# Preparation of Nanoparticle-Loaded Microbubbles via an Electrohydrodynamic Atomization Process
## 3 Results and Discussion
### Effect of Nanoparticle Concentrations on the Microbubbles  
To further evaluate the effect of nanoparticle loading on the microbubbles, stability analysis of microbubbles loaded with different concentrations of silica nanoparticles were carried out. Figure 5 shows representative optical micrographs of microbubbles loaded  
Figure 4: Variation of normalized microbubble number against time. Figure 3: Representative optical micrographs of microbubbles with nanoparticles. (**A**) \(t=0\) min. (**B**) \(t=5\) min. (**C**) \(t=10\) min. (**D**) \(t=15\) min. with various concentrations of nanoparticles. The operating conditions were as follows: flow rate of liquid (\(Q_{l}\)), 10 mL/h; flow rate of gas (\(Q_{g}\)), 60 mL/h, and applied voltage 11 kV. As shown in Figure 5, with the increasing nanoparticle concentration, the microbubbles become more uniform. More microbubbles were produced when \(C_{NP}\) = 2 mg/mL. However, as shown in Figure 6, when the nanoparticle concentration increased from 0.5 mg/mL to 1 mg/mL, the reduction rate of bubble number tended to slow down. Further increasing the nanoparticle concentration to 2 mg/mL resulted in an intensification of bubble number reduction. This indicates that the concentration of nanoparticles has a significant impact on the stability of microbubbles, and loading an appropriate amount of nanoparticles is helpful in improving the stability of microbubbles. Figure 7 shows the effect of nanoparticle concentration on the average diameter of microbubbles. It can be clearly observed that when nanoparticles are loaded in the shell layer, the average size of microbubbles decreases. When the concentration of silica nanoparticles was 0.5 mg/mL, the average size of microbubbles was 100.32 \(\pm\) 5.49 \(\upmu\)m. With the increase of silica concentration, the average size of microbubbles increased gradually. When the concentration of silica nanoparticles was 1 mg/mL, the average size of microbubbles was 131.43 \(\pm\) 5.28 \(\upmu\)m. After increasing the concentration of silica nanoparticles to 2 mg/mL, the average size of microbubbles increased slightly (135.66 \(\pm\) 5.57 \(\upmu\)m). The average size of microbubbles without loading nanoparticles was 196.83 \(\pm\) 13.14 \(\upmu\)m under the same operating conditions. In addition, the size of microbubbles without loading nanoparticles increased gradually at the first 10 min. However, the size of microbubbles loaded with nanoparticles seemed more stable. Figure 5: Representative optical micrographs of microbubbles loaded with various concentration of nanoparticles. (**a**) \(C_{NP}\) = 0.5 mg/mL; (**b**) \(C_{NP}\) = 1 mg/mL; (**c**) \(C_{NP}\) = 2 mg/mL. Figure 6: Variation of normalized bubble number with time when loading nanoparticles with different concentrations. Figure 7: Variation of average bubble diameter over time for microbubbles loaded with various concentrations of nanoparticle. At time 0, the initial sizes of microbubbles at different loading concentrations were as follows: 175.19 \(\pm\) 14.59 \(\upmu\)m (0 mg/mL), 98.41 \(\pm\) 5.57 \(\upmu\)m (0.5 mg/mL), 127.23 \(\pm\) 5.45 \(\upmu\)m (1 mg/mL), 137.98 \(\pm\) 5.28 \(\upmu\)m (2 mg/mL). When nanoparticles are introduced into the shell layer of microbubbles under the electric field, the introduction of nanoparticles may induce the accumulation of charge on the microbubbles. It is generally accepted that the charge on the surface makes the bubble more stable due to a repulsion force [37; 38]. The high-density charge makes the microbubbles easier to break into finer particles under the condition of the electric field [39], which may be one of the reasons for the reduction of microbubbles size. At the same time, we speculate that the existence of nanoparticles limits the Oswaldo ripening effect in the microbubble environment to a certain extent, which is also the reason why the size of microbubbles does not increase or fall sharply after the introduction of nanoparticles.