Characteristics of Ultrafine Bubbles (Bulk Nanobubbles) and Their Application to Particle-Related Technology

Abstract

Ultrafine bubbles (bulk nanobubbles), small bubbles less than 1µm in diameter, have attracted academic and industrial attention because of their numerous advantages, including their chemical-free nature and extraordinarily long lifetime. The long lifetime is related to the much higher Brownian motion velocity than buoyancy. The reason why ultrafine bubbles can endure under stable conditions is still unclear, even though their inside is highly pressured. They have several characteristics, such as pH-dependent surface charge and reduction in friction. They are also closely related to ultrasound. Ultrafine bubbles are generated and removed by selecting the ultrasonic frequency. Reaction and separation using ultrasonic cavitation and atomization, respectively, are enhanced by ultrafine bubbles. They can produce hollow nanoparticles, enhance adsorption on activated carbon, and clean solid surfaces. This review discusses the fundamental and ultrasonic characteristics of ultrafine bubbles and their application to particle-related technology: fine particle synthesis, adsorption, desorption, extraction, cleaning, and prevention of fouling.