## 1 Introduction  
In recent years, the efficient utilization of woody biomass, a renewable resource, has been strongly emphasized as a key step toward achieving a sustainable society. Among various types of biomasses, waste wood remains largely underutilized, with its primary use still limited to fuel consumption [1, 2, 3]. This highlights an urgent need to explore alternative applications. Liquefaction, a chemical conversion technology for woody biomass, has garnered attention as a promising method for transforming wood into valuable chemical feedstocks. Since the 1980s, intensive research has been conducted on liquefaction techniques using solvents such as phenols and polyethylene glycol, leading to advancements in understanding reaction mechanisms and optimizing process conditions [4, 5, 6, 7]. Notably, progress has been made in the development of functional polymers and adhesives derived from liquefied wood products [4, 7, 8, 9]. However, these technologies remain confined to the laboratory scale. At the same time, conventional liquefaction processes face several challenges, particularly in terms of reaction efficiency and environmental impact [10, 11, 12, 13, 14, 15, 16]. The rigid crystalline structures of lignin and cellulose, the primary components of wood, present significant barriers to efficient liquefaction. In particular, the rigid crystalline structure of cellulose, in addition to the strong bonds between each component, inevitably slows down the liquefaction process. Therefore, if these bonds and crystalline structures can be modified into states more conducive to liquefaction, the efficiency of the liquefaction process would naturally be enhanced. To solve this matter, traditional pretreatment methods, including strong acid or alkali treatments and high-temperature, high-pressure processing, have been investigated. However, these approaches are associated with high capital and operational costs as well as considerable environmental burdens. Against this backdrop, the development of more efficient and environmentally friendly pretreatment technologies has become a critical research focus for advancing the practical implementation of wood liquefaction processes [17]. Nanobubble technology, which enables the stable generation and maintenance of ultrafine bubbles with diameters less than 100 nm, has recently garnered attention across various industries as an innovative solution. Nanobubbles have unique physicochemical properties such as high specific surface area, long-term stability, high gas dissolution efficiency and strong oxidative potential [18; 19; 20; 21; 22]. The mechanism of organic matter decomposition is driven by free radicals generated during the disappearance of nanobubbles, involving various gases. Among these, ozone nanobubbles have been shown to possess several advantages over conventional ozone treatments. First, the utilization efficiency of ozone is improved by more than tenfold, which is attributed to the prolonged retention time enabled by nanobubble stability. Second, enhanced bubble stability allows for sustained oxidative effects, ensuring that ozone's reactive capabilities are maintained over an extended period. Furthermore, the ultrafine size of nanobubbles facilitates uniform treatment even in deeper regions, achieving high treatment efficiency even at low concentrations [21; 23; 24; 25]. The generation of nanobubbles can be achieved through various methods, each utilizing different physical and chemical principles. The major methods include ultrasonic cavitation [26], hydrodynamic cavitation [27; 28; 29; 30], porous material diffusion [31; 32], and solvent exchange techniques [28]. Each method has its advantages and disadvantages. The ultrasonic cavitation method generates nanobubbles by exposing a liquid to high-frequency ultrasonic waves, typically in the range of 20 kHz to several MHz. The ultrasonic waves induce alternating high-pressure and low-pressure cycles in the liquid, leading to the formation, growth, and subsequent collapse of microbubbles.