Editing Electrohydrodynamics

{{Short description|Study of electrically conducting fluids in the presence of electric fields}}
{{Use American English|date = February 2019}}
'''Electrohydrodynamics''' ('''EHD'''), also known as '''electro-fluid-dynamics''' ('''EFD''') or '''electrokinetics''', is the study of the [[dynamics (mechanics)|dynamics]] of [[electrically charged]] fluids.<ref name="Castellanos">{{cite book | author=Castellanos, A. | title=Electrohydrodynamics | year=1998 | author-link=Antonio Castellanos Mata }}</ref><ref name=":0" /> Electrohydrodynamics (EHD) is a joint domain of electrodynamics and fluid dynamics mainly focused on the '''fluid 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 the '''electric fields''' and '''charged particles''' or '''polarization effects''' within the fluid.<ref name=":0">{{Cite journal |last1=Iranshahi |first1=Kamran |last2=Defraeye |first2=Thijs |date=2024 |title=Electrohydrodynamics and its applications: Recent advances and future perspectives |journal=International Journal of Heat and Mass Transfer |volume=232 |doi=10.1016/j.ijheatmasstransfer.2024.125895|doi-access=free |bibcode=2024IJHMT.23225895I |hdl=20.500.11850/683872 |hdl-access=free }}</ref> The generation and movement of '''charge carriers (ions)''' in a fluid subjected to an electric field are the underlying physics of all EHD-based technologies.
[[File:EHD Wiki Iranshhai et al.tif|thumb|Electrohydrodynamics employed for drying applications (EHD Drying)<ref name=":0" />.|353x353px]]


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):

<math>F_e= \rho_e \overrightarrow{E} - {1 \over 2}\varepsilon_{0}\overrightarrow{E}^2\triangledown\varepsilon_r + {1 \over 2}\varepsilon_{0}\triangledown\Bigl(\overrightarrow{E}^2 \rho_{f}\left ( \frac{\partial \varepsilon_{r}}{\partial \rho_{f}} \right ) \Bigr)</math><ref name=":0" />

This electrical force is then inserted in [[Navier–Stokes equations|Navier-Stokes]] equation, as a body (volumetric) force.[[File:EHD Wiki 2 Iranshhai et al.tif|thumb|Electrohydrodynamics employed for [[Plasma actuator|Airflow control]] and [[Electrospinning]] applications.]]EHD covers the following types of particle and fluid transport mechanisms: [[electrophoresis]], [[Electrohydrodynamics#Electrokinesis|electrokinesis]], [[dielectrophoresis]], [[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 field]]s (ESF's) create [[hydrostatic pressure]] (HSP, or motion) in [[dielectric media]]. When such media are [[fluid]]s, a [[Fluid dynamics|flow]] is produced. If the dielectric is a [[vacuum]] or a [[solid]], no flow is produced. Such flow can be directed against the [[electrode]]s, generally to move the electrodes. In such case, the moving structure acts as an [[electric motor]]. Practical fields of interest of EHD are the common [[air ioniser]], [[electrohydrodynamic thruster]]s 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 ==
{{Redirect|Electrokinesis|the ability to manipulate electricity|List of psychic abilities}}
'''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 <ref>Wall, Staffan. "The history of electrokinetic phenomena." Current Opinion in Colloid & Interface Science 15.3 (2010): 119-124.</ref> 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.<ref name="Wired">{{Cite news
	| magazine = [[Wired Magazine]]
	| date = August 2003
	| title = The Antigravity Underground
	| last = Thompson
	| first = Clive
	| url = https://www.wired.com/wired/archive/11.08/pwr_antigravity.html
	}}</ref> The flow rate in such a mechanism is linear in the [[electric field]]. Electrokinesis is of considerable practical importance in [[microfluidics]],<ref name=Chang>{{cite book |author1=Chang, H.C. |author2=Yeo, L. | title=Electrokinetically Driven Microfluidics and Nanofluidics | year=2009 | publisher =[[Cambridge University Press]] }}</ref><ref name=Kirby>{{cite book| author=Kirby, B.J.| title=Micro- and Nanoscale Fluid Mechanics: Transport in Microfluidic Devices.| url=http://www.kirbyresearch.com/textbook| year=2010| publisher=Cambridge University Press| isbn=978-0-521-11903-0| access-date=2010-02-13| archive-date=2019-04-28| archive-url=https://web.archive.org/web/20190428234717/http://www.kirbyresearch.com/textbook/| url-status=dead}}</ref><ref name=Bruus>{{cite book | author=Bruus, H. | title=Theoretical Microfluidics | year=2007 | publisher =[[Oxford University Press]] }}</ref> 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
<math display="block">F = \frac{I d}{k} </math>
where, <math>F </math> is the resulting force, measured in [[newton (unit)|newtons]], <math>I </math> is the current, measured in [[ampere]]s, <math>d </math> is the distance between electrodes, measured in metres, and <math>k </math> is the ion mobility coefficient of the dielectric fluid, measured in m<sup>2</sup>/(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.<ref name=patkesn1>{{cite book
  | last = Patterson 
  | first = Michael 
  |author2=Kesner, Raymond 
  | title = Electrical Stimulation Research Techniques
  | publisher = [[Academic Press]]
  | year = 1981
  | isbn = 0-12-547440-7}}</ref><ref>{{cite journal
  | last = Elul 
  | first = R.J. 
  | title = Fixed charge in the cell membrane
  | journal = The Journal of Physiology 
 | year = 1967
  | volume = 189 
 | issue = 3 
 | pages = 351–365 
 | doi = 10.1113/jphysiol.1967.sp008173 
 | pmid = 6040152 
 | pmc = 1396124 
 }}</ref>  It is 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]] discussed a method to convert hydrodynamic to [[energy conversion|electrical energy]] by exploiting the natural electrokinetic properties of a liquid such as ordinary [[tap water]], by pumping fluid through tiny micro-channels with a pressure difference.<ref>{{cite journal|last1=Yang|first1=Jun|last2=Lu|first2=Fuzhi|last3=Kostiuk|first3=Larry W.|last4=Kwok|first4=Daniel Y.|title=Electrokinetic microchannel battery by means of electrokinetic and microfluidic phenomena|journal=Journal of Micromechanics and Microengineering|volume=13|issue=6|pages=963–970|language=en|doi=10.1088/0960-1317/13/6/320|date=1 January 2003|bibcode=2003JMiMi..13..963Y|s2cid=250922353 }}</ref> 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 high [[pressure]]. 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.
This [[streaming potential]], water-flow phenomenon was discovered in 1859 by German physicist [[Georg Hermann Quincke]]. {{citation needed|date=April 2017}}<ref name=Kirby/><ref name=Bruus/><ref name=Levich>{{cite book | author=Levich, V.I. | title=Physicochemical Hydrodynamics | year=1962 }}</ref>

== Electrokinetic instabilities ==

The fluid flows in [[microfluidic]] and nanofluidic devices are often stable and strongly damped by viscous forces (with [[Reynolds number]]s of order unity or smaller). However, heterogeneous ionic conductivity fields in the presence of applied [[electric field]]s 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 instability|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 (physics)|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 [[double layer (interfacial)#Electrical double layers|electric double layer]], that can generate temporal, convective, and absolute flow instabilities. Electrokinetic flows with conductivity gradients become unstable when the [[electroviscous effects|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".

== Misc ==
Liquids can be printed at nanoscale by pyro-EHD.<ref>{{Cite journal | doi = 10.1038/nnano.2010.82| pmid = 20453855| title = Dispensing nano–pico droplets and liquid patterning by pyroelectrodynamic shooting| journal = Nature Nanotechnology| volume = 5| issue = 6| pages = 429–435| year = 2010| last1 = Ferraro | first1 = P.| last2 = Coppola | first2 = S.| last3 = Grilli | first3 = S.| last4 = Paturzo | first4 = M.| last5 = Vespini | first5 = V.|bibcode = 2010NatNa...5..429F }}</ref>

== See also ==
* [[Magnetohydrodynamic drive]]
* [[Magnetohydrodynamics]]
* [[Electrodynamic droplet deformation]]
* [[Electrospray]]
* [[Electrokinetic phenomena]]
* [[Optoelectrofluidics]]
* [[Electrostatic precipitator]]
* [[List of textbooks in electromagnetism]]

==References==
{{Reflist}}

==External links==
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*[https://web.archive.org/web/20060822054546/http://www.mece.ualberta.ca/staff/Kostiuk/Kostiuk_index.htm Dr. Larry Kostiuk's website].
*[https://www.sciencedaily.com/releases/2003/10/031020054036.htm Science-daily article about the discovery].
*[http://news.bbc.co.uk/2/hi/technology/3201030.stm BBC article with graphics].

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[[Category:Electrodynamics]]
[[Category:Energy conversion]]
[[Category:Fluid dynamics]]