Measurement and characterization of bulk nanobubbles by nanoparticle tracking analysis method

The size of the bulk nanobubbles (typically 100 nm-200 nm in diameter) is below the optical resolution of the typical microscopes, which makes it difficult to make a direct observation of them and differentiate between the nanoparticles and the nanobubbles. In this work, a nanoparticle tracking analysis (NTA) system is developed for tracking and sizing the bulk nanobubbles, and more broadly, the Brownian diffusing particles. We demonstrate the capabilities of the NTA in characterizing the nanoparticles through systematic validation of accuracy and resolution. We particularly exhibit its unique advantages in detecting the polydisperse nanoparticle populations. Then the nucleation and the thermodynamic stability of the bulk nanobubbles are systematically studied by the NTA system. The bulk nanobubbles smaller than 600 nm in size are generated via the ultrasonication method. It is shown that both the ultrasonic cavitation treatment time and amplitude, essentially, the energy input, favor the nucleation of the bulk nanobubbles with a more concentrated size distribution, a higher concentration, and a smaller hydrodynamic size. The temperature dependence of the bulk nanobubbles over a wide range from 25°C to 70°C is explored. It is found that the nanobubble first shrinks in size significantly and then the size keeps approximately constant with the increasing temperature, showing a narrow size distribution. The transition temperature is around 45°C. Further, as an open system, the NTA is a potential tool in the study of the dynamic behavior of the bulk nanobubbles in situ, such as the coalescence and the collapse.