Probing the Structure and Rheology of Suspensions Exhibiting Reversible Intermediate-Range Clustering

Under certain conditions, the sub-micron-sized particles of a colloidal suspension can aggregate into equilibrium clusters that (1) self-terminate their growth, (2) are small enough to avoid sedimentation, and (3) can be readily de-aggregated. Such clusters emerge owing to a competition between opposing short- and long-range forces between the particles, and the ability to precisely tune in these forces to purposely induce clustering has implications for, e.g., the stabilization of high-concentration protein dispersions with medical applications. However, there remain many open questions regarding how precisely to identify clustering given only information about pair structural correlations (from, e.g., x-ray scattering data) and the implications that reversible clustering has upon dynamics and suspension rheology. Using molecular simulations and analytical theory, we validate a physically-intuitive criterion for detecting cluster formation based on the thermal correlation length, which can be used given knowledge of only the static structure factor. We also highlight stark dynamic differences between systems that form amorphous clusters versus those that exhibit microcrystallization under quiescent conditions. Finally, using non-equilibrium simulations that incorporate hydrodynamic coupling, we elucidate how reversible clustering influences rheological responses to applied shear for given bonding strengths and aggregate sizes at rest.