Microfluidic Investigation of the Ion-Specific Effect on Bubble Coalescence in Salt Solutions

Abstract

A microfluidic method was developed to study the ion-specific effect on bubble coalescence in salt solutions. Compared with other reported methods, microfluidics provides a more direct and accurate means of measuring bubble coalescence in salt solutions. We analyzed the coalescence time and approach velocity between bubbles and used simulation to investigate the pressure evolution during the coalescence process. The coalescence time of the three salt solutions decreased initially and then increased as the concentration of the salt solution was increased. The concentration with the shortest coalescence time is considered as the transition concentration (TC) and exhibits ion-specific. At the TC, the change in coalescence time indicates a shift in the effect of salt on bubble coalescence from facilitation to initial inhibition. Meanwhile, it can be seen that the sodium halide solutions significantly inhibit the bubble coalescence and the inhibition capability follows the order NaCl > NaBr > NaI. The results of the approach velocity show that the coalescence time decreases with increasing approach velocity, as well as the approach velocity was strongly influenced by concentration. The approach velocity undergoes a significant change at the TC. Furthermore, simulations of bubble coalescence in the microchannel indicate that the vertical pressure gradient at the center point of the bubble pairs increases as bubbles approach, driving liquid film drainage until bubble coalescence. The pressure at the center of the bubble pair reaches the maximum when the bubbles have first coalesced. It was further revealed that the concentration of the salt solution has a significant impact on the maximum pressure, as evidenced by the observed trend of decreasing pressure values with increasing concentrations.

Coalescence of Two Equal-Sized Bubbles with Air Injection

“…Bubble coalescence plays a crucial role in the bubble size and void fraction distribution in gas–liquid two-phase flow systems and thus heat and mass transfer efficiency. , When two bubbles come into contact and merge, a kind of hydrodynamic singularity arises. , Subsequently, an air bridge (often called a “neck”) forms, connecting the two merging bubbles. Finite-approach velocities, arbitrary surface shapes, and fluid properties contribute to the variety. − The neck formation and temporal evolution are critical features in the study of bubble coalescence. In addition to its theoretical significance, a precise understanding of bubble coalescence improves the predictive accuracy of system performance in engineering. − The neck expansion resulting from the quasi-static bubble coalescence has been extensively studied. ,, In this case, the bubbles approach each other slowly, and no gas is introduced during the merging process.…”