## 3 Result and Discussion
### Fabrication of Microsphere through the ATPS  
In order to rebuild the phase boundary at the droplet, primarily, the interface of the ATPS should close to the initial oil/water interface (Figure 2b), which means that the volume of the aqueous phases inside the oil droplet needs to correspond approximately to the volume of the water channels (the pore volume of the polymerized particles). Furthermore, the volume of the external water phase should be determined by the optimal ratio of the inner phase to the outer phase.  
Thus, through calculation, the volume of external water was about 9 times the internal volume. According to the phase diagram, the volume ratio could be determined by the segmentation of tie-line. (Figure 3) In consideration of the stabilization effect and the ease of the after-treatment of the products, the concentration of PVA would be about 3% of the total continuous phase. Therefore, the system concentration of the water phase (\(C_{\text{total}}\)) and the concentration in the two water phases (\(C_{\text{inner}}\) and \(C_{\text{outer}}\)) can be deduced from the figures in Table 2.  
When the concentration of each phase was confirmed, the microspheres could be fabricated. By the two-step method, the phase boundary could be established theoretically. In our experiment, we measured the concentration ratio of the PEG/PVA in the outside water, and the result was nearly 1.5, which was in agreement with the theoretical values of \(C_{\text{outer}}\) and bigger than \(C_{\text{total}}\). This result means that the concentrations of the inner phase and outer phase were not the same (if they were the same, the concentration would be \(C_{\text{total}}\)), and confirms the formation of the ATPS.  
After polymerization, the SEM photo showed that the particle size was one tenth of the microspheres made by the original method (Figure 4a,b), and, for the small microspheres with a diameter of about 10 \(\upmu\)m totally aggregated, the aggregation was difficult to disperse so that the size distribution was larger than could be observed. The yield of the reaction was 87% because the small particles were lost in the after-treatment. In addition to the interface effect, another main factor for the aggregation was due to the bigger surface energy of the small particle. Thus, we had to adjust the particle size. Considering that the stirring speed had a significant influence on the particle size, the polymerization was processed with a speed from 150 to 70 rpm. The particle size was from 15 to 25 \(\upmu\)m and an aggregated degree was reduced compared with the control group prepared with the same recipe but without the formation of the ATPS (Figures 4c,d and 5). However, we should ascertain whether the existence of the aggregation was due to the small particle or not. Therefore, the next work was aimed at the optimization of the particle size and porous structure of the microspheres in order to find the effect of the ATPS when ruling out the influence of small particles.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Concentration** & **PEG(_w/w_)** & **PVA(_w/w_)** & **PEG/PVA** \\ \hline \(C_{\text{inner}}\) & 1.6\% & 7.1\% & 0.23 \\ \(C_{\text{outer}}\) & 3.5\% & 2.3\% & 1.52 \\ \(C_{\text{total}}\) & 3.3\% & 2.8\% & 1.18 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The concentration of materials in the aqueous two-phase system (ATPS).  
Figure 4: The photographs of the microspheres made at different stirring speeds. Stirring speed 150 rpm, swelling time 10 min (**a**) SEM image (\(\times\)500) and (**b**) particle size distribution (dispersed). Stirring speed 70 rpm, swelling time 10 min (**c**) SEM image (\(\times\)500) and (**d**) particle size distribution (dispersed).  
Figure 5: (**a**) The SEM image (\(\times\)100), (**b**) size distribution (dispersed), and (**c**) pore distribution of control group.