Does size matter? Elasticity of compressed suspensions of colloidal- and granular-scale microgels

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Microgels are particles consisting of a cross-linked polymer network swollen with solvent. Most colloids consist of incompressible solids suspended in solution, whereas microgels can be compressed and deformed. The degree of microgel deformation is governed by the cross-link density of the polymer network. Due to their compressibility, microgels can achieve much higher packing densities (jammed packing) compared to the hard spheres inherent to many typical colloidal solutions. When microgel particles become jammed, they are unable to rearrange in response to an applied stress, giving rise to an effective glass transition. However, it has been noted that thermally induced particle rearrangement can occur in these systems. It is difficult to describe the nature of these interactions because particle properties are not easily isolated from interparticle interactions.

In an effort to elucidate the thermal effects on elasticity of microgels, the author explores the elasticity and rheology of colloidal and granular microgels. By comparing microgels of widely varying sizes the effects of interparticle interactions can be accounted for. Due to the small size of colloidal microgels, Brownian interactions play a significant role in the stress response in the particle - hence their stress response will be a combination of intraparticle and interparticle interactions. However, for the larger granular microgels, it is hypothesized that Brownian interactions will play a negligible role in the stress response, eliminating the interparticle component of the stress response. By comparing the stress response of the colloidal microgels to the granular microgels in both compression and shear, the stress responses arising from intraparticle and interparticle interactions will be separated and evaluated.


Colloidal microgels where made of poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAAm-co-AAc)) via precipiation polymerization. Three different cross-link densities were created in the microgels - 0.1, 1.0, and 10 wt% relative to monomer concentration. Granular microgels were made from p(NIPAAm-co-AAc) as well, except particle size was maintained between 100-400 microns via emulsion polymerization. A second type of granular microgel of similar size was made from polyacrylamide again via emulsion polymerization.

Isotropic compression was performed on the colloidal and granular microgels of identical composition by way of osmostic pressure. The microgels were emmersed in a solution of polyethylene glycol (PEG) of known osmotic pressure and stressed between 1-200kPa. Shear stress was inflicted on all specimen types via a plate-plate rheometer.

Effect of Particle Size on Compressibility