Pls. help about Free surface flow

AT

I modelled the change of the droplet shape when it hit the solid wall. I currently used a multiphase free surface with a liquid domain (droplet and air) and a solid domain (for wall). The droplet is initially at very high temperature and in liquid form. The solid is at room temperature.

When the droplet hit the substrate, there was heat transfer between droplet and the wall. The droplet is cool down and solidify. The phase change is also modelled.

Now i want to extend my model to further step. In fact, when the droplet hit the wall, the wall is heated. Some water on the wall will be released. This water vapor has very high pressure. So I suppose that this pressure will deteriorate the droplet shape. however, i have no idea how to descibe this pressure in the model.

I tried to use another fluid domain 2 (water) which is between two above domains. So I have a fluid fluid interface. Then I defined a pressure change at the interface. however, the model did not run in solver. if i changed this fluid domain exactly as the above fluid domain, it works but always mixing at the interface. Anybody know why we can not defind two fluids with different materials. and how to avoid the mixing at the fluid fluid interface?

I really appreciate anybody which can give me an idea about my problem. thank you in advance.