## 3 Results
### pH Test in Millimeter Channels
An analysis of the pH-sensitive film in Anycubic Basic resin millireactors over 20 cycles suggests improved mixing with the implementation of static mixers, as indicated by a smaller range of \(H\) values at later times. The pH sensor film remained stable throughout the experiment, consistently showing color changes, suggesting the potential for long-term use in continuous pH monitoring processes. In the next part of the cyclic experiment, the color change was monitored in the millireactors without static mixers made of High Temp resin. As in the previous experiment, one cycle involved introducing the acid at a higher flow rate of 500 uL/min, while in the subsequent cycle, the base was introduced at 500 uL/min. The results shown in Figure 8 indicate that the mixing of solutions does not occur in the millireactor channels, as evidenced by the increase in the \(H\) values during cycles with excess acid and the decrease in the \(H\) values during cycles with excess base. Therefore, there is a color shift from red to light blue and vice versa in cycles. However, the color change is subtle due to the lower transparency of the millireactor. This reduced change is reflected in the smaller range of \(H\) value variation. Figure 8: Graphical representation of the \(H\) values’ dependence on time, with error bars, in the middle of the millireactor made of High Temp resin during cycles of alternating acid (A) and base (B) flow. Figure 7: Graphical representation of the dependence of \(H\) values on time, with error bars, in the middle of the millireactor with static mixers made of Anycubic Basic resin in acid (A) and base (B) flow exchange cycles. As shown in Figure 9, the H values distinctly increase during cycles with excess acid and decrease during cycles with excess base. This occurs due to a color shift from red to light purple. However, the color change is less pronounced compared to that in the millireactor without static mixers, as indicated by the smaller range of \(H\) value variations, from 17\({}^{\circ}\) to 21\({}^{\circ}\). This suggests that mixing between the two solutions improves as they pass through the static mixers. The examination of the pH sensor film in High Temp resin millireactors over 20 cycles shows that the use of millireactors with static mixers improves mixing. The film remained stable throughout the entire experiment and exhibited a color change over the course of the study. The change in the millireactors made of High Temp resin was visible to the naked eye, but not as clearly as that in the Anycubic Basic resin millireactors due to the reduced transparency of the High Temp resin and the yellowing of the resin. A comparative assessment of millireactors fabricated from Formlabs High Temp and Anycubic Basic resins was performed to examine their impact on sensor film performance. While both materials allowed for effective sensor integration, the higher transparency of Anycubic Basic resin facilitated more distinct colorimetric detection, whereas the Formlabs High Temp resin exhibited reduced visibility due to its yellowish tint. Despite these differences, the pH-sensitive films remained functional in both systems, demonstrating their versatility across different 3D printing materials. Reversibility, expressed as the relative standard deviation of the maximum hue value in the dominantly basic flow, was found to be 2.4-3.3% for all measurements. These calculations indicate excellent reversibility. Given the color change in the film throughout the experiment and its excellent stability, it can be concluded that the film may be used for longer-term testing, such as monitoring pH in continuous processes. The mixing index calculations based on RGB color model analysis demonstrate the effectiveness of the static mixer in enhancing mixing efficiency within millireactors. The methodology for the calculation was adopted from [PERSON] et al. [35].