Study of the local critical coalescence concentration (l-CCC) of alcohols and salts at bubble formation in two-phase systems

The acoustic emissions generated by bubbles when they form are well understood and can be easily measured using a hydrophone and amplifier. Bubbles emit an audible sound not only when they form, but also when two or more coalesce. In this case, however, the amplitude of the sound is higher than after bubble formation. The difference in amplitude is enough to tell between bubble formation and bubble coalescence. Based on this property, the capability of alcohols and salts to prevent coalescence right after bubble formation at a capillary tube was studied. In general, the higher the gas flow rate through the capillary the more intense the collisions between subsequent bubbles, which eventually leads to coalescence, hence a higher reagent concentration in the system is needed to protect the bubbles against it. The reagent concentration at which coalescence is prevented can be seen as a local critical coalescence concentration (l-CCC) at the gas flow rate tested. This allows generating a curve of l-CCC vs. gas flow rate that can be used for comparison between different reagents. The paper presents results of l-CCC curves for alcohols and salts. The l-CCC curves show a comparable effect on coalescence prevention between 0.4 M NaCl and 8 ppm MIBC (a common frother), which is in agreement with the literature (Quinn et al., 2007).