Wedge-shaped viscous fingering near the critical point of phase separation in a partially miscible system

Well-known viscous fingering (VF) occurs in a porous medium or in Hele-Shaw cell, which is a thin gap between two parallel plates, when a more viscous fluid is displaced by a less viscous one because of hydrodynamically unstable situation. The classical VF can be divided into two depending on whether two fluids are immiscible or fully miscible. We have shown droplets pattern in a partially miscible system and suggested the mechanism of droplets formation. Here, we discovered wedge-shaped (top-pointed) pattern and Kunai (Ninja-knife) pattern of viscous fingering near the critical point of phase separation in the partially miscible system, as which we employed aqueous two phase system (consisting of polyethylene glycol, sodium sulfate and water system) although our used solutions are Newtonian fluids without any chemical reactions. These patterns have never observed in classical miscible and immiscible systems. We suggest that these patterns should be induced by the competition among the rate of mutual solubility, the rate of phase separation and the flow rate.