If you are modeling a mixture of water and ethylene glycol (water & ethylene glycol) in a system and you have a system operating at temperatures below 0 degrees C (freezing point of water), you may encounter a warning message indicating that "solid phase" has been detected.
As the system has a temperature of below 0 degrees C, water in this case would be identified as solid as FluidFlow doesn't take into account chemical changes in fluid characteristics as a result of actually mixing it with ethylene glycol.
Please note, the software will however estimate physical properties for the component fluids in the mixture and solve accordingly.
It is also worth noting that the software doesn't make provision for mass transfer between phases due to solubility of a gas for example. Similarly, it wouldn't be possible to make a mixture of say hydrogen chloride and water and expect to get the properties of hydrochloric acid.
Design Note:
An important point here is that, if you define the physical properties of this mixture in the fluids Database as a "Pure Newtonian" fluid, the software will model the system more accurately and below 0 degrees C.