Hi Robert, yes! I definitely have an interface. The primary phase is vapor (to_species) and I patch the liquid zone at the initialization step. I can plot the contour of the phases and I have liquid in the liquid zone and vapor everywhere else. The interface itself shows a gradient so it should be satisfying the if-statement condition. I initially thought that patching it would create a step change at the interface so the if-statement would not work but it seems to apply a gradient to those cells.