Abstract:
Classical viscous fingering (VF) instability, the formation of finger-like interfacial
patterns, occurs when a less viscous fluid displaces a more viscous one in porous
media in immiscible and fully miscible systems. However, the dynamics in partially
miscible fluid pairs, exhibiting a phase separation due to its finite solubility into
each other, has not been largely understood so far. This study has succeeded in
experimentally changing the solution system from immiscible to fully miscible or
partially miscible by varying the compositions of the components in an aqueous
two-phase system (ATPS) while leaving the viscosities relatively unchanged at room
temperature and atmospheric pressure. Here, we have experimentally discovered a new
topological transition of VF instability by performing a Hele-Shaw cell experiment
using the partially miscible system. The finger formation in the investigated partially
miscible system changes to the generation of spontaneously moving multiple droplets.
Through additional experimental investigations, we determine that such anomalous
VF dynamics is driven by thermodynamic instability such as phase separation due to
spinodal decomposition and Korteweg convection induced by compositional gradient
during such phase separation. We perform the numerical simulation by coupling
hydrodynamics with such chemical thermodynamics and the spontaneously moving
droplet dynamics is obtained, which is in good agreement with the experimental
investigations of the ATPS. This numerical result strongly supports our claim that the
origin of such anomalous VF dynamics is thermodynamic instability.
