Figure 1

Schematic representation of polymer-metal contact/separation electrification and the corresponding oscilloscope signals (electrical potential) for the individual events of contact and separation. Electrical potential signals are bipolar (+ and −) at contact and unipolar at separation. In the experimental setup, contacting surfaces are; a metal base electrode (BE) covered with a thick (100 μm) polymer film and a metal electrode (ME, Al, Cu, or stainless steel). The charges created during contact and separation measured as electrical potential on the metal electrodes were collected using 100 mega ohm oscilloscope probes connected to a two-channel oscilloscope (OWON SDS7072, 70 MHz, 2 + 1 Channel, 1 GS/s). The channels of the oscilloscope can measure electrical signals independently. The distance between the electrodes was adjusted by an x-y stage and separation force was provided by a spring. A microprocessor (Arduino nano) was used to set the frequency of tapping and the force was applied by a solenoid actuator. See SI for further details of the setup and Movie S1 for the simultaneous visual observation of the contact and separation, and their corresponding electrical signals.