A double film model describing the finite-miscibility structure of interfaces was established. Without losing generality, the viscosity stratified flow was taken as an example in this approach. The double film model was validated by the break interface model and single film model proposed by previous researchers. The results indicate that the viscosity profile within the interface film of fluids has strong effects on the flow stability and the double film model is a universal model to describe the interface film structure of liquid-liquid flow. The interface break model and single film model are two special cases of the double film model. Furthermore, the double film model was adopted to investigate the stability of liquid-liquid stratified flow. The effect of liquid-liquid miscibility, film thickness, interface location and viscosity ratio on the stability of stratified flow was outlined.

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