SONIC: A Speed of Sound Measurement for Nanobubble Characterization

Nanobubbles (NBs)─gas inclusions in water with diameters <1 μm─are of growing interest because of their unique properties and their potential for transformative applications. For example, it has been reported that NBs exist in water over long periods (i.e., weeks to months) and can act as free gas reservoirs. However, NBs are a source of scientific debate, particularly regarding characterization methods. Conventional methods, such as dynamic light scattering, nanoparticle tracking analysis, and nanoflow cytometry, cannot distinguish between nanoparticles and NBs since they are insensitive to the differences of the physical properties of the materials. However, acoustic (speed of sound) measurements can be used to quantify NBs because they rely on the compressibility dependence of gases (κ_gas) which is considerably larger than liquids (κ_water) and solids. In the present work, a speed of sound measurement for nanobubble characterization (SONIC) was designed and developed to probe the compressibility variations diagnostic to NBs in water. NBs in water act as acoustic scatters that reduce the speed of sound relative to the bubble-free water. This decrease in the speed of sound can only be attributed to the existence of gas bubbles due to the strong compressibility dependence that solid nanoparticles lack. The results obtained from the acoustic measurements are compared with the observations from nanoparticle tracking analysis to confirm the existence of NBs in water. SONIC was validated in water with different molalities of NaCl (aq), and in the presence of solid nanoparticles of similar size and concentration to the NBs. SONIC is the first technique that addresses an important bottleneck of NB characterization by providing accurate and selective characterization of NBs in complex water mixtures that will help the behavior of NBs to be better understood and accelerate their application in many fields.