Formation and Shear Yielding Dynamics of Poly( N ‐isopropylacrylamide)‐Based Colloidal Gels

This review summarizes our recent studies on the formation criteria of attractive colloidal gels from poly( N -isopropylacrylamide)-based microgel suspensions and the shear yielding behavior of the dilute colloidal gels. Long-term stable colloidal gels are achieved by appropriately balancing attractive interparticle interactions via hydrophobicity and repulsive interparticle interactions via surface electric charge, specifically by controlling pH, ionic strength, and the amount of charged group on network chains. Within the parameters that achieve colloidal gelation, the gel modulus can be tuned over three orders of magnitude by varying the degree of electric repulsion screening. Dilute colloidal gels with particle volume fractions of approximately 0.1 exhibit yielding when the applied strain exceeds a threshold value. This shear yielding is characterized by strain-induced phase separation into a condensed gel and water, accompanied by a stress-independent delay time ( t D ). The stress-independent t D indicates that the yielding observed here represents a previously unidentified type, as t D in the yielding of conventional colloidal gels is typically stress-dependent.