Electrospray
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 (usually a glass or metallic capillary).
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 the late 16th century William Gilbert[1] 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.
His prediction that a droplet reaching this limit would throw out fine jets of liquid was confirmed experimentally more than 100 years later.
[4] In 1914, John Zeleny published work on the behaviour of fluid droplets at the end of glass capillaries.
[5] This report presents experimental evidence for several electrospray operating regimes (dripping, burst, pulsating, and cone-jet).
[6] A few years later, Zeleny captured the first time-lapse images of the dynamic liquid meniscus.
[7] Between 1964 and 1969 Sir Geoffrey Ingram Taylor produced the theoretical underpinning of electrospraying.
He further worked with J. R. Melcher to develop the "leaky dielectric model" for conducting fluids.
[11] The number of publications about electrospray started rising significantly around 1990 (as shown in the figure on the right) when John Fenn (2002 Nobel Prize in Chemistry) and others discovered electrospray ionization for mass spectrometry.
To simplify the discussion, the following paragraphs will address the case of a positive electrospray with the high voltage applied to a metallic emitter.
Under the effect of surface tension, the liquid meniscus assumes a semi-spherical shape at the tip of the emitter.
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.
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.
[16][17] Ions are produced by field evaporation at tip of the Taylor cone.
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 .
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.
Electrochemical reduction of ions to atoms and in situ assembly was believed to be the mechanism of nano structure formation.
Electrospray has garnered attention in the field of drug delivery, and it has been used to fabricate drug carriers including polymer microparticles used in immunotherapy[19] as well as lipoplexes used for nucleic acid delivery.
Particulate suspended in air can be charged by aerosol 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.
