How to define a mixture of two miscible liquids at the inlet?

Hello,

I have a question about how to define a mixture of two miscible liquids (for instance water and ethanol) at the inlet.

My 2D geometry is as follows: I have two parallel rectangles (representing a channel through which liquid flows) and the two parallel rectangles are connected by a narrow third rectangle so that they are interconnected and it looks like the capital letter i.

The liquids flowing in the upper and bottem parallel rectangle are immiscible (for instance heptane at the top and water at the bottem) and I want to model the interface where these two liquids meet (in the third rectangle which interconnecteds the upper and bottem channel). To do this I use the "Two-Phase Flow, Phase Field" model from the physics tree.

The problem however is that I do not want to define a pure liquid at one of the two inlets (for instance not just water), but I want to define a mixture of two liquids (ethanol and water for instance) so that a part of the ethanol for example disolves into the other liquid (heptane) and you have some kind of extraction and how it affects the liquid-liquid interface.

Does anyone has some hints on how to do this?

Jonas