Ultrafine bubbles (bulk nanobubbles), small bubbles less than 1 µm in diameter, have attracted academic and industrial attention because they have numerous advantages, including chemical-free nature and extraordinarily long lifetime. Since Brownian motion velocity is much higher than buoyancy, ultrafine bubble lifetime is more than several months. But stable mechanism is still unclear, even though their inside is highly pressured. They have several characteristics, such as pH-dependent surface charge and reduction in friction. Ultrafine bubbles are generated and removed by selecting the ultrasonic frequency. This review discusses the fundamental characteristics of ultrafine bubbles and their applications. Ultrafine bubbles effectively remove proteins, dyes, and inorganic salts from solid surfaces. Ultrafine bubbles promote adsorption, desorption and extraction. Gold nanoparticles, nanocomposites of CaCO₃ and pulp fiber, and follow nanoparticles are synthesized using ultrafine bubbles. Thus, ultrafine bubbles have been shown to be powerful, environmentally friendly, safe, clean, simple, and inexpensive methods.

Nanobubbles (NBs), with their unique physicochemical properties and promising applications, have become an important research topic. Generation of monodispersed bulk NBs with specified gas content remains a challenge. We developed a simple method for generating bulk NBs, using porous alumina films with ordered straight nano-scaled holes. Nanoparticle tracking analysis (NTA) is used to confirm NB formation. The NTA data demonstrates that NB size is distributed discretely with a uniform increment factor of √2. To explain the observed characteristic size distribution of NBs, we propose a simple model in which two NBs of the same size are assumed to preferentially coalesce.

In home life, the use of finebubble technology is incleasing, and because of their high cleaning effect, the technology is expected to easily save water and reduce the amount of chemicals used. We believe it has high compatibility with the SDGs (Sustainable Development Goles : adopted by the United Nations in 2015).

Plastic materials are widely used for automotive parts, electronic devices, and faucet parts because they are lighter than metals and have superior workability and productivity. However, due to their poor weather resistance, their surfaces must be painted or plated. As a pretreatment for plating, it is most important to roughen the surface to obtain strong adhesion through an anchor effect. The conventional method is to use chemicals, but the use of chemicals is now severely restricted by the REACH regulation in the EU due to the environmental impact of wastewater and effluent treatment.

We have obtained the same or higher adhesion strength than conventional methods by using a pretreatment method with ozone fine bubbles, which has almost no environmental damages.

The paper presents the results of electrical property measurements for submicron-scale bubbles (Ultra Fine Bubble, UFB) as a property that includes correlation with water. The relationship between the concentration of UFBs and average size and the electrical conductivity of water is shown, confirming that the presence of UFBs generated by oxygen gas decreases the electrical conductivity of water. In addition, the internal pressure was directly detected using the perturbation angle correlation method, and a lower internal pressure (about 3.4 atmospheres) than theoretical was measured. We proposed the use of ultrasonic waves as a simple detection method for bubbles, including the distinction between UFBs and impurities, and are still measuring the bubbles.

The International Organization for Standardization has standardized “fine bubble” for bubbles less than 100 µm diameter. Even when the same amount of gas turns into bubbles, due to its fineness, the number of fine bubbles is much larger than that of easily visible cm-size bubbles. As a result, the total bubble surface area is expanded, and the chemical reaction efficiency at the gas-liquid interface is dramatically improved. If air is contained within, it will increase the amount of dissolved oxygen in the water. For this reason, the use of fine bubbles has been growing in the agriculture and fisheries industries, where dissolved oxygen is essential. For example, it is used for irrigation in the agricultural industry and for densification processes in the fishery industry. In each case, it is utilized to solve problems in the field.