2.2. Fabrication of U-Tube L-S TENG

The U-tube LS-TENG, consistent with the device in [24], was assembled and used to conduct experiments. We chose a U-shaped tube with a thickness of 1.0 mm and an inner diameter of 6.0 mm (I.D. = 6.0 mm) to construct the U-tube TENG. Two conductive copper foils (50 µm thick) with a length of 6 cm (L = 6.0 cm) and a gap length of 8 cm (L_gap = 8.0 cm) were glued to the two columns of the U-tube TENG. During the pasting process, conductive copper wires that have been scraped off the paint were pasted between the U-tube wall and the copper foil. Then, we made the U-tube TENG for the experiment. In the experiment, we sequentially filled the liquid purchased above into the U-tube in a volume of 5.0 mL (V_liquid = 5.0 mL), and the liquid level was located in the center of the two Cu tapes.

A Hitachi SU8010 field-emission SEM (Hitachi High-Tech, Shanghai, China) was used to measure the inner surface morphology of the FEP and PTFE tubes. A mechanical rocker arm with adjustable swing frequency was used to swing the U-tube to achieve power generation. We used programmable electrometers (Keithley 6514 Electrostatic Meter, Tektronix, Shanghai, China) to test the open-circuit voltage (V_oc), short-circuit current (I_sc), and transferred charge (Q). The software platform was constructed based on LabView, which can realize real-time data acquisition and storage with a computer. In addition, some of the measured data were filtered. The FFT low pass filter (LPF) in the origin software was used to perform the data filtering. The LPF passes through low-frequency signals while blocking high-frequency signals so that the wave shape can be seen obviously. The filtering parameters are considered based on the wave shape.