrodes is used to produce a net flow of fluid. The flow produced is approximately uniform at a distance from the electrodes that is greater than the periodicity of the electrode array. The potential and frequency dependence of this flow is reported and compared to a simple model. This method of producing fluid flow differs from electrical and thermal traveling-wave techniques as only a low voltage is required and the electrode construction is much simpler.