Charge transfer upon collision

Despite the fact that contact electrification and triboelectricity has been observed for more than 2000 years the underlying physics is still an active research area. However, it is not only of academic interest, it is of major importance in applications. On the one hand discharges following contact electrification may cause severe explosions of flammable liquids or dust. On the other hand, there are many useful purposes, like exhaust gas cleaning etc.

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Fig. 1: Principle of experiment.
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We have developed a novel experimental technique which enables us to study the charge transfer between a free-falling spherical particle and a surface with unprecedented precision. It relies on self-built charge amplifiers providing a resolution < 1 fC and < 1 µs. The principal setup is shown in Fig. 1. Spherical particles enter a parallel plate capacitor and at each plate the induced as well as the transferred charges are measured as a function of time. The setup is mounted in an ultrahigh vacuum chamber allowing vacuum or different gas atmospheres at variable pressures. In Fig. 2 a photograph of the actual experiment is shown. Currently, we build up a new experimental chamber in which in situ preparation of plate and spheres are possible.

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Fig. 2: Photograph of the experiment.
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Typical data are plotted in Fig. 3. They are recorded from the bottom Cu plate while a Au sphere is bouncing many times on it. The green line represents an almost perfect simulation of the experimental transient (black line). From the contact times the distance between the Au sphere and the bottom plate, the height, can be calculated. Moreover, we can extract easily the inelastic mechanical energy transfer. The electric charge on the sphere may be calculated from the curvature of the charge curve. The inset in Fig. 3 gives the results for the presented experiment showing that the initial charge changes sign upon the first contact and ranges around a few 100 fC.

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Fig. 3: Typical data for a multiple contact due to bouncing of a Au sphere on the Cu plate. The green line shows the simulation.
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Due to the ultra-sensitive detection and the ultra-high time resolution each part of the charge curve can be enlarged by orders of magnitude as shown in Fig. 4 for the first contact. The red line shows the data and the black line the simulation. The metallic contact is clearly visible as a horizontal line. The contact time of a few microseconds agrees well with Hertz theory. The bending of the curve before and after contact is due to the electric polarization of the sphere. It shifts the center of charge out of the middle of the sphere as schematically depicted in the figure.

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Fig. 4: Detail of the data for the first contact in Fig. 3.
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