Multiphase coexistence capacity in complex fluids

Complex fluids like the cytoplasm comprise hundreds of species organized into many coexisting phases. How molecular interactions, reflecting sequence, design, or functional constraints, dictate multiphase coexistence is a major open question. To answer this, we consider models of multicomponent fluids with both designed and random interactions. When crosstalk is introduced, we show that coexisting phases lose specificity beyond a common threshold. In a sequence model, we demonstrate that phase capacity is limited by sequence length and grows logarithmically/linearly with sequence length/number. These results provide a general route to program multiphase coexistence from molecular features.