## 3 Results and Discussion  
### Electricity Generation of DC-FluTENG  
To evaluate the electrical characteristics of the DC-FluTENG, DI water droplets (about 1.06 mL) are delivered periodically into the system at a flow rate of 113 mL/min. Here, the aluminum electrode is used. Due to the continuous flow of water droplets, a stable open-circuit voltage (Voc) and short-circuit current (Isc) are continuously generated, as shown in Figure 3a,b. As regards these figures, both voltage and current signals are obtained as the distinct characteristics of pulsed direct current. It can be observed that, with a single droplet, DC-FluTENG can produce an average Voc of \(-8.1\) V and an average Isc of \(-0.757\) uA, corresponding to a maximum charge transfer of 78 nC (calculated in short-circuit mode). Herein, the negative sign (\(-\)) represents the direction of the electric current according to the electrical measurement setup, that is, from the ground to the electrode. The inset images present the correspondence between typical pulses of Voc and Isc and the step-by-step process mentioned in the above section. Obviously, the pulse outputs are observed when water and the electrode are in contact, in which the signal exhibits a large acceleration from zero and deceleration to zero at the beginning and the end of the contact-separation process, respectively; during the contact, the output signal fluctuates around an average value; otherwise, no electrical signal is produced in the whole circuit. Figure 3: Concept demonstration of DC–FluTENG. (**a**) Open–circuit voltage and (**b**) short–circuit current generated by DC–FluTENG. Inset images: enlarged view of single–voltage and –current peaks. (**c**) Output voltage measured with various resistances in the external circuit. (**d**) Dependences of voltage, current, and instantaneous power on the load resistance in a range from 10 k\(\Omega\) to 50 M\(\Omega\). Measured with DI water at a flow rate of 113 mL/min and using an aluminum electrode. To confirm the DC output characteristics of the device, the electrical responses of DC-FluTENG are examined by connecting a resistor with a range of load resistance between the electrode and the ground. The output voltages plotted in Figure 3c clearly show the DC characteristics of the output voltage with all negative pulses. Notice that the magnitude of the output voltage is considerably increased when increasing the load resistance from kiloohm to megaohm level. The changes of average magnitudes of both voltage and current based on the load resistance imply an opposite trend, as reported in Figure 3d. Regarding the increase of load resistance from 10 k\(\Omega\) to 50 M\(\Omega\), the magnitude of output voltage increases from 9 mV to 8.7 V; meanwhile, the peak current decreases from 0.9 to 0.173 uA, respectively. This figure also displays the summarization of the instantaneous power (\(P_{ins}\)) concerning loading resistances. By varying the load resistance from 10 k\(\Omega\) to 10 M\(\Omega\), the value of \(P_{ins}\) dramatically increases from 8 nW to 3.27 uW. However, as the load resistance exceeds 10 M\(\Omega\), \(P_{ins}\) is reduced and reaches a value of 1.53 uW at 50 M\(\Omega\). Thereby, we believe that there certainly exists an optimal load resistance; here, it is 10 M\(\Omega\), where the maximum output power is achieved. Thereby, DC-FluTENG can produce a maximum peak power of 3.27 uW at a flow rate of 113 mL/min, corresponding to a power density of 0.327 W/m\({}^{2}\), calculated based on the total contact area between the water droplet and the electrode. In regard to another aspect, to verify that the DC outputs are generated due to the direct contact of water with the electrode, comparison experiments were conducted by covering the electrode with an insulating layer, such as polyvinyl chloride, resulting in a control single-electrode-based TENG, as shown in Figure 4a. The performance of this control-TENG was studied, and the results are shown in Figure 4b.