## 2 Materials and Methods
### Characterization and Experiments  
To assess the functionality and pH operating range of the film based on litmus and TiO\({}_{2}\), ten solutions with pH values ranging from 2 to 11 were prepared. These solutions were formulated by diluting 0.1 M hydrochloric acid and 0.1 M sodium hydroxide with distilled water. The pH values of the solutions were measured using a pH/mV/Ion/Temp Meter BT-675 from Boeco (Hamburg, Germany). After the films were dried, the test plates were immersed in the solutions of varying pH values, and color changes were monitored at intervals of 10 s, 1 min, 5 min, and 10 min. All test plates were photographed using an iPhone 14 at specific time points, and the images were processed using the Color Picker AR application to determine the RGB and the hue (\(H\) value) of the film's color. The photographs of the test plates and the millireators were imported in the Color Picker AR application, where areas designated for color analysis were defined manually. The application displayed RGB (red, green, blue) and \(H\) values (hue). Additionally, the \(H\) values can be calculated from the recorded RGB following the formula below (Formula (1)) [34]. \[ \begin{array}{ll}\text{(A)}&H=60^{\circ}\times[(G^{\prime}-B^{\prime})/(R^{ \prime}-B^{\prime})]\text{ for R }\geq\text{G}\geq\text{B}\\ \text{(B)}&H=60^{\circ}\times[(2-(R^{\prime}-B^{\prime})/(G^{\prime}-B^{\prime })]\text{ for G }>\text{R}\geq\text{B}\\ \text{(C)}&H=60^{\circ}\times[2+(B^{\prime}-R^{\prime})/(G^{\prime}-R^{\prime })]\text{ for G }>\text{B}\geq\text{R}\\ \text{(D)}&H=60^{\circ}\times[4-(G^{\prime}-R^{\prime})/(B^{\prime}-R^{\prime })]\text{ for B }>\text{G}>\text{R}\\ \text{(E)}&H=60^{\circ}\times[4+(R^{\prime}-G^{\prime})/(B^{\prime}-G^{\prime })]\text{ for B }\geq\text{R}\geq\text{G}\\ \text{(F)}&H=60^{\circ}\times[6-(B^{\prime}-G^{\prime})/(R^{\prime}-G^{\prime })]\text{ for R }\geq\text{B}\geq\text{G}\end{array}  \tag{1}\]  
where R\({}^{\prime}\)G\({}^{\prime}\)B\({}^{\prime}\) values were calculated from [34]:  
R\({}^{\prime}\) = (R value)/255  
G\({}^{\prime}\) = (G value)/255  
B\({}^{\prime}\) = (B value)/255  
The tested films were applied to test plates measuring 50 mm \(\times\) 10 mm \(\times\) 1.5 mm using the previously described method. Material characterization techniques, such as contact angle measurements with the OCA 20 goniometer from DataPhysics, were employed to evaluate the impact of the films on the surface properties. Contact angles were measured for the films based on the test plates coated with litmus film (with added TiO\({}_{2}\)) and on the High Temp and Anycubic Basic resin plates without films, which served as references. Water and diiodomethane were used as test liquids. The contact angle was measured after 60-80 ms using a fitting based on the Young-Laplace equation. Each fluid was tested on every test plate at least three times. The films within the channels of the millireactor and their performance in the millireactor channels were evaluated by passing solutions of different pH values using Aladdin AL-1000 syringe pumps (World Precision Instruments, Sarasota, FL, USA) (Figure 3). In the first part of the experiment, solutions of the following pH combinations--2 and 7; 7 and 11; and 2 and 11--were simultaneously passed through the millireactor channels at a flow rate of 500 \(\mu\)L/min. When changing the pH of the solutions, it was necessary to rinse the millireactor channels with distilled water to remove any residual acid or base. The millireactors were photographed after 10 s, 30 s, 1 min, and 5 min. The color change in the films was monitored through image processing using the Color Picker AR application. In the second part of the experiment, the films and their reusability were assessed using a cyclic flow of pH solutions. A combination of pH 2 and pH 11 solutions was employed due to the most pronounced color difference. The solutions simultaneously flowed through the millireactor channels, with one solution consistently dominating the flow.