Microstructure of dense colloid-polymer suspensions and gels

A systematic experimental study of polymer-induced changes of the collective structure of model hard-sphere nanocolloids in the fluid and gel states has been carried out using ultra-small-angle x-ray scattering. The focus is on small, non-adsorbing polymer depletants where a direct transition from the homogeneous fluid phase to a nonequilibrium gel state occurs with increasing polymer additions. As the polymer concentration is increased in the homogeneous fluid phase, the low angle concentration fluctuations monotonically increase, the characteristic interparticle separation decreases and tends to saturate, and the intensity of the cage order peak varies in a non-monotonic manner. These equilibrium structural changes depend in a systematic fashion on colloid volume fraction and polymer-colloid size asymmetry, and are in near quantitative agreement with the parameter-free polymer reference interaction site model theory calculations. By combining the accurate equilibrium theory with experimental observations, the loss of ergodicity and nonequilibrium structure formation in the gel state can be deduced. Abrupt departures between theory and experiment on the -2-3 particle diameter and greater length scales are observed as the gel boundary is traversed. The liquid-like local cage structure is arrested. Intermediate scale fluctuations are suppressed suggesting the formation of small, compact clusters. Large amplitude, Porod-like fluctuations emerge on large length scales due to quenched heterogeneities which are analysed using a random two-phase composite model. By combining the results of all the scattering experiments and theoretical calculations a qualitative real space picture of the gel microstructure is constructed, and its mechanical consequences are qualitatively discussed.