The vertical contact-separation mode was the first invented operation
mode for TENGs.27–29 A physical contact between the two
dielectric films with distinct electron affinity (at least one is
insulative) creates oppositely charged surfaces. Once the two
surfaces are separated by a gap, a potential drop is created between
the electrodes deposited on the top and bottom surfaces of two
dielectric films, as demonstrated in Fig. 2a. If the two electrodes are
electrically connected by a load, free electrons in one electrode
would flow to the other electrode in order to balance the electrostatic
field. Once the gap is closed, the potential drop created by
triboelectric charges disappears, the induced electrons will flow
back. A periodic contact and separation between the two materials
drives the induced electrons to flow back and forth between the two
electrodes, resulting in an AC output in the external circuit.