What Is Electrokinetics?

Electrokinetics is the study of the motion of charged particles, ions, and fluid phases that arises from the interaction of electric fields with the electrical double layer formed at interfaces between solid surfaces and liquid electrolytes. When an electric field is applied to a system containing charged interfaces, the mobile counter-ions accumulated near each surface experience a direct Coulomb force, and their motion drags the surrounding fluid with them, generating a variety of coupled transport phenomena. The discipline draws on classical electrostatics, physical chemistry of surfaces, and low-Reynolds-number fluid mechanics, and is closely related to electrohydrodynamics, which treats electrically driven flows in weakly conducting bulk media rather than in charged-double-layer systems.

The foundational phenomena were discovered over two centuries ago. Ferdinand Reuss observed in 1809 that applying voltage across a porous clay plug caused water to flow through it (electroosmosis) and that clay particles suspended in water migrated toward an electrode (electrophoresis). Modern electrokinetic theory developed quantitatively from the Gouy-Chapman-Stern model of the electric double layer, whose structure and thickness set the length scale for all electrokinetic transport.

Electrophoresis

Electrophoresis is the movement of charged particles or macromolecules through a fluid under an applied electric field. A particle bearing a surface charge accumulates a diffuse counter-ion cloud; in the presence of an external field, the particle migrates at a velocity proportional to its electrophoretic mobility, which depends on its surface charge density, size, and the ionic strength of the surrounding medium. Gel electrophoresis, which separates nucleic acids and proteins by size through a sieving matrix, is one of the most widely used techniques in molecular biology. Capillary electrophoresis conducts separations in narrow fused-silica capillaries at high field strengths, achieving peak efficiencies that far exceed what column chromatography can provide. IEEE Transactions on Mechatronics coverage of electrokinetics in micro devices examines how these principles scale into miniaturized analytical systems.

Electroosmosis

Electroosmosis is the bulk flow of a liquid electrolyte relative to a stationary charged surface when an electric field is applied tangentially to that surface. The mechanism relies on the excess mobile charge in the diffuse part of the electric double layer: the applied field acts on these ions, which are not bound to the surface, and their motion entrains the viscous fluid in a plug-flow profile with no velocity gradient across the channel cross-section. This flat velocity profile contrasts sharply with the parabolic profile of pressure-driven flow and makes electroosmosis particularly advantageous for transporting samples without the dispersion that pressure-driven pumping introduces. In microfluidic lab-on-a-chip devices, electroosmotic pumping eliminates the need for external pressure sources, allowing valveless fluid routing by simply switching electrode voltages. A detailed treatment of electroosmotic flow from microfluidics to nanofluidics shows how the technique extends to channels narrower than the Debye length, where new transport regimes emerge.

Dielectrophoresis

Dielectrophoresis (DEP) is a force exerted on polarizable particles by a non-uniform electric field, even when the particles carry no net charge. The mechanism arises from the differential polarizability between the particle and its surrounding medium: if the particle polarizes more strongly than the medium, it experiences a positive DEP force toward regions of stronger field; if the medium polarizes more strongly, the particle is repelled toward field minima. Because DEP response depends on the electrical properties of particles at specific frequencies, it can differentiate living from dead cells, cancer cells from healthy cells, or different species of bacteria without chemical labels. When combined with electroosmotic and electrophoretic flows on a microfluidic chip, DEP enables fully integrated sorting, trapping, and analysis of biological particles. PMC's review of electric-field-driven pumping situates dielectrophoresis within the broader electrokinetic toolkit used in lab-on-chip design.

Applications

Electrokinetics has applications in a wide range of fields, including:

• Gel and capillary electrophoresis for DNA sequencing, protein analysis, and forensic identification
• Lab-on-a-chip systems for point-of-care diagnostics and environmental monitoring
• Electrokinetic soil remediation, using applied fields to mobilize heavy-metal contaminants from contaminated ground
• Microfluidic cell sorting and single-cell analysis in biomedical research
• Electroosmotic pumping in inkjet print heads and precision liquid dispensing systems