I'd engineered a binary fluid by mixing in different proportions and found that the thermophysical properties results come in between the properties of the constituent parts. Does this situation exists or not?
In some (even numerous) cases your observations is true. But we know many examples when the mixture behaviour become qualitatively different in comparison with components one. Best regards, Serge
In mixtures involving no chemical reaction, the physical properties such as density, specific heat of the mixture in most cases will be as you expected. However, it has to be verified whether the law of mixtures holds good for thermal conductivity and viscosity.
You need to read textbooks where you will find good information about the behavior of real mixtures. You will learn first about ideal behavior but most important about excess properties.
Books dealing about chemical engineering and thermodynamics from Stan Sandler, from J M Prausnitz or from A. Firozabadi are highly recommended.