## 3 Results and Discussion
### Effect of Nanobubbling on Wood Liquefaction
The GPC analysis revealed that, excluding the solvent peaks corresponding to PEG400 and glycerin (from 7-9 min), the liquefied products derived from ozone nanobubble-treated wood meal contained a higher proportion of high-molecular-weight components (\(\mathrm{Mw}\coloneqq 2190\)), as indicated by the peak observed around the 6-min mark. Despite this, the residue rate remained low, suggesting that ozone nanobubble treatment may alter the structural characteristics of the wood meal, making it more soluble in the solvent system. This phenomenon is similarly observed for the high-molecular-weight peak appearing around 5 min, and the maintained state of dissolution suggests potential differences in conditions such as the branched structure of the molecules and the hydrogen bonding of the hydroxyl groups. Alternatively, the recondensation mechanism may be modified, resulting in a structure that is more readily soluble in polyol solvents such as PEG and glycerin. However, because GPC analysis alone provides limited insight into these structural changes, further evaluations were conducted, including hydroxyl (OH) number measurements and viscosity analysis of the liquefied wood. The results are shown in Table 6.  
At room temperature, ozone nanobubble treatment resulted in only a slight increase in hydroxyl numbers. However, treatments at 50 \({}^{\circ}\)C and those combining ozone nanobubbles with ultrasonic processing exhibited significantly higher hydroxyl numbers. Previously, prolonged liquefaction was found to promote the depolymerization of wood components, leading to an increase in hydroxyl numbers. This suggests that ozone nanobubble treatment may facilitate the depolymerization of wood components, making them more reactive. On the other hand, despite the GPC results indicating a higher proportion of high-molecular-weight components, the viscosity of the liquefied wood was lower than that \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Liquefied Products** &
\begin{tabular}{c} **OH Number** \\ **(KOH/mg)** \\ \end{tabular} & **Residue (wt\%)** & **Viscosity (cP)** \\ \hline Untreated & 341 & 14.7 & 684 \\ \hline Nanobubble @RT & 387 & 0.4 & 246 \\ \hline Nanobubble @50 \({}^{\circ}\)C & 557 & 2.9 & 232 \\ \hline +Ultrasonic @RT & 581 & 7.5 & 198 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The comparison of the OH numbers, residues, and viscosities of untreated and nanobubble-treated samples. Figure 7: GPC chromatography of untreated and nanobubble-treated liquefied wood samples. of untreated samples. This suggests that the number of hydrogen bonds formed within the liquefied wood may have been reduced, indicating changes in its chemical structure. These effects were particularly pronounced under conditions that combined nanobubble treatment with ultrasonic processing, further correlating with the peak shifts observed around 1100 cm\({}^{-1}\) in the FT-IR spectra. Based on these findings, for wood meal pretreatment, the most effective approach for achieving a low-residue, low-viscosity liquefied product is the combination of the ozone nanobubble and ultrasonic treatments rather than ozone nanobubbles alone. Thus, the ability to achieve a low residue rate within a shorter reaction time while obtaining a highly reactive liquefied wood with a high hydroxyl number and process-friendly low viscosity demonstrates the industrial potential of ozone nanobubble treatment. However, further process optimization, including more effective treatment conditions and the integration of nanobubble treatment with the liquefaction process, will be necessary for maximizing its effectiveness.