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Self-charging of sprays

Our research on the charging mechanism of sprays have been published in the journal Scientific Reports, a link to the article can be found here. We investigated all the parameters that can affect the charging efficiency, such as the nozzle hole diameter, the properties of the nozzle wall and the liquid flow rate. By adapting the surface of the nozzle wall, we could increase the charging or change the polarity from positive to negative. This control on the charging properties is of paramount importance for the use of the sprays as an ionization source for mass spectrometry.
We discovered that the amount of charging is set by the liquid flow near the nozzle wall (Figure 1). At the nozzle the liquid velocity goes from zero to the maximum velocity within a small layer, this layer is called the boundary layer, and it is responsible for the transport of a net charge. With this knowledge we could develop a theory that is able to predict the amount of charging with great accuracy.

Figure 1 The boundary layer theory, the velocity gradient in the boundary layer transports a net charge. Image source: Scientific Reports

For a very large range of liquid flow rates, and different nozzle configurations the charging efficiency can be determined (Figure 2). Knowing the charging mechanism allows us to optimize the ionization process and shows us how different solvent mixtures will behave.
Figure 2 The electric current, that is the amount of charging can be directly predicted. Image source: Scientific Reports