## 2 Materials and Methods
### Choice and Calibration of Pumps  
There are several types of pumps which are used in microfluidics, e.g., syringe, peristaltic, piston, plunger pumps, etc. Hydrodynamics plays a crucial role regarding mixing intensity [16]. For example, chaotic flow rate modulation can significantly enhance convective mixing [47]. It can dramatically shorten the mixing time, resulting in greater conversion values. However, at the same time, flow rate \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Activation Energy** **(kJ/mol)**} & \multirow{2}{*}{**Pre-Exponential** **Factor (L/(mol-s))**} & \multicolumn{2}{c}{**Reaction Rate Constant (L/(mol-s)) at**} & \multirow{2}{*}{**Source**} \\ \cline{3-5}  & & **17 \({}^{\circ}\)C** & **22 \({}^{\circ}\)C** & **32 \({}^{\circ}\)C** \\ \hline
41.40 & 2.18 \(\times\) 10 [FOOTNOTE:]Footnote : footnotemark: [ENDFOOTNOTE] & 0.077 & 0.103 & 0.178 & [45] \\
48.00 & 2.86 \(\times\) 10 [FOOTNOTE:]Footnote : footnotemark: [ENDFOOTNOTE] & 0.065 & 0.091 & 0.173 & [46] \\
45.38 & 1.00 \(\times\) 10 [FOOTNOTE:]Footnote : footnotemark: [ENDFOOTNOTE] & 0.068 & 0.093 & 0.171 & present study \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Kinetic data of ethyl acetate saponification by sodium hydroxide. Figure 3: Arrhenius plot for saponification of ethyl acetate. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Measurement**} & \multicolumn{3}{c}{**Reaction Rate Constant, (L/(mol-s))**} \\ \cline{2-4}  & **17 \({}^{\circ}\)C** & **22 \({}^{\circ}\)C** & **32 \({}^{\circ}\)C** \\ \hline
1 & 0.0603 & 0.0990 & 0.1549 \\
2 & 0.0617 & 0.0967 & 0.1605 \\
3 & 0.0616 & 0.0950 & 0.1  
fluctuations can add several uncertainties to the measurement results, making them unreliable. In such a case, the mixing rate also depends on the modulation wave form, frequency and amplitude [47]. The pumps, in which the volume flow rate is constant and steady and where no pulsations appear, provide more reliable experimental results. For this purpose three different pumping systems based on distinct pumping principles were tested (Table 5). The operating characteristics of the investigated pumps differ in the presence of pulsations, their frequency and amplitude (Figure 4). The REGLO-CPF (Wertheim, Germany) piston pump uses \"No-Valve\" technology, where a ceramic piston synchronously rotates and reciprocates in a precisely mated ceramic cylinder. Whilst rotating, the piston moves up and down inside the ceramic chamber, filling the piston flat with a media (moving up) and pumping out fluid (moving down). According to the pumping principle, a piston pump, which has a piston rotation frequency of up to several tens of rounds per minute, shows a highly oscillating flow rate characteristic. Figure 5 shows the mixing process of two water solutions in the chicane microreactor at different time points. The solutions were inked with different colors and pumped using two piston pumps at volume flow rates of 1 and 6 mL/min each. The distribution of colored solutions changes over time, indicating an unsteady process. It is evident that the flow pulsations that appear essentially affect the mixing process and provide a more intensive mass transfer in comparison with cases where there are no flow rate pulsations. Moreover, the higher the set-volume flow rate, the more frequently the piston head rotates, biasing the experimental data. On the contrary, the application of an NE-300 pulsation-free syringe pump or an S 1610 two-syringe pumping system ensures that the flow pattern mainly stays steady. Because of the rapid switching valve which is inside the S 1610 pump, there is a periodic short jump of flow rate, as schematically indicated in Figure 4.  
Although piston pumps may lead to a more efficient mixing grade, flow pulsations contribute to an uncontrolled effect on the mixing process, which is not easy to reproduce at a particular moment in time to get a comparison of mixing in different reactor systems.