Magnesium drives a switch-like behavior in RNA droplets, creating distinct RNA coacervates. (a) Solution turbidity measurements of a sequential MgCl2 titration showing a window of miscibility (150–250 mM MgCl2), indicative of droplet switching. ([RP3] = 500 µM, 2x polyU wt/wt). (b) Theoretical free energy curvature vs. salt volume fraction calculated using a model (SI Note 1) based on Voorn-Overbeek theory of complex coacervation in the context of Flory-Huggins mean field theory. (c) Confocal fluorescence images of Mg2+-dependent droplet switching. ([RP3] = 500 µM, 2x polyU wt/wt, 0.5 µM RP3-AF594, 0.5 µM FAM-UGAAGGAC).