Electrohydrodynamics



Electrohydrodynamics (EHD), also known as electro-fluid-dynamics (EFD) or electrokinetics, is the study of the dynamics of electrically conducting electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.

In the first instance, shaped electrostatic fields create hydrostatic pressure (or motion) in dielectric media. When such media are 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 a potential difference by electrodes. In such case, the structure acts as an electrical generator.

Electrokinesis

Electrokinesis is the particle or microfluidics, since 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:

F = \frac{Id}{k}     
where
F  is the resulting force, measured in Newtons,
I  is the current flow, measured in amperes,
d  is the distance between electrodes, measured in metres, and
k  is the ion mobility coefficient of the dielectric fluid, measured in m2/Volt sec.

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 damaged to neurons by inciting movement in their membranes.[1][2] It is also discussed in R.J.Elul's "Fixed charge in the cell membrane" (1967).

Water electrokinetics

In October 2003, Dr. Daniel Kwok, Dr. Larry Kostiuk and two graduate students from the University of Alberta revealed a new 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 microchannels with a pressure difference. 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 microchannels. When water travels over a surface, the ions that it is made up of "rub" against the solid, leaving the surface slightly charged. Kinetic energy from the moving ions would be thus converted to electrical energy. Although the power generated from a single channel is extremely small, millions of parallel channels can be used to increase the power output. Actually, this phenomenon, called streaming potential, has been well-known for about 200 years.[citation needed]

Electrokinetic Instabilities

The fluid flow in Rayleigh-Taylor instabilities.

Electrokinetic instabilities 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.

For more information see:

  • Electrokinetic Instability Work in the Posner Research Lab
  • Electrokinetic Instability work by Juan G. Santiago and coworkers

References

  1. ^ Patterson, Michael; Kesner, Raymond (1981). Electrical Stimulation Research Techniques. Academic Press. ISBN 0-12-547440-7. 
  2. ^ Elul, R.J. (1967). Fixed charge in the cell membrane. 

See also

  • RMCybernetics - Electrohydrodynamic Propulsion and how to make an EHD Thruster at home
  • Dr. Larry Kostiuk's website.
  • Science-daily article about the discovery.
  • BBC article with graphics.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Electrohydrodynamics". A list of authors is available in Wikipedia.