The Determination of the Location of Contact Electrification-Induced Discharge Events
Contact electrification - the transfer of charges from one object to another when brought into contact and separated - is ubiquitous and has been known for a very long time. Yet, there are still fundamental questions that are not fully understood, such as the role of friction in the transfer of charges. Contact electrification is associated with friction, and yet it is not known if friction is the mechanism for contact electrification or merely incidental to the pressures required to bring the two surfaces together. The authors reported a system of a steel sphere rolling in a circular path on a disc made of organic insulator. A rotating bar magnet drives the rolling of the steel sphere. As the steel sphere rolls around the organic insulator, it will pick up positive charges (ions) from the organic insulator and when the electric field due to the excess charges exceed the dielectric field breakdown, there will be a discharge of positive ions back to the insulator. By placing electrodes beneath the organic insulator, the authors were able to track and pinpoint the location of these discharge events. The authors were then able to study more closely the conditions for discharge events. The schematics of the set-up in shown below.
The electrodes are able to measure the amount of charge on the steel ball by measuring the amount of charge induced on the electrode which can then be measured using an electrometer. To be more accurate, the amount of charge measured by the electrode, Q, is a sum of the amount of charges on the sphere, Qs, and the amount of charges on the insulator near the electrode, Qdne, i.e. Q = Qs + Qdne. The signal measured each time the steel passes through the electrode is a sharp peak, similar to figure 2(c). The peak corresponds to Qs + Qdne, while the base correspond to Qdne. When the steel ball is sufficiently far away, the induced charge due to the ball is insignificant. Figure 2(a) tracks the position of peak (grey line) and base (purple line) as the steel ball passed through electrode. When the steel ball accumulates charges, the peaks and baseline will increase and decrease monotically with time respectively. Discontinuities in either the peak or the baseline correspond to discharge events, marked with coloured arrows on figure 2(a). In this set-up, the disc has been divided into four parts: A, O, B and F (Figure 2 d). Interestingly, we can attribute in which part (A, O, B or F) the discharge occurred just by looking at the general shape of the discontinuity. For example, if there is only discontinuity in the peak, but not the baseline, the discharge event must happen in part F (black arrows) and if there is only discontinuity in the baseline and not the peak, the discharge event must happen in part O. The maximum amount of charge that the metal sphere can hold can also be measured once the measurement is corrected for Qdne (figure 2b).
The conditions for discharge events can then be studied in greater details. For example, it was found that PDMS that had been plasma treated promotes discharge of ions. Figure 3a) shows how only certain part of the disc is subjected to plasma treatment and figure 3b) summarizes the frequency of discharge events in different parts of the disc. For all cases, it was found that discharge events mostly occured in the plasma-treated surface (boxed purple).
Using more electrodes, the authors were able to pinpoint the location of the discharge events even more accurately, as illustrated in figure 4. Once again, they showed that the majority of discharge events occured in plasma-treated surface.