Sensing changes of surface properties of oxygen nano-bubbles in water by the ripplon surface laser-light scattering

Micro and nano-sized bubbles containing in water have been attracting increasing attention from wide variety of research fields because of their remarkable bioactive effects. Although many application studies have been conducted, the fundamental understanding such as existence and stability of especially nano-bubbles in water is still unsatisfactory. The purpose of the present study is to detect the changes of surface properties (surface tension and viscosity) of nano-bubbles in water by the newly developed ripplon surface laser-light scattering apparatus. The present measurement technique is able to detect nanometer-order-amplitude surface wave usually regarded as ripplons excited by thermal fluctuations. Because of this technique's nanometer-order sensitivity to surface properties, the present apparatus has the potential to detect the possible presence of nano-bubbles in the vicinity of water surface. We have observed temporal change of surface tension and viscosity of ultrapure water containing oxygen nano-bubbles in a closed container by the ripplon surface laser-light scattering apparatus for 2 weeks. When the bubbles was generated for at least 90 minutes by using a micro and nano bubble generator, the apparent surface tension and viscosity of ultrapure water containing oxygen nano-bubbles was decreased up to about 23% and was increased up to about 270% relative to those of reference water, respectively. It can be speculated from the present experimental evidence that the nano-bubbles are existing in the vicinity of water surface in a closed container.