Particle charge

The particle charging describes physical mechanisms for electrical charging of gas-borne particles ( aerosol ).

Charging mechanisms

Static charge

Static charging describes the natural charging of particles in separation processes in which liquids or solids are broken up. The charging takes place here without the presence of additional charge carriers and is of secondary importance for aerosol technology due to the difficult to estimate charge state of the particles. Static charging can also be broken down into three areas: electrolytic charging (liquids), atomizing charging (liquids), and contact charging (non-metallic solids).

Diffusion charging

The diffusion charging describes the electrical charging of particles due to Brownian motion and the resulting collisions between particles and charge carriers (ions and electrons). Depending on whether unipolar or bipolar charge carriers are present, one also speaks of unipolar or bipolar diffusion charging.

Field charging

The field charging describes the electrical charge of particles in an electric field in the presence of unipolar charge carriers (ions or electrons).

Charge limit

If one assumes that the charges are distributed homogeneously on the surface, a particle can only take up a certain total charge without the charges being released again by the resulting Coulomb forces.

The smallest charge state of a particle is zero, i.e. i.e. it is electrically neutral. In the case of solid particles, the maximum charge state (charge limit) is determined by the electric field that is generated by the surface charges and is usually specified as a multiple of the elementary charge.

${\displaystyle n_{\text{L}}={\frac {d_{\text{P}}^{2}\cdot E_{\text{L}}\cdot \pi \cdot \epsilon _{0}}{e}}}$

Here, E _{L is} the surface boundary field strength from which a spontaneous emission of the charge carriers begins (similar to the corona discharge ). The level of this field strength depends on whether the particle is negatively or positively charged and can be estimated at 9.1 × 10 ⁸  V / m for negatively charged particles and 2.1 × 10 ¹⁰  V / m for positively charged particles. In the case of negative charging, the emission of electrons begins earlier with a comparatively lower field strength than the ion emission in the case of positive charging.

A distinction still has to be made as to whether the particles are solid or liquid. The surface of drops can be stretched and deformed by surface charges, depending on the surface tension of the liquid. The maximum charge is here by the Rayleigh limit determined

${\displaystyle n_{\text{L}}={\frac {\sqrt {8\cdot \pi ^{2}\cdot \epsilon _{0}\cdot \gamma \cdot d_{\text{P}}^{3}}}{e}}}$,

where γ is the surface tension of the liquid. If this limit is exceeded, the drop disintegrates into smaller fractions due to the high Coulomb forces, in which the limit is again undershot due to the increased total surface area.

In the presence of an external electric field, the field strengths are superimposed so that the conditions for the spontaneous emission of charge carriers change. In this case, a higher charge of the particles is possible ( field charge ), but here there is no bipolar Boltzmann charge distribution, but a unipolar charge distribution, which u. a. depends on the polarity of the applied voltage.

literature

• William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 .

Individual evidence

1. ^ William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 .