(a) Phase diagram of PEG 8000-Na2SO4-water system and solution systems used in the present study. Region I is a one-phase region and region II is a two-phase region. When the concentration is set in red triangle, the solution is separated into two phases, phase L and phase H. The solution compositions used in the immiscible, fully miscible, and partially miscible systems are indicated by red, black, and green circles, respectively. (b) When a more viscous liquid is injected from a single point into a Hele-Shaw cell (a gap between two parallel flat plates with a very small distance b (= 0.3 mm)) filled with a less viscous liquid, their interface becomes hydrodynamically stable and spreads out in a circle. (c) Hydrodynamically stable displacement in the immiscible, fully miscible, and partially miscible systems for a given flow rate. The pictures are taken when the longest radius of the displacement pattern is 42 mm and the injection time…



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