Droplet jet fission

Fig. 11.13. Illustration of droplet jet fission. The average number of charges on a droplet, the radii of the droplets [pm], and the timescale of events are assigned. The inset shows a drawing of droplet jet fission based on an actual flash microphotograph. Reproduced from Ref. [49] by permission of the authors.

As a consequence, the droplet breaks up into a stream of smaller droplets, each one continuing to shrink by evaporation until the Rayleigh stability limit is reached again. The process of droplet fission is repeated several times and it is called uneven fission or droplet jet fission [5,6],... 

Fig. 32.18 Droplet evolution due to solvent evaporation and droplet jet fission (Reprinted with...

The process of droplet jet fission starts on a macroscopic scale and eventually leads to states that might be regarded as large clusters or multiply solvated ions. The final step of the creation of isolated gas phase ions from these multi-molecular entities is not addressed by this model, however. 

Once the droplets have undergone a few jet fissions, they are so small that they have not been directly observed through experiments as mentioned before. This has led to the creation of two main theories to explain what happens to these small, highly charged droplets. 

The first theory is the charged residue model (CRM) first proposed by Dole [31], who was one of the first people to study the gas phase ion production in an electrospray. In Dole s model, the droplets undergo jet fission until very small droplets, on the order of a few nanometers, are created that contain only single ions. Continuing solvent evaporation from these drops yields a single gas phase ion. 

The second theory is based on the ion evaporation model (IBM) proposed by Iribame and Thompson [32]. In IBM, after the radii of the charged droplets have reduced to the order of tens of nanometers, due to solvent evaporation and jet fission, direct ion emission to the gas phase from the droplets becomes possible. The theory states that IBM becomes dominant over jet fission for droplets of radii R < 10 nm [29]. Figure 32.19 illustrates the two different theories of gas phase ion evolution in an electrospray. 

The evaporation progressively forms smaller and smaller droplets with the eventual release of ions into the vapor phase. The desolvation causes the density of the electric field of the droplets to increase to the point where disintegration occurs. There are two models for the disintegration, Coulombic and jet fissions. When the repulsive Coulombic forces between the like charges on the surface of the droplets exceed the forces attributable to surface tension (the Rayleigh instability Umit), the... 

On-Chip Electrospray, Fig. 1 Taylor cone formation at the tip of a 100 pm capillary across which a 3 kV DC voltage is applied. A slcmdca jet issues from the tip of the TaylOT cone as a result of Rayleigh fission, subsequently breaking up to fium aerosol droplets... 

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