Electrohydrodynamics (EHD), also known aselectro-fluid-dynamics (EFD) orelectrokinetics, is the study of thedynamics ofelectrically charged fluids.[1][2] Electrohydrodynamics (EHD) is a joint domain of electrodynamics and fluid dynamics mainly focused on thefluid motion induced by electric fields. EHD, in its simplest form, involves the application of an electric field to a fluid medium, resulting in fluid flow, form, or properties manipulation. These mechanisms arise from the interaction between theelectric fields andcharged particles orpolarization effects within the fluid.[2] The generation and movement ofcharge carriers (ions) in a fluid subjected to an electric field are the underlying physics of all EHD-based technologies.
The electric forces acting on particles consist of electrostatic (Coulomb) and electrophoresis force (first term in the following equation)., dielectrophoretic force (second term in the following equation), and electrostrictive force (third term in the following equation):
This electrical force is then inserted inNavier-Stokes equation, as a body (volumetric) force.
EHD covers the following types of particle and fluid transport mechanisms:electrophoresis,electrokinesis,dielectrophoresis,electro-osmosis, andelectrorotation. In general, the phenomena relate to the direct conversion ofelectrical energy intokinetic energy, andvice versa.
In the first instance, shapedelectrostatic fields (ESF's) createhydrostatic pressure (HSP, or motion) indielectric media. When such media arefluids, aflow is produced. If the dielectric is avacuum or asolid, no flow is produced. Such flow can be directed against theelectrodes, generally to move the electrodes. In such case, the moving structure acts as anelectric motor. Practical fields of interest of EHD are the commonair ioniser,electrohydrodynamic thrusters and EHD cooling systems.
In the second instance, the converse takes place. A powered flow of medium within a shaped electrostatic field adds energy to the system which is picked up as apotential difference by electrodes. In such case, the structure acts as anelectrical generator.
Electrokinesis is the particle orfluid transport produced by an electric field acting on a fluid having a net mobile charge. (See -kinesis for explanation and further uses of the -kinesis suffix.)Electrokinesis was first observed by Ferdinand Frederic Reuss during 1808, in theelectrophoresis of clay particles[3] The effect was also noticed and publicized in the 1920s byThomas Townsend Brown which he called theBiefeld–Brown effect, although he seems to have misidentified it as an electric field acting on gravity.[4] The flow rate in such a mechanism is linear in theelectric field. Electrokinesis is of considerable practical importance inmicrofluidics,[5][6][7] because it offers a way to manipulate and convey fluids in microsystems using only electric fields, with no moving parts.
The force acting on the fluid, is given by the equationwhere, is the resulting force, measured innewtons, is the current, measured inamperes, is the distance between electrodes, measured in metres, and is the ion mobility coefficient of the dielectric fluid, measured in m2/(V·s).
If the electrodes are free to move within the fluid, while keeping their distance fixed from each other, then such a force will actually propel the electrodes with respect to the fluid.
Electrokinesis has also been observed in biology, where it was found to cause physical damage to neurons by inciting movement in their membranes.[8][9] It is discussed in R. J. Elul's "Fixed charge in the cell membrane" (1967).
In October 2003, Dr. Daniel Kwok, Dr. Larry Kostiuk and two graduate students from theUniversity of Alberta discussed a method to convert hydrodynamic toelectrical energy by exploiting the natural electrokinetic properties of a liquid such as ordinarytap water, by pumping fluid through tiny micro-channels with a pressure difference.[10] This technology could lead to a practical and clean energy storage device, replacing batteries for devices such as mobile phones or calculators which would be charged up by simply compressing water to highpressure. Pressure would then be released on demand, for the fluid to flow through micro-channels. When water travels, or streams over a surface, the ions in the water "rub" against the solid, leaving the surface slightly charged. Kinetic energy from the moving ions would thus be converted to electrical energy. Although the power generated from a single channel is extremely small, millions of parallel micro-channels can be used to increase the power output.Thisstreaming potential, water-flow phenomenon was discovered in 1859 by German physicistGeorg Hermann Quincke.[citation needed][6][7][11]
The fluid flows inmicrofluidic and nanofluidic devices are often stable and strongly damped by viscous forces (withReynolds numbers of order unity or smaller). However, heterogeneous ionic conductivity fields in the presence of appliedelectric fields can, under certain conditions, generate an unstable flow field owing toelectrokinetic instabilities (EKI). Conductivity gradients are prevalent in on-chip electrokinetic processes such as preconcentration methods (e.g. field amplified sample stacking andisoelectric focusing), multidimensional assays, and systems with poorly specified sample chemistry. The dynamics and periodic morphology ofelectrokinetic instabilities are similar to other systems withRayleigh–Taylor instabilities. The particular case of a flat plane geometry with homogeneous ions injection in the bottom side leads to a mathematical frame identical to theRayleigh–Bénard convection.
EKI's can be leveraged for rapidmixing or can cause undesirable dispersion in sample injection, separation and stacking. These instabilities are caused by a coupling of electric fields and ionic conductivity gradients that results in an electric body force. This coupling results in an electric body force in the bulk liquid, outside theelectric double layer, that can generate temporal, convective, and absolute flow instabilities. Electrokinetic flows with conductivity gradients become unstable when theelectroviscous stretching and folding of conductivity interfaces grows faster than the dissipative effect of molecular diffusion.
Since these flows are characterized by low velocities and small length scales, the Reynolds number is below 0.01 and the flow islaminar. The onset of instability in these flows is best described as an electric "Rayleigh number".
Liquids can be printed at nanoscale by pyro-EHD.[12]
