Electrospray
Encyclopedia
The 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 (usually a glass or metallic capillary). 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.
In 1882, Lord Rayleigh
theoretically estimated the maximum amount of charge a liquid droplet could carry; this is now known as the "Rayleigh limit". His prediction that a droplet reaching this limit would throw out fine jets of liquid was confirmed experimentally more than 100 years later.
In 1914, John Zeleny
published work on the behaviour of fluid droplets at the end of glass capillaries. This report presents experimental evidence for several electrospray operating regimes (dripping, burst, pulsating, and cone-jet). A few years later, Zeleny captured the first time-lapse images of the dynamic liquid meniscus.
Between 1964 and 1969 Sir Geoffrey Ingram Taylor
produced the theoretical underpinning of electrospraying. Taylor modeled the shape of the cone formed by the fluid droplet under the effect of an electric field; this characteristic droplet shape is now known as the Taylor cone
. He further worked with J. R. Melcher to develop the "leaky dielectric model" for conducting fluids.
from a grounded counter-electrode. The liquid being sprayed is characterized by its viscosity 
, surface tension 
, conductivity 
, and relative permittivity 
.
will induce the electric field:
where
is the liquid radius of curvature. This field leads to liquid polarization: the negative/positive charge carriers migrate toward/away from the electrode where the voltage is applied. At voltages below a certain threshold, the liquid quickly reaches a new equilibrium geometry with a smaller radius of curvature.
described the theoretical shape of this cone based on the assumptions that (1) the surface of the cone is an equipotential surface and (2) the cone exists in a steady state equilibrium. To meet both of these criteria the electric field must have azimuthal symmetry and have
dependence to balance the surface tension and produce the cone. The solution to this problem is:
where
(equipotential surface) exists at a value of 
(regardless of R) producing an equipotential cone. The magic angle necessary for 
for all R is a zero of the Legendre polynomial of order 1/2, 
. There is only one zero between 0 and 
at 130.7099°, which is the complement of the Taylor's now famous 49.3° angle.
becomes larger than the charge relaxation time
. The undefined symbols stand for characteristic length 
and vacuum permittivity 
. Due to intrinsic varicose instability, the charged liquid jet ejected through the cone apex breaks into small charged droplets, which are radially dispersed by the space-charge.
Electrospray became widely used as ionization source for mass spectrometry after the Fenn group successfully demonstrated its use as ion source for the analysis of large biomolecules.
Similarly to the standard electrospray, the application of high voltage to a polymer solution can result in the formation of a cone-jet geometry. If the jet turns into very fine fibers instead of breaking into small droplets, the process is known as electrospinning .
Electrospray techniques are used to control satellite
s, since the fine-controllable particle ejection allows precise and effective thrusts.
, for example to deposit single particles on surfaces. This is done by spraying colloids on average containing only one particle per droplet. The solvent evaporates, leaving an aerosol
stream of single particles of the desired type. The ionizing property of the process is not crucial for the application but may be used in electrostatic precipitation
of the particles.
Particulates suspended in air can be charged by the aerosol generated by an electrospray, manipulated by an electric field and collected on a grounded electrode. This approach minimizes the production of ozone
which is common to other types of air purifiers.
Taylor cone
A Taylor cone refers to the cone observed in electrospinning, electrospraying and hydrodynamic spray processes from which a jet of charged particles emanates above a threshold voltage...
, 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.
History
In the late 16th century William Gilbert set out to describe the behaviour of magnetic and electrostatic phenomena. He observed that, in the presence of a charged piece of amber, a drop of water deformed into a cone. This effect is clearly related to electrosprays, even though Gilbert did not record any observation related to liquid dispersion under the effect of the electric field.In 1882, Lord Rayleigh
John Strutt, 3rd Baron Rayleigh
John William Strutt, 3rd Baron Rayleigh, OM was an English physicist who, with William Ramsay, discovered the element argon, an achievement for which he earned the Nobel Prize for Physics in 1904...
theoretically estimated the maximum amount of charge a liquid droplet could carry; this is now known as the "Rayleigh limit". His prediction that a droplet reaching this limit would throw out fine jets of liquid was confirmed experimentally more than 100 years later.
In 1914, John Zeleny
John Zeleny
John Zeleny was a Czech-American physicist at the University of Minnesota, who in 1911 invented the Zeleny electroscope. He also studied the effect of an electric field on a liquid meniscus....
published work on the behaviour of fluid droplets at the end of glass capillaries. This report presents experimental evidence for several electrospray operating regimes (dripping, burst, pulsating, and cone-jet). A few years later, Zeleny captured the first time-lapse images of the dynamic liquid meniscus.
Between 1964 and 1969 Sir Geoffrey Ingram Taylor
Geoffrey Ingram Taylor
Sir Geoffrey Ingram Taylor OM was a British physicist, mathematician and expert on fluid dynamics and wave theory. His biographer and one-time student, George Batchelor, described him as "one of the most notable scientists of this century".-Biography:Taylor was born in St. John's Wood, London...
produced the theoretical underpinning of electrospraying. Taylor modeled the shape of the cone formed by the fluid droplet under the effect of an electric field; this characteristic droplet shape is now known as the Taylor cone
Taylor cone
A Taylor cone refers to the cone observed in electrospinning, electrospraying and hydrodynamic spray processes from which a jet of charged particles emanates above a threshold voltage...
. He further worked with J. R. Melcher to develop the "leaky dielectric model" for conducting fluids.
Mechanism
To simplify the discussion, the following paragraphs will address the case of a positive electrospray with the high voltage applied to a metallic emitter. A classical electrospray setup is considered, with the emitter situated at a distance from a grounded counter-electrode. The liquid being sprayed is characterized by its viscosity , surface tension , conductivity , and relative permittivity .Effect of small electric fields on liquid menisci
Under the effect of surface tension, the liquid meniscus assumes a semi-spherical shape at the tip of the emitter. Application of the positive voltage will induce the electric field:where
is the liquid radius of curvature. This field leads to liquid polarization: the negative/positive charge carriers migrate toward/away from the electrode where the voltage is applied. At voltages below a certain threshold, the liquid quickly reaches a new equilibrium geometry with a smaller radius of curvature.The Taylor cone
Voltages above the threshold draw the liquid into a cone. Sir Geoffrey Ingram TaylorGeoffrey Ingram Taylor
Sir Geoffrey Ingram Taylor OM was a British physicist, mathematician and expert on fluid dynamics and wave theory. His biographer and one-time student, George Batchelor, described him as "one of the most notable scientists of this century".-Biography:Taylor was born in St. John's Wood, London...
described the theoretical shape of this cone based on the assumptions that (1) the surface of the cone is an equipotential surface and (2) the cone exists in a steady state equilibrium. To meet both of these criteria the electric field must have azimuthal symmetry and have
dependence to balance the surface tension and produce the cone. The solution to this problem is:where
(equipotential surface) exists at a value of (regardless of R) producing an equipotential cone. The magic angle necessary for for all R is a zero of the Legendre polynomial of order 1/2, . There is only one zero between 0 and at 130.7099°, which is the complement of the Taylor's now famous 49.3° angle.Singularity development
The apex of the conical meniscus cannot become infinitelly small. A singularity develops when the hydrodynamic relaxation timeRelaxation time
In the physical sciences, relaxation usually means the return of a perturbed system into equilibrium.Each relaxation process can be characterized by a relaxation time τ...
becomes larger than the charge relaxation timeRelaxation time
In the physical sciences, relaxation usually means the return of a perturbed system into equilibrium.Each relaxation process can be characterized by a relaxation time τ...
. The undefined symbols stand for characteristic length and vacuum permittivity . Due to intrinsic varicose instability, the charged liquid jet ejected through the cone apex breaks into small charged droplets, which are radially dispersed by the space-charge.Closing the electrical circuit
The charged liquid is ejected through the cone apex and captured on the counter electrode as charged droplets or positive ions. To balance the charge loss, the excess negative charge is neutralized electrochemically at the emitter. Imbalances between the amount of charge generated electrochemically and the amount of charge lost at the cone apex can lead to several electrospray operating regimes. For cone-jet electrosprays, the potential at the metal/liquid interface self-regulates to generate the same amount of charge as that lost through the cone apex.Electrospray ionization
- see also the main article on Electrospray ionizationElectrospray ionizationElectrospray ionization is a technique used in mass spectrometry to produce ions. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized...
Electrospray became widely used as ionization source for mass spectrometry after the Fenn group successfully demonstrated its use as ion source for the analysis of large biomolecules.
Electrospinning
- see also the main article on ElectrospinningElectrospinningElectrospinning uses an electrical charge to draw very fine fibres from a liquid. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers. The process does not require the use of coagulation chemistry or high temperatures to produce solid...
Similarly to the standard electrospray, the application of high voltage to a polymer solution can result in the formation of a cone-jet geometry. If the jet turns into very fine fibers instead of breaking into small droplets, the process is known as electrospinning .
Colloid thrusters
- see also the main article on Colloid thrusterColloid thruster]A colloid thruster is a type of thruster which uses electrostatic acceleration of charged liquid droplets for propulsion. It is closely related to electrospray ionization and other hydrodynamic spraying processes. In a colloid thruster charged liquid droplets are produced by an electrospray...
s
Electrospray techniques are used to control satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s, since the fine-controllable particle ejection allows precise and effective thrusts.
Deposition of particles for nanostructures
Electrospray may be used in nanotechnologyNanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
, for example to deposit single particles on surfaces. This is done by spraying colloids on average containing only one particle per droplet. The solvent evaporates, leaving an aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
stream of single particles of the desired type. The ionizing property of the process is not crucial for the application but may be used in electrostatic precipitation
Electrostatic precipitator
An electrostatic precipitator , or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas using the force of an induced electrostatic charge...
of the particles.
Air purifiers
- see also the main article on Air purifierAir purifierAn air purifier is a device which removes contaminants from the air. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke...
s
Particulates suspended in air can be charged by the aerosol generated by an electrospray, manipulated by an electric field and collected on a grounded electrode. This approach minimizes the production of ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
which is common to other types of air purifiers.