Electrohydrodynamics

Electrokinesis is the particle or fluid 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 the electrophoresis of clay particles [2] The effect was also noticed and publicized in the 1920s by Thomas Townsend Brown which he called the Biefeld–Brown effect, although he seems to have misidentified it as an electric field acting on gravity.[3] The flow rate in such a mechanism is linear in the electric field. Electrokinesis is of considerable practical importance in microfluidics,[4][5][6] 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 equation

$${\displaystyle F={\frac {Id}{k}}}$$

where, ${\displaystyle F}$ is the resulting force, measured in newtons, ${\displaystyle I}$ is the current, measured in amperes, ${\displaystyle d}$ is the distance between electrodes, measured in metres, and ${\displaystyle k}$ 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.[7][8] 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 the University of Alberta discussed a method of hydrodynamic to electrical energy conversion by exploiting the natural electrokinetic properties of a liquid such as ordinary tap water, by pumping fluids through tiny micro-channels with a pressure difference.[9] This technology could some day provide a practical and clean energy storage device, replacing today's batteries, for devices such as mobile phones or calculators which would be charged up by simply pumping water to high pressure. Pressure would then be released on demand, for fluid flow to take place over the micro-channels. When water travels, or streams over a surface, the ions of which water is made "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. This streaming potential, water-flow phenomenon was discovered in 1859 by German physicist Georg Hermann Quincke. [5][6][10]

The fluid flows in microfluidic and nanofluidic devices are often stable and strongly damped by viscous forces (with Reynolds numbers of order unity or smaller). However, heterogeneous ionic conductivity fields in the presence of applied electric fields can, under certain conditions, generate an unstable flow field owing to electrokinetic instabilities (EKI). Conductivity gradients are prevalent in on-chip electrokinetic processes such as preconcentration methods (e.g. field amplified sample stacking and isoelectric focusing), multidimensional assays, and systems with poorly specified sample chemistry. The dynamics and periodic morphology of electrokinetic instabilities are similar to other systems with Rayleigh–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 the Rayleigh–Bénard convection.

EKI's can be leveraged for rapid mixing 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 the electric double layer, that can generate temporal, convective, and absolute flow instabilities. Electrokinetic flows with conductivity gradients become unstable when the electroviscous 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 is laminar. The onset of instability in these flows is best described as an electric "Rayleigh number".

Liquids can be printed at nanoscale by pyro-EHD.[11]

• Magnetohydrodynamic drive
• Magnetohydrodynamics
• Electrospray
• Electrokinetic phenomena
• Optoelectrofluidics
• Electrostatic precipitator

• Dr. Larry Kostiuk's website.
• Science-daily article about the discovery.
• BBC article with graphics.