I am studying about triboelectricity, and I have a question why some open circuit voltages some out as single-peak and some output voltages comes out as double-peak.

1. The first one has only one peak which shows both the voltage for contacting and separating in one peak. For example, the positive slope represents pressing, and the negative slope in the peak represents releasing.

2. The second one has two peaks that have opposite signs to each other. For example, when pressing, the voltage shows distinct positive peak. Then the voltage comes to equilibrium. And then when released, the voltage has a negative peak. After that, the voltage comes to equilibrium.

I want to know what is the difference between those two.

Thank you.