I suspect that Photo Flo does not fully evaporate. However, if you use it as directed it will be sufficient to absorb into the emulsion and vanish when dry.

Photo Flo is mainly the nonionic surfactant called Triton X 100 and either propylene glycol (PF200) or ethylene glycol (PF600). The dilution ratios are different for these versions of Photo Flo, so if you mess up it will leave a residue.