Electrohydrodynamics

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Electrohydrodynamics (EHD), also known as electro-fluid-dynamics (EFD) or electrokinetics, is the study of the dynamics of electrically charged fluids. [1] It is the study of the motions of ionized particles or molecules and their interactions with electric fields and the surrounding fluid. The term may be considered to be synonymous with the rather elaborate electrostrictive hydrodynamics. ESHD covers the following types of particle and fluid transport mechanisms: electrophoresis, electrokinesis, dielectrophoresis, electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.

Contents

In the first instance, shaped electrostatic fields (ESF's) create hydrostatic pressure (HSP, or motion) in dielectric media. When such media are fluids, a flow is produced. If the dielectric is a vacuum or a solid, no flow is produced. Such flow can be directed against the electrodes, 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 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 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

where, is the resulting force, measured in newtons, is the current, measured in amperes, 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. [7] [8] 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 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. [ citation needed ] [5] [6] [10]

Electrokinetic instabilities

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".

Misc

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

See also

Related Research Articles

The Biefeld–Brown effect is an electrical phenomenon that produces an ionic wind that transfers its momentum to surrounding neutral particles. It describes a force observed on an asymmetric capacitor when high voltage is applied to the capacitor's electrodes. Once suitably charged up to high DC potentials, a thrust at the negative terminal, pushing it away from the positive terminal, is generated. The effect was named by inventor Thomas Townsend Brown who claimed that he did a series of experiments with professor of astronomy Paul Alfred Biefeld, a former teacher of Brown whom Brown claimed was his mentor and co-experimenter at Denison University in Ohio.

<span class="mw-page-title-main">Corona discharge</span> Ionization of air around a high-voltage conductor

A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.

<span class="mw-page-title-main">Electro-osmosis</span> Movement of liquid through a conduit due to electric potential

In chemistry, electro-osmotic flow is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit. Because electro-osmotic velocities are independent of conduit size, as long as the electrical double layer is much smaller than the characteristic length scale of the channel, electro-osmotic flow will have little effect. Electro-osmotic flow is most significant when in small channels, and is an essential component in chemical separation techniques, notably capillary electrophoresis. Electro-osmotic flow can occur in natural unfiltered water, as well as buffered solutions.

Electrowetting is the modification of the wetting properties of a surface with an applied electric field.

<span class="mw-page-title-main">Zeta potential</span> Electrokinetic potential in colloidal dispersions

Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface.

An ion-propelled aircraft or ionocraft is an aircraft that uses electrohydrodynamics (EHD) to provide lift or thrust in the air without requiring combustion or moving parts. Current designs do not produce sufficient thrust for manned flight or useful loads.

<span class="mw-page-title-main">Dielectrophoresis</span> Particle motion in a non-uniform electric field due to dipole-field interactions

Dielectrophoresis (DEP) is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles' electrical properties, on the particles' shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires. Furthermore, a study of the change in DEP force as a function of frequency can allow the electrical properties of the particle to be elucidated.

A surface charge is an electric charge present on a two-dimensional surface. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge distribution on the surface. The electric potential is continuous across a surface charge and the electric field is discontinuous, but not infinite; this is unless the surface charge consists of a dipole layer. In comparison, the potential and electric field both diverge at any point charge or linear charge.

The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter. Ideally the liquid reaching the emitter tip forms a Taylor cone, which emits a liquid jet through its apex. Varicose waves on the surface of the jet lead to the formation of small and highly charged liquid droplets, which are radially dispersed due to Coulomb repulsion.

A streaming current and streaming potential are two interrelated electrokinetic phenomena studied in the areas of surface chemistry and electrochemistry. They are an electric current or potential which originates when an electrolyte is driven by a pressure gradient through a channel or porous plug with charged walls.

Electroacoustic phenomena arise when ultrasound propagates through a fluid containing ions. The associated particle motion generates electric signals because ions have electric charge. This coupling between ultrasound and electric field is called electroacoustic phenomena. The fluid might be a simple Newtonian liquid, or complex heterogeneous dispersion, emulsion or even a porous body. There are several different electroacoustic effects depending on the nature of the fluid.

The Dukhin number is a dimensionless quantity that characterizes the contribution of the surface conductivity to various electrokinetic and electroacoustic effects, as well as to electrical conductivity and permittivity of fluid heterogeneous systems. The number was named after Stanislav and Andrei Dukhin.

<span class="mw-page-title-main">Double layer (surface science)</span> Molecular interface between a surface and a fluid

In surface science, a double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions which are adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids, or in porous bodies filled with fluid, or in a fast flow over a flat surface. The term heterogeneous here means a fluid containing particles. Particles can be solid, liquid or gas bubbles with sizes on the scale of a micrometer or nanometer. There is a common source of all these effects—the so-called interfacial 'double layer' of charges. Influence of an external force on the diffuse layer generates tangential motion of a fluid with respect to an adjacent charged surface. This force might be electric, pressure gradient, concentration gradient, or gravity. In addition, the moving phase might be either continuous fluid or dispersed phase.

Sedimentation potential occurs when dispersed particles move under the influence of either gravity or centrifugation or electricity in a medium. This motion disrupts the equilibrium symmetry of the particle's double layer. While the particle moves, the ions in the electric double layer lag behind due to the liquid flow. This causes a slight displacement between the surface charge and the electric charge of the diffuse layer. As a result, the moving particle creates a dipole moment. The sum of all of the dipoles generates an electric field which is called sedimentation potential. It can be measured with an open electrical circuit, which is also called sedimentation current.

Nanofluidic circuitry is a nanotechnology aiming for control of fluids in nanometer scale. Due to the effect of an electrical double layer within the fluid channel, the behavior of nanofluid is observed to be significantly different compared with its microfluidic counterparts. Its typical characteristic dimensions fall within the range of 1–100 nm. At least one dimension of the structure is in nanoscopic scale. Phenomena of fluids in nano-scale structure are discovered to be of different properties in electrochemistry and fluid dynamics.

<span class="mw-page-title-main">Induced-charge electrokinetics</span>

Induced-charge electrokinetics in physics is the electrically driven fluid flow and particle motion in a liquid electrolyte. Consider a metal particle in contact with an aqueous solution in a chamber/channel. If different voltages apply to the end of this chamber/channel, electric field will generate in this chamber/channel. This applied electric field passes through this metal particle and causes the free charges inside the particle migrate under the skin of particle. As a result of this migration, the negative charges move to the side which is close to the positive voltage while the positive charges move to the opposite side of the particle. These charges under the skin of the conducting particle attract the counter-ions of the aqueous solution; thus, the electric double layer (EDL) forms around the particle. The EDL sign on the surface of the conducting particle changes from positive to negative and the distribution of the charges varies along the particle geometry. Due to these variations, the EDL is non-uniform and has different signs. Thus, the induced zeta potential around the particle, and consequently slip velocity on the surface of the particle, vary as a function of the local electric field. Differences in magnitude and direction of slip velocity on the surface of the conducting particle effects the flow pattern around this particle and causes micro vortices. Yasaman Daghighi and Dongqing Li, for the first time, experimentally illustrated these induced vortices around a 1.2 mm diameter carbon-steel sphere under the 40V/cm direct current (DC) external electric filed. Chenhui Peng et al. also experimentally showed the patterns of electro-osmotic flow around an Au sphere when alternating current (AC) is involved . Electrokinetics here refers to a branch of science related to the motion and reaction of charged particles to the applied electric filed and its effects on its environment. It is sometimes referred as non-linear electrokinetic phenomena as well.

<span class="mw-page-title-main">Antonio Castellanos Mata</span> Spanish physicist

Antonio Castellanos Mata was a Spanish physicist.

A flowFET is a microfluidic component which allows the rate of flow of liquid in a microfluidic channel to be modulated by the electrical potential applied to it. In this way, it behaves as a microfluidic analogue to the field effect transistor, except that in the flowFET the flow of liquid takes the place of the flow of electric current. Indeed, the name of the flowFET is derived from the naming convention of electronic FETs.

References

  1. ↑ Castellanos, A. (1998). Electrohydrodynamics.
  2. ↑ Wall, Staffan. "The history of electrokinetic phenomena." Current Opinion in Colloid & Interface Science 15.3 (2010): 119-124.
  3. ↑ Thompson, Clive (August 2003). "The Antigravity Underground". Wired Magazine .
  4. ↑ Chang, H.C.; Yeo, L. (2009). Electrokinetically Driven Microfluidics and Nanofluidics. Cambridge University Press.
  5. 1 2 Kirby, B.J. (2010). Micro- and Nanoscale Fluid Mechanics: Transport in Microfluidic Devices. Cambridge University Press. ISBN   978-0-521-11903-0.
  6. 1 2 Bruus, H. (2007). Theoretical Microfluidics. Oxford University Press.
  7. ↑ Patterson, Michael; Kesner, Raymond (1981). Electrical Stimulation Research Techniques. Academic Press. ISBN   0-12-547440-7.
  8. ↑ Elul, R.J. (1967). Fixed charge in the cell membrane. PMID   6040152.
  9. ↑ Yang, Jun; Lu, Fuzhi; Kostiuk, Larry W.; Kwok, Daniel Y. (1 January 2003). "Electrokinetic microchannel battery by means of electrokinetic and microfluidic phenomena". Journal of Micromechanics and Microengineering. 13 (6): 963–970. Bibcode:2003JMiMi..13..963Y. doi:10.1088/0960-1317/13/6/320. S2CID   250922353.
  10. ↑ Levich, V.I. (1962). Physicochemical Hydrodynamics.
  11. ↑ Ferraro, P.; Coppola, S.; Grilli, S.; Paturzo, M.; Vespini, V. (2010). "Dispensing nano–pico droplets and liquid patterning by pyroelectrodynamic shooting". Nature Nanotechnology. 5 (6): 429–435. Bibcode:2010NatNa...5..429F. doi:10.1038/nnano.2010.82. PMID   20453855.
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