Influence of temperature on bulk nanobubble generation by ultrasonication

The effects of liquid temperature on bulk nanobubbles (BNBs) generation using ultrasound was investigated. When liquid at different temperatures and relatively similar dissolved oxygen concentration (25 °C: 8.29 ppm; 50 °C: 8.11 ppm; and 75 °C: 7.78 ppm) were sonicated, a high number of negatively charged (−13 to −21 mV) BNBs were consistently generated within a few minutes. As the temperature of the liquid increased, it is expected that the concentration of BNBs considerably increased due to a lot of nucleation site and strong collapse of tiny bubbles. On the other hand, in a liquid with relatively low temperature, a high number of large bubbles were formed and the BNBs concentration considerably decreased. It is suggested that it has less nucleation sites compared to a liquid with higher temperature; thus, the bubble growth was accelerated by rectified diffusion and coalescence, owing to less nucleation sites versus dissolved gas concentration. [Display omitted] •To generate NBs, ultrasound was irradiated in DI water with different temperature.•The NB concentration decreased with a decreased in temperature of DI water because of a low nucleation site and high bubble growth rate.•The large sized bubbles attenuated the ultrasound and intensity of the bubble collapse required for the NBs generation.•In the high-temperature DI water, the NBs were easily generated because of the higher intensity of bubble collapse.