  Solubility and interfacial energy are two fundamental parameters underlying
the competitive nucleation of polymorphs. However, solubility measurement of
metastable phases comes with a risk of solventmediated transformations which
can render the results unreliable. In this work, we present a rapid
microfluidic technique for measuring aqueous solubility of the metastable form
using KDP Phase IV as a model system. This bracketing approach involves
analyzing the dissolution behavior of crystals in contact with supersaturated
microdroplets generated via evaporation. Then, with the help of our recently
developed nucleation time measurement technique, together with Mersmann
calculation of interfacial energies from solubilities, we were able to access
the interfacial energies of both metastable and stable phases. To gain further
insights into the observed nucleation behavior, we employed the Classical
Nucleation Theory (CNT) to model the competition of polymorphs using our
measured solubility and calculated interfacial energies. The results show that
the stable form is favored at lower supersaturation while the metastable form
is favored at higher supersaturation, in good agreement with our observations
and experimental reports in the literature. Overall, our microfluidic approach
allows access to unprecedentedly deep levels of supersaturation and reveals an
interesting interplay between thermodynamics and kinetics in polymorphic
nucleation. The experimental methods and insights presented herein can be of
great interest, notably in the mineral processing and pharmaceutical industry.
