## 3 Results
### Influence of AFM Chip Materials on Charge Generation
#### 3.1.2 Influence of HOPG AFM Chip on Charge Generation in Water Flow  
\(\Delta q\)(\(t\)) dependencies were obtained in a repeated series of experiments in the absence and in the presence of HOPG chip near the tip nozzle. Insignificant variation between the obtained \(\Delta q\)(\(t\)) curves \begin{table}
\begin{tabular}{c c c} \hline \hline
**Investigated Material** & \multicolumn{2}{c}{\(\Delta q\)/\(q\), \%} \\ \hline  & \(l\) = 4 mm & \(l\) = 1 cm \\ \hline Mica & 28 & 14 \\ HOPG & 41 & 12 \\ polypropylene tube & - & 204 \\ polypropylene tube + water & - & 33 \\ polypropylene tube + BSA (\(10^{-15}\) M) & - & 33 \\ polypropylene tube + BSA (\(10^{-4}\) M) & - & 35 \\ polypropylene tube + C\({}_{2}\)H\({}_{5}\)OH & - & 33 \\ \hline \hline \end{tabular} \(\Delta q\)/\(q\) is the relative change in the value of charge accumulated in the cell in the presence of an investigated material near the injector nozzle; \(l\) is the distance between the material and the injector nozzle.  
\end{table} 
Table 1: Values of charge accumulated in the cell in the presence of various materials, nC. Figure 4: The dependencies of the relative change in the value of charge (\(\Delta q\)/\(q\)), generated in water, on the distance between the injector nozzle and the investigated material: mica (1) or highly oriented pyrolytic graphite (HOPG) (2). T = 35 \({}^{\circ}\)C.  
were observed within this experimental series. For this reason, Figures 5 and 6 display averaged data (\(\Delta q(t)\) dependencies) obtained in these experiments in the absence and in the presence of HOPG chip placed at either \(l=4\) mm (Figure 5) or \(l=1\) cm (Figure 6) distance from the injector nozzle. Figure 5: The effect of the presence of HOPG on the charge generation in water. Averaged \(\Delta q(t)\) dependencies obtained upon pumping of water through the injector. Experimental conditions: without HOPG (**1**); with HOPG (**2**). The distance between the injector nozzle and the HOPG chip is \(l=4\) mm. T = 35 \({}^{\circ}\)C. The data of one and the same experimental series obtained using one and the same tube-pipe-tip set (see the Materials and Methods section) are presented. Figure 6: The effect of the presence of HOPG on the charge generation in water. Averaged \(\Delta q(t)\) dependencies obtained upon pumping of water through the injector. Experimental conditions: without HOPG (**1**); with HOPG (**2**). The distance between the injector nozzle and the HOPG chip is \(l=10\) mm. T = 35 \({}^{\circ}\)C. The data of one and the same experimental series obtained using one and the same tube-pipe-tip set (see the Materials and Methods section) are presented. As seen from Figure 5, an increased rate of charge accumulation is observed when a HOPG AFM chip is placed near the injector nozzle--as compared to the case without the chip. The accumulation of the generated charge in the measuring cell was linearly dependent on time. The charge accumulation rate within the series of three repeated experiments without HOPG was 21 to 22 nC in 7 min, the average value was (22 \(\pm\) 1) nC in 7 min. In the presence of HOPG, an increased level of charge accumulation (30 to 32 nC) was observed within the respective series of three repeated experiments, with an average value of (31 \(\pm\) 1) nC in 7 min. Figure 4 displays the dependence of the relative change in the value of charge, accumulated in the measuring cell, on the distance between the investigated material and the injector nozzle (\(\Delta q/q(l)\) dependence). From the data presented in this figure, one can see that at \(l\) = 2 cm no influence (within the error) of the presence of the HOPG chip on the charge generation is observed. Table 1 (see the Discussion section) lists the values of the relative change (\(\Delta q/q\)) in the charge accumulated in the cell in the presence of various materials near the injector nozzle.