Electrohydrodynamics
Encyclopedia
Electrohydrodynamics, also known as electro-fluid-dynamics (EFD) or electrokinetics, is the study of the dynamics
of electrically charged fluids. It is the study of the motions of ion
ised particles or molecules and their interactions with electric field
s and the surrounding fluid. The term may be considered to be synonymous with the rather elaborate electrostrictive hydrodynamics. EHD 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.
In the first instance, shaped electrostatic fields create hydrostatic pressure (or motion) in dielectric media. When such media are fluid
s, a 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
.
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 Reuss in 1809 and has been studied extensively since the 19th century. Such study is known as electrohydrodynamics or electrokinetics, and was documented by Thomas Townsend Brown
in 1921. It was later refined in scientific terms during the 1930s in conjunction with Dr. Paul Alfred Biefeld
. The flow rate in such a mechanism is linear in the electric field
. Electrokinesis is of considerable practical importance in microfluidics
, 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:
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. It is also discussed in R.J.Elul's "Fixed charge in the cell membrane" (1967).
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 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.
This phenomenon is called streaming potential and was discovered in 1859.
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 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.
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
of electrically charged fluids. It is the study of the motions of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ised particles or molecules and their interactions with electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s and the surrounding fluid. The term may be considered to be synonymous with the rather elaborate electrostrictive hydrodynamics. EHD covers the following types of particle and fluid transport mechanisms:Electrophoresis
Electrophoresis
Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
, electrokinesis, dielectrophoresis
Dielectrophoresis
Dielectrophoresis 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...
, electro-osmosis
Electro-osmosis
Electroosmotic flow is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit...
, and electrorotation
Electrorotation
Electrorotation is the circular movement of an electrically polarized particle. Similar to the slip of an electric motor, it can arise from a phase lag between an applied rotating electric field and the respective relaxation processes and may thus be used to investigate the processes or, if these...
. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
, and vice versa.
In the first instance, shaped electrostatic fields create hydrostatic pressure (or motion) in dielectric media. When such media are fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
s, a flow
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
is produced. If the dielectric is a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
or a solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
, no flow is produced. Such flow can be directed against the electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s, generally to move the electrodes. In such case, the moving structure acts as an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. Practical fields of interest of EHD are the common air ioniser
Air ioniser
An air ioniser is a device that uses high voltage to ionise air molecules. Negative ions, or anions, are particles with one or more extra electrons, conferring a net negative charge to the particle. Cations are positive ions missing one or more electrons, resulting in a net positive charge...
, Electrohydrodynamic thruster
Electrohydrodynamic thruster
An electrohydrodynamic thruster is a high voltage device that propels air or fluids to achieve relative motion. EHD thrusters, unlike ion thrusters, do not carry their own propellant and thus cannot operate in space or vacuum....
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
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
.
Electrokinesis
Electrokinesis is the particle or fluidFluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
transport produced by an electric field acting on a fluid having a net mobile charge. (See -kinesis
-kinesis
-kinesis, from Greek "κίνησις", movement, motion, is a suffix that denotes movement. The word Kinesis itself similarly indicates movement, particularly in biology.-Cytokinesis:...
for explanation and further uses of the kinesis suffix.) Electrokinesis was first observed by Reuss in 1809 and has been studied extensively since the 19th century. Such study is known as electrohydrodynamics or electrokinetics, and was documented by Thomas Townsend Brown
Thomas Townsend Brown
Thomas Townsend Brown was an American physicist.-Early and middle years:Brown was born in Zanesville, Ohio; his parents were Lewis K. and Mary Townsend Brown. In 1921, Brown discovered what was later called the Biefeld-Brown effect while experimenting with a Coolidge X-ray tube. This is a vacuum...
in 1921. It was later refined in scientific terms during the 1930s in conjunction with Dr. Paul Alfred Biefeld
Paul Alfred Biefeld
Dr. Paul Alfred Biefeld was born in Jöhstadt, Saxony, Germany. He was the son of Heinrich and Wilhelmina Biefeld, he moved to the United States in 1881. Biefeld received his B.S. in Electrical Engineering at the University of Wisconsin in 1894. He received his Ph.D...
. The flow rate in such a mechanism is linear in the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. Electrokinesis is of considerable practical importance in microfluidics
Microfluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale.Typically, micro means one of the following features:* small volumes...
, 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:
 |
|
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. 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 AlbertaUniversity of Alberta
The University of Alberta is a public research university located in Edmonton, Alberta, Canada. Founded in 1908 by Alexander Cameron Rutherford, the first premier of Alberta and Henry Marshall Tory, its first president, it is widely recognized as one of the best universities in Canada...
discussed a method of hydrodynamic to electrical energy conversion
Energy conversion
Transforming energy is when the energy changes into another form.In physics, the term energy describes the capacity to produce changes within a system, without regard to limitations in transformation imposed by entropy...
by exploiting the natural electrokinetic properties of a liquid such as ordinary tap water
Tap water
Tap water is a principal component of "indoor plumbing", which became available in urban areas of the developed world during the last quarter of the 19th century, and common during the mid-20th century...
, 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
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
. 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.
This phenomenon is called streaming potential and was discovered in 1859.
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 fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
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
Isoelectric focusing
Isoelectric focusing , also known as electrofocusing, is a technique for separating different molecules by their electric charge differences...
), 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.
Electrokinetic instabilities can be leveraged for rapid mixing
Mixing (physics)
In physics, a dynamical system is said to be mixing if the phase space of the system becomes strongly intertwined, according to at least one of several mathematical definitions. For example, a measure-preserving transformation T is said to be strong mixing ifwhenever A and B are any measurable...
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
Electroviscous effects
Electroviscous effects, in chemistry of colloids and surface chemistry, according to an IUPAC definition, are the effects of the particle surface charge on viscosity of a fluid....
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.
See also
- Magnetohydrodynamic driveMagnetohydrodynamic driveA magnetohydrodynamic drive or MHD propulsor is a method for propelling seagoing vessels using only electric and magnetic fields with no moving parts, using magnetohydrodynamics. The working principle involves electrification of the propellant which can then be directed by a magnetic field,...
- MagnetohydrodynamicsMagnetohydrodynamicsMagnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
- ElectrosprayElectrosprayThe name electrospray is used for a device that employs electricity to disperse a liquid or for the fine aerosol resulted in this process. The method is sometimes improperly called electrohydrodynamic atomization. High voltage is applied to a liquid supplied through an emitter...
- Electrokinetic phenomenaElectrokinetic phenomenaElectrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...
- OptoelectrofluidicsOptoelectrofluidicsOptoelectrofluidics, also known as optically induced electrohydrodynamics, refers to the study of the motions of particles or molecules and their interactions with optically-induced electric field and the surrounding fluid....
External links
- Blaze Labs Research - Introduction to EHD thrusters
- 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.
- Electrokinetic Instability Work in the Posner Research Lab
- Electrokinetic Instability work by Juan G. Santiago and coworkers