We describe a complete microfluidic screening platform for protein crystallization that integrates fully programmable on-chip combinatorial mixing with dense assay storage using two-phase flows. This device supports rational crystallization strategies based on systematic solubility phase space mapping followed by nanoliter volume crystallization screening. Sequential formulation of arbitrary chemical mixtures and injection of nanoliter volume droplets into an immiscible carrier fluid creates a water/oil emulsion with independent control over droplet chemistry, size, frequency, and spacing. Long-term incubation of droplets with controlled dehydration is used to induce crystallization.