Abstract
Surface tension of any fluid is crucial for multiphase systems and is often manipulated during industrial processes by introducing surfactants. Normally, water used in an industry includes impurity such as salt. In this study, effect of NaCl concentrations on surface tension of aqueous solution, which is treated with bubbling process of CO_2 or N_2, was investigated under microwave irradiation. It has found that surface tension decreased quickly, while temperature increased during microwave. Once the radiation is turned off, the temperature returns rapidly as expected. However, surface tension recovery was much slower than temperature. Moreover, the reduction amounts obtained by non-thermal effect depend on salt concentrations and gas type. For example, effect of dissolved gas was obtained for CO_2 gas. Solubility of CO_2 is higher than that of N_2, and larger amount of CO_2 gas is released during microwave heating. The release prevents formation of nano-bubbles or cluster network of water molecules, which play important roles for surface tension reduction during microwave irradiation because the growth of larger bubbles captures smaller bubble. On the other hand, N_2 is not dissolved so much. Bubble formation of N_2 does not affect nano-bubble formation so much. Accordingly, similar non-thermal effect for surface tension reduction was obtained in case of N_2 gas.
