Magnesium concentration alters the fluidity of heterotypic and homotypic droplets. (a) Scaled FRAP in heterotypic RP3-polyU droplets as a function of [MgCl2]. ([RP3] = 500 μM, 0.6x polyU wt/wt) The blue line is a visual guide. (b) Scaled FRAP in homotypic polyU droplets as a function of [MgCl2]. (2 mg/mL polyU, 10% PEG) The red line is a visual guide. (c) Cartoon representation of switch-like behavior between heterotypic and homotypic droplets as a function of increasing cation concentration. The dotted lines represent the phase boundaries of the distinct droplet types. The arrows indicate that these phase boundaries will move under changing conditions and are capable of overlapping.