Observation of Kinetic and Structural Scalings during Slow Coalescence of Nanobubbles in an Aqueous Solution

The addition of salt can induce the slow coalescence of nanobubbles (approximately 100 nm) in an aqueous solution of alpha-cyclodextrin (alpha-CD). A combination of static and dynamic laser light scattering was used to follow the coalescence. Our results reveal that its kinetic and structural properties follow some scaling laws; namely, the average size ( ) of the nanobubbles is related to their average mass ( ) and the coalescence time (t) as approximately (d(f)) and approximately t(gamma) with two salt-concentration-dependent scaling exponents (d(f) and gamma). For a lower sodium chloride concentration (C(NaCl) = 40 mM), gamma = 0.13 +/- 0.01 and d(f) = 1.71 +/- 0.02. The increase of C(NaCl) to 80 mM results in gamma = 0.32 +/- 0.01 and d(f) = 1.99 +/- 0.01. The whole process has two main stages: the aggregation and the coalescence. At the lower C(NaCl), the process essentially stops in the aggregation stage with some limited coalescence. At higher C(NaCl), coalescence occurs after the aggregation and results in large bubbles.