Cone-jet mode

The cone-jet mode described is only one of the possible ES modes. For a qualitative description of other modes and the cone-jet mode see Cloupeau.41 The cone-jet mode is most often used in ESMS. It is also the best charact ized mode... 

Theoretical39 and semiempirical44 equations have been derived which predict approximately, the current /, the radius R, and charge q of average initial droplets produced in the cone-jet mode by ES. Very useful are equations 15-17 which relate the above to important experimental variables,44... 

Gomez, A. The electrospray and its application to targeted drug inhalation. Respir. Care 2002, 47 (12), 1419-1433. Cloupeau, M. Prunet-Foch, B. Electrostatic spraying of liquids in cone-jet mode. J. Electrostal. 1989, 22, 135-159. 

Cloupeau, M. Prunet-Foch, B. Electrostatic spraying of liquids in cone-jet mode. J. Electronics 1989, 22, 135-159. 

Other less-common deposition methods include electrospraying in a stable cone-jet mode to generate highly reproducible spots of as little as 50-pL volumes [33] and a laser transfer technique that allows the accurate deposition of picoliter volumes of proteins onto the solid surface [34]. 

Figures 22.4c and d). ° Generally, liquids with high conductivity produce fine particles in cone-jet mode at low flow rates liquids with low conductivity have the opposite effect. Investigations have shown that a minimum solution conductivity of 0.01 pSm i is desired for EDHA processing. ... 

Figure 22.5 shows the acting force in formation of the conical jet. In Taylor cone-jet mode, the force on the surface of the cone is assumed to be at equilibrium at all points except near the apex, where the jet of charged fluid accelerates under the tangential forces. In this mode, three... 

FIGURE 22.5 Forces acting on the Taylor cone in EHDA process. (Reprinted from J. Aerosol ScL, 30(7), Hartman, R.P.A., Bruimer, D.J., Camelot, D.M.A., Marijnissen, J.C.M., and Scarlett, B., Electrohydrodynamic atomization in the cone-jet mode physical modeling of the liquid cone and jet, 823-849. Copyright 1999, with permission from Elsevier.)... 

Xie, J. and C.-H. Wang. Encapsulation of proteins in biodegradable polymeric microparticles using electrospray in the Taylor cone-jet mode. Biotechnology and Bioengineering 97(5) (2007) 1278-1290. 

Tang, K. and A. Gomez. Monodisperse electrosprays of low electric conductivity liquids in the cone-jet mode. Journal of Colloid and Interface Science 184(2) (1996) 500-511. 

BEM can be extended to compute interior fluxes by taking the derivative of the velocity potential from the governing equation (18.4). Yoon et al. [3] computed the interior fluxes of the electrostatic field outside an electrified jet whose symmetric sector configuration is shown in Fig. 18.14. Assuming symmetry, they computed a sector of the multi-jets, which can produce extremely small (a few or sub-micron) droplets. The multi-jet mode enables multiple cone-jet operation (about 5 10 jets), which in turn increases the flow rate without sacrificing basic features of the eletrospray s cone-jet mode [82]. 

Fig. 18.14 Multiple cone-jet mode, its symmetric sector, and the interior electric field computation using the BEM [3] (Courtesy of Elsevier)...

These relationships are based on theoretical reasoning and experiments. In 32.4 /(e/eo) is a numerical function that has been tabulated and for liquids whose dielectric constant is e/eq > 40, /(e/eq) 80 [10]. These relations are valid when the electrospray is operated in the cone-jet mode [10] which is a particular mode of electrospray operation which will be discussed in a following section. Figure 32.3 shows experimental results that correlate well with the above relations for electrospray mean droplet size. 

Fig. 32.8 Pictures showing the pulsating cone jet mode of electrospray operation. Experiment conditions 2,500 V applied at emitter orifice, 2.4 pL/min flow rate of 75% methanol aqueous solution, 10 mm ground distance from emitter...

This mode of electrospray operation is one of the most interesting and studied cases. Cone-jet mode is when the meniscus at the liquid/air interface permanently takes the form of a Taylor cone and emits a steady jet of liquid from its apex. This jet of liquid travels a certain distance below the cone apex before breaking up into droplets. Depending on the fluid properties, the shape of the Taylor cone varies quite dynamically, as shown in Fig. 32.9. 

Fig. 32.9 Different fonns of the meniscus in the cone-jet mode (Reprinted with permission [17])...

Once the cone-jet electrospray mode has been established, and the electric field is further increased in strength, several more jets may begin to be emitted from the capillary tip simultaneously. In this case the Taylor cone shrinks and splits up between the various emitted jets. This is referred to as a multi jet or multi-cone jet mode. 

Droplet Size and Distribution Characteristics in Cone-Jet Mode... 

The operation of the electrospray in the cone-jet mode is achieved only when the operational and fluid parameters are within certain ranges as already shown in... 

The main factors affecting the stability island of an electrospray system is the fluid properties themselves in particular the surface tension and conductivity. The fluid surface tension directly affects the ability of a fluid to atomize an electrospray because it opposes the force applied by the ions at the fluid/air interface. Therefore, increasing the surface tension will increase the required field strength to establish a cone-jet mode. The conductivity of the fluid used has the effect of shifting the electrospray stability island to a narrower range and also to lower flow rates. This is shown in Fig. 32.12. 

In Fig. 32.12 there are three sets of top and bottom curves. The top curves show the maximum voltage (field strength) at which a stable cone-jet mode can... 

The effect of increasing the conductivity of the electrospray fluid decreases the flow rate and also narrows the electric field strength range at which a stable cone-jet mode can be established. Another effect of increasing cmiductivity is that the filament jet radius emitted from the cone tip decreases and this in turn decreases the droplet size produced [11]. 

The effect of increasing the electrospray fluid surface tension tends to increase the electric field strength required to create a stable cone-jet mode. If a fluid, such as distilled water ( 0.074 N m at 20°C), has a sufficiently large surface tension, the ionization potential of the air around the electrospray is reached before any kind of electrospray is created. Therefore a stable cone-jet mode is difficult and almost impossible to create without the use of some sheathing gas around the electrospray system. 

Sheath gas flow around the emitter orifice can be utilized when a stable electrospray in the cone-jet mode is difficult to achieve. One reason why a stable cone-jet mode electrospray can be difficult to obtain in a mass spectrometer application is an increased flow rate. An increase in flow rate causes the electrospray to operate outside the island of stability mentioned previously (Fig. 32.12). The effect that the sheath gas has on the electrospray is increased breakup of the spray droplets through the transfer of kinetic energy. 

For perfectly conducting liquids, Taylor [7] showed that a conical meniscus with a half angle of 49.3° is produced by considering the static equilibrium balance between the capillary and Maxwell stresses in Eqs. 3, 6, and 17. In the perfect conducting limit, the drop is held at constant potential, and hence, the gas phase electric field at the meniscus interface is predominantly in the normal direction. It can then be shown that the normal gas phase electric field scales as in which R is the meniscus radius which then stipulates from Eq. 7 that the Maxwell pressure Pm n,g scales as HR, therefore exactly balancing the azimuthal capillary pressure pc ylR for all values of R. This exact balance, and absence of a length scale selection, is responsible for the formation of a static Taylor cone (Fig. 1) in the dominant cone-jet mode in DC electrosprays [8]. 

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