Decrease in the Surface Tension of Nanobubble Dispersion in Water: Results of Surface Excess of Bulk Nanobubbles at Interfaces

The effect of nanobubbles (NBs) on the surface tension of liquid was investigated by three methods of different measuring principles, pendant drop (PD), Wilhelmy, and du Nou''y methods, over a wide range of number concentration of bulk NBs (BNBs). In all of the three methods, the surface tension decreased in proportion to the number concentration of BNBs and the proportional constant was different among the three methods. Such behavior was inferred to be caused by the surface excess of BNBs at the gas-liquid or solid-liquid interface. In the PD method, the hydrophobic interaction between BNBs and air around a drop seems to cause the surface excess of BNBs along the surface of water drops. It brings about a subtle change in its profile, resulting in the decrease in surface tension, which takes a time of hundreds of seconds. Meanwhile, in the Wilhelmy and du Nou''y methods, electrostatic attractive force between BNBs and a plate or ring is a likely cause of surface excess at the solid-liquid interface, resulting in the instantaneous decrease in surface tension. This study also provides a practical methodology of comparison for surface tension of NB dispersion: surface tension shall be compared among different samples with the same measurement method. Especially in the PD method, retention time of droplets before measurement shall be the same among samples.

Automated summary

The effect of nanobubbles (NBs) on liquid surface tension was studied using three different methods, namely pendant drop (PD), Wilhelmy, and du Nou'y methods, over a wide range of bulk NBs (BNBs) concentrations. In all three methods, the surface tension decreased proportionally with the BNBs concentration, although the proportional constant varied. This behavior was attributed to the surface excess of BNBs at the gas-liquid or solid-liquid interface. The decrease in surface tension was observed to be gradual in the PD method, taking several hundreds of seconds, while it was instant in the Wilhelmy and du Nou'y methods, likely due to electrostatic forces at the solid-liquid interface.