Rheological aspects of colloidal gels in thermoresponsive microgel suspensions: formation, structure, and linear and nonlinear viscoelasticity

This review focuses on the rheological aspects of colloidal gels that are a three-dimensional sparse network made of aggregated attractive particles formed in the aqueous suspensions of microgels composed of thermoresponsive polymers. Heating changes the dominant interparticle interactions from repulsive to attractive because of the hydrophilic-to-hydrophobic transition. Under appropriate conditions, the hydrophobic microgel suspensions form colloidal gels behave as a yield fluid. The elastic and yielding features of the colloidal gels are considerably different from those of the repulsive glass which is formed by the dense packing of the hydrophilic microgels at low temperatures. The thermoresponsive microgel suspensions undergoing colloidal gelation have attracted much attention from not only the academic interests but also the potentials as a functional suspension because they show interesting and marked changes in viscoelasticity when subjected to temperature variation. We discuss the criteria and dynamics of colloidal gelation, the structure, and linear and nonlinear viscoelasticity of the colloid gels with an emphasis on the results of the experimental studies.