Optical density inferences in aqueous solution with embedded micro/nano bubbles: A reminder for the emerging green bubble cleantech

As an environmental friendly cleantech, micro/nano bubbles (MNBs) have been increasingly used for pollution control in cleaner production and water renewability improvement, and many works used optical density (OD) as proxy for the concentration of chemical/biological substances in liquid samples. This study reported that MNBs induced considerable light scattering and impacted the measured OD of liquid samples. The experimental results showed that bubble aeration increased the OD at wavelengths of 300–600 nm by 0.14–0.50 in commonly used bio-cultural medium and deionized water. The scattered light that fell outside the effective forward range increased the OD, and this increase was dependent on the bubble size and light wavelength. The Mie calculations showed that the scattering of smaller bubble is stronger and results in more OD increase. By defining the OD deviation <0.01, the critical thresholds of bubble size were calculated using the Kepler conjecture and Beer-Lambert’s law. For the commonly used wavelength 600 nm, the OD increase can be neglected only for bubbles with diameter >263 μm in water. The finite-difference time-domain simulation illustrated the highly nonuniform electric field near the bubble, and verified the higher scattering efficiencies at shorter wavelength. De-bubbling samples from MNB systems via short-term sonication eliminated light scattering. Findings from this work delineated the optical properties of MNBs, highlighted resulting impact on optical measurement and proposed a de-bubbling method in water treatment.