Clusters of multi-valent molecules are often very liquid, because the affinities of many bimolecular interaction (Mayer et al., 2009) are quite modest, implying that off-rates are relatively rapid. However, by increasing concentrations of each molecule type it is possible to form aggregates that undergo sol-gel transition (Li et al 2012). One challenge when working with physiological proteins, rather than constructs, is to identify the affinities and valency of each molecule type. Titration experiments with two molecule types covering the sol-gel transition region may help identify such affinities and valencies.
W. H. Stockmayer developed an analytical solution predicting the sol-gel transition in systems of two molecule types (A and B) under the equal reactivity assumption. This means that each site type a in molecule A has equal probability of binding to any site type b in all molecules B. Similarly, each site b has equal probability of binding any site type a (Journal of Polymer Science, V9: 69-71, 1952).
Below is the phase diagram for systems of two molecule types. Notice how the shape of the phase diagram changes with valences vA and vB and the dissociation constant between each site pair KD.