## 2 Results and Discussion
### Effect of Flow Conditions on the Morphology and Dimensions of the Microcapsules  
The relationship between the flow conditions and the microcapsule dimensions was investigated. This is because, in general, the dimensions of microcapsules generated using microchannels vary depending on the flow rate [17]. To simplify the control parameters, the inner oil phase flow rate, Qi, was fixed at a constant value (2 mL/h). In this study, to investigate whether the shell thickness (difficult to control in the interface polymerization and sol-gel methods) could be significantly varied, the flow rate was set such that the diameter (outer diameter) of the microcapsules remained approximately constant, whereas  
Figure 1: Schematic diagram of the formation of oil-in-water-in-oil (O/W/O) microcapsules in a microchannel. Figure 2: Microcapsule formation behavior under different flow conditions. The inset shows micrographs of the upstream and downstream microchannels. The flow rate of the inner oil phase was 2.0 mL/h. Scale bars are 300 μm. the core size (linked to the shell size) could be significantly varied. Specifically, the outer oil phase flow rate Qo was varied such that the ratio of the total flow rate of the inner oil phase and aqueous phase to the outer oil phase flow rate was constant according to the following equation:  
\[Q_{o}=2(Q_{i}+Q_{a}) \tag{1}\]  
The aqueous phase flow rate Qa was varied in the range of 1-6 mL/h, as shown in Figure 2. Under all the flow conditions described above, the formation of single-core microcapsules was confirmed in the microchannel. To examine the morphology and dimensions of the microcapsules, they were collected in a dish and observed under a microscope. The collected microcapsules were mostly single-core microcapsules, with some containing multiple cores formed by the coalescence of single-core microcapsules, and others consisting solely of the aqueous phase with the core removed. First, we measured the microcapsule diameter (outer diameter) and the core diameter of the collected single-core microcapsules (Figure 3). Shell thickness was calculated as the average value of the microcapsule outer diameter minus the average value of the core diameter divided by 2. In reality, because of the density difference between the inner oil and aqueous phases, the core is not centered within the microcapsule (in this case, the core floats upward). Therefore, the shell thickness formed around the core was uneven, depending on the location (in this case, thinner at the top and thicker at the bottom). Here, we defined the shell thickness as \"effective shell thickness\". The coefficient of variation for the single-core microcapsules was 1.3-4.6% for the microcapsule outer diameter and 1.0-5.3% for the core diameter, indicating a narrow distribution. The higher the aqueous phase flow rate, the smaller the core diameter. However, the outer diameter of the microcapsules remained almost the same. The decrease in the core diameter is suggested to be due to an increase in the shear force exerted by the aqueous phase on the inner oil phase as the aqueous phase flow rate increased, thereby promoting the shear of the inner oil phase. Additionally, the fact that the outer diameter of the microcapsules remained nearly constant suggests that increasing the aqueous phase flow rate made the aqueous phase more resistant to shear by the outer oil phase, thereby increasing shell volume (thickness). In summary, within the flow rate range set in this study, the outer diameter of the microcapsules remained nearly constant, whereas the core dimensions could be controlled in response to changes in the flow rate. Next, we investigated the ratio of single-core microcapsules to the total number of collected microcapsules (that is, the single-core rate) (Figure 4). The single-core rate increased at higher aqueous phase flow rates. Correlating this with Figure 3, the single-core rate increases as the core size decreases.