Soft particle pastes (SPPs) are composed of deformable particles randomly packed into a dense suspension. Examples of constituent soft particles include polymer coated colloids (R~20 µm), polyelectrolyte microgels (R~200 nm) and star polymers (R~10 nm). In spite of the differences in the particle sizes and source of elasticity, all of these materials show similar rheological characteristics and aging behavior. That is, their microstructure and rheological properties change slowly with time without apparent end.

A pairwise interaction theory is developed to predict the elastic moduli, shear stress and normal stress differences of soft-particle pastes. The theory is based on the equilibrium radial distribution function and its perturbation due to flow, which is convoluted with the appropriate elastic and hydrodynamic pairwise interaction to compute the properties of interest. As part of this theory, a methodology is presented to compute a priori the radial distribution function for the quiescent or “equilibrium” glassy SPPs based on free volume considerations and energetic constraints.

The theory is compared to experimental observations and computational simulations of the viscoelastic properties of the paste.