Stability of Nanobubbles

Abstract With stable existence in liquids for over several weeks, nanobubbles have an extensive range of applications across many fields of science and engineering. For an effective and functional use of these bubbles, it is important to know the reason for their long-term stability. Therefore, a comprehensive laboratory investigation was performed to determine bubble size distributions and zeta potentials of nanobubbles, first with four different gases (test series I), then with different salt concentrations, pH levels, and temperatures of the solution (test series II). Experimental results from test series I showed that the average bubble size depended on the gas solubility in water, and zeta potential depended on the ability of the gas to generate OH − ions at the water/gas interface. Experimental results from test series II showed that bubbles with high negative zeta potentials can be generated in solutions of high pH, low temperatures, and low salt concentrations. The high pH solutions produced smaller but stable nanobubbles. Bubble diameter slightly increased with increasing salt concentration. However, bubble size did not show considerable dependence on solution temperature. Long-term tests showed that with time zeta potential of bubbles decreased while the bubble size increased. Even though bubble sizes are expected to decrease with time due to gas diffusion, results indicate increased bubble sizes. This is because of decrease in zeta potential and bubble movement due to Brownian motion which causes bubble coalescence over time to form larger bubbles.