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. 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 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 1750 the French clergyman and physicist Jean-Antoine Nollet noted water flowing from a vessel would aerosolize if the vessel was electrified and placed near electrical ground.
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. 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.
The number of publications about electrospray started rising significantly around 1990 when John Fenn and others discovered electrospray ionization for mass spectrometry.
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 Taylor described the theoretical shape of this cone based on the assumptions that the surface of the cone is equipotential and 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 exists at a value of producing an equipotential cone. The angle necessary for for all R values is a zero of the Legendre polynomial of order 1/2,. There is only one zero between 0 and at 130.7°, which is the complement of the Taylor's now famous 49.3° angle.