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). 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.

History

In the late 16th century

In 1750 the French clergyman and physicist Jean-Antoine (Abbé) Nollet noted water flowing from a vessel would aerosolize if the vessel was electrified and placed near electrical ground.^[2]

In 1882,

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). A few years later, Zeleny captured the first time-lapse images of the dynamic liquid meniscus.^[6]

Between 1964 and 1969

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 ${\displaystyle d\,}$ from a grounded counter-electrode. The liquid being sprayed is characterized by its viscosity ${\displaystyle (\mu )\,}$, surface tension ${\displaystyle (\gamma )\,}$, conductivity ${\displaystyle (\kappa )\,}$, and relative permittivity ${\displaystyle (\epsilon _{r})\,}$.

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 ${\displaystyle V\,}$ will induce the electric field:^[11]

${\displaystyle E={2V \over r\ln(4d/r)}}$

where ${\displaystyle r\,}$ 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

symmetry and have ${\displaystyle R^{1/2}\,}$ dependence to balance the surface tension and produce the cone. The solution to this problem is:

${\displaystyle V=V_{0}+AR^{1/2}P_{1/2}(\cos \theta _{0})\,}$

where ${\displaystyle V=V_{0}\,}$ (equipotential surface) exists at a value of ${\displaystyle \theta _{0}}$ (regardless of R) producing an equipotential cone. The magic angle necessary for ${\displaystyle V=V_{0}\,}$ for all R is a zero of the

Legendre polynomial

of order 1/2, ${\displaystyle P_{1/2}(\cos \theta _{0})\,}$. There is only one zero between 0 and ${\displaystyle \pi \,}$ 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 infinitely small. A singularity develops when the hydrodynamic

relaxation time

${\displaystyle \tau _{H}={\mu r \over \gamma }}$ becomes larger than the charge

relaxation time

${\displaystyle \tau _{C}={\epsilon _{r}\epsilon _{0} \over \kappa }}$.

${\displaystyle (r)\,}$

${\displaystyle (\epsilon _{0})\,}$

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.^[13]

Applications

Electrospray ionization

Electrospray became widely used as ionization source for mass spectrometry after the

A liquid metal ion source (LMIS) uses electrospray in conjunction with liquid metal to form ions.^[15][16] Ions are produced by field evaporation at tip of the Taylor cone. Ions from a LMIS are used in ion implantation and in focused ion beam instruments.

Electrospinning

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

Electrospray techniques are used as low thrust

of the particles.

Deposition of ions as precursors for nanoparticles and nanostructures

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Instead of depositing nanoparticles, nanoparticles and nano structures can also fabricated in situ by depositing metal ions to desired locations. Electrochemical reduction of ions to atoms and in situ assembly was believed to be the mechanism of nano structure formation.

Fabrication of drug carriers

Electrospray has garnered attention in the field of drug delivery, and it has been used to fabricate drug carriers including polymer microparticles used in

Electrospray is used in some air purifiers. 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.

See also

• Flow focusing

References