Evaporation-Driven Water-in-Water Droplet Formation.We present new observations of aqueous two-phase system (ATPS) thermodynamic and interfacial phenomena that occur inside sessile droplets due to water evaporation.  To understand this phenomenon, we first study the secondary W/W droplet formation process on different substrate materials, namely, glass, polycarbonate (PC), thermoplastic elastomer (TPE), poly(dimethylsiloxane)-coated glass slide (PDMS substrate), and Teflon-coated glass slide (Teflon substrate), and show that secondary W/W droplet formation arises only in lower-contact-angle substrates near the three-phase contact line. 
THERM