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Abstract
Nanobubbles (NBs)βgas inclusions in water with diameters < 1 Β΅mβare 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 (π
πππ ) which is considerably larger than liquids (π
π€ππ‘ππ) 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 an accurate and selective characterization on NBs in complex water mixtures that will help understand the behaviour of NBs and accelerate their application in many fields.
Supplementary materials
Title
Supplementary Information for SONIC Paper
Description
Additional text is included that provides in detail the development of equations, the cross-correlation method, and the calibration of the acoustic cell; figures that depict a schematic of the nanobubble generator, a flowchart of the how to use SONIC, results from Type βAβ design, a typical output of an acoustic measurement, results of the Tyndall effect of nanobubble samples and controls, video frames of NB water samples and controls; and a table with time-of-flight measurements at different temperatures.
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