Effect of gas generation by chemical reaction on viscous fingering in a Hele-Shaw cell

Herein, the effect of gas-bubble generation by a chemical reaction on viscous fingering (VF) is investigated using a Hele-Shaw cell in a miscible two-phase liquid. Sodium bicarbonate (NaHCO3) and citric acid (C6H8O7) solutions were used as displacing and displaced fluids, respectively. As factors affecting the displacement pattern with gas bubbles, four characteristic times of displacement, chemical reaction, bubble nucleation, and bubble coalescence, as well as the viscosity ratio, were discussed. In the experiments conducted herein, the characteristic time of the chemical reaction was shorter than those of other characteristic factors. Bubble coalescence occurred quickly, and the coalescence time was almost the same as the nucleation time. Therefore, if the displacement time changes with the injection flow rate, then the flow pattern changes depending on the competition between the displacement and nucleation times. When the displacement time was shorter than the nucleation time, the bubble generation did not follow the onset of VF. First, a VF pattern was formed, and small gas bubbles were then generated in the mixture inside the fingers. On the backbone of the fingers, small gas bubbles lined up and grew bigger with time. Moreover, when the nucleation time was lower than the displacement time, the bubbles coalesced more rapidly, thereby inducing outward flow with gas nucleation in addition to fluid injection. These gas bubbles prevented the mixing of the displacing and displaced fluids. Furthermore, the effects of C6H8O7 concentration and the viscosity ratio were discussed from the viewpoint of the characteristic time.