## 3 Results and Discussion
### Effect of Nanobubbling on Wood Liquefaction  
To evaluate the liquefaction rate of ozone nanobubble-treated wood meal, liquefaction was conducted under conditions previously identified in our research as yielding a low residue rate. These liquefaction conditions included a 3% sulfuric acid catalyst, a PEG-to-glycerin ratio of 7:3, a solvent-to-wood meal ratio of 5:1 (by weight), and a liquefaction temperature of 150 \({}^{\circ}\)C. The extent to which the residue rate varied depended on the presence or absence of ozone nanobubble treatment was assessed, and the results of the residue rate measurements are presented in Figure 5.  
In the first 30 min of liquefaction, no significant difference was observed between the ozone nanobubble-treated wood meal and untreated wood meal. However, from 30 to 180 min, the residue rate of the ozone nanobubble-treated wood meal was noticeably lower. In the liquefaction mechanism of wood, hemicellulose and lignin liquefy first, followed by cellulose. Considering this, ozone nanobubble treatment appears to enhance the liquefaction of cellulose specifically. In addition, the time required to reach a residue rate of 10% was 180 min for untreated wood meal, whereas it was reduced to 90 min for the ozone nanobubble-treated wood meal, effectively halving the required time. This suggests that the energy consumption for liquefaction is also reduced by half while simultaneously facilitating the production of low-residue liquefied wood. Under this assumption, nanobubble treatment enables us to attain a more eco-friendly liquefaction condition. Figure 4: XRD spectra of untreated and nanobubble-treated wood meal samples. \begin{table}
\begin{tabular}{c c c} \hline
**XRD Sample** & **Crystallinity (\%)** & **Amorphous (\%)** \\ \hline Untreated & 37.7 & 62.3 \\ \hline Nanobubble @RT & 43.4 & 56.6 \\ \hline Nanobubble @50 \({}^{\circ}\)C & 40.3 & 59.7 \\ \hline +Ultrasonic @RT & 49.7 & 50.3 \\ \hline \end{tabular}
\end{table} 
Table 5: Results of the calculations of the crystallinity for each sample. These findings are consistent with the XRD crystallinity measurements and the observed decrease in hemicellulose content, further supporting the fact that ozone nanobubble treatment enhances the liquefaction efficiency of wood meal. To assess how this treatment affects the physical properties of the liquefied wood itself, further analyses were conducted using FT-IR and GPC for molecular weight determination, as well as hydroxyl numbers and viscosity measurements. The FT-IR spectra of these liquefied wood samples are shown in Figure 6.  
The FT-IR spectra of liquefied wood derived from ozone nanobubble-treated and untreated wood meal revealed notable changes, particularly around 1000 cm\({}^{-1}\). Specifically, the absorption peaks associated with the C-O bond in cellulose (1070 cm\({}^{-1}\)) and the C-O-C bond (1150 cm\({}^{-1}\)) exhibited variations. These changes are likely attributable to alterations in cellulose crystallinity and molecular interactions such as hydrogen bonding. Consistent with previous findings, this suggests a reduction in the amorphous fraction and a loosening of the rigid crystalline structure. Notably, in Sample 3, which underwent both nanobubble treatment and ultrasonic processing, the peak intensity ratios changed significantly. This distinctive spectral shift further supports the characteristic effects of nanobubble treatment. Figure 5: Liquefaction rate of wood meal with different nanobubble treatments. Figure 6: FT-IR spectra of the liquefied wood from untreated wood and nanobubble-treated wood (Nanobubble sample #1: nanobubbling at room temperature; nanobubble sample #2: nanobubbling at 50 \({}^{\circ}\)C; and nanobubble #3: nanobubbling and ultrasonicating at room temperature). Figure 7 shows the comparison of GPC chromatography between non-nanobubble treatment and nanobubble treatment at room temperature.