The jamming and unjamming transition in poly(N-isopropylacrylamide) microgel suspensions

The jamming and unjamming transition in thermo-sensitive poly(N-isopropylacrylamide) (PNIPAm) microgel suspensions was observed by rheology with three independent variables of temperature T, microgel weight fraction w, and the shear stress σ. The effective volume fraction φeff, depending on both T and w, was adopted to interpret this transition. In the jamming state, the plateau modulus G ′ p followed the same power law to φeff with varying T and w, indicating that the change of the interaction potential with temperature was insignificant. However, the volume fraction φc at the jamming transition decreased with increasing w because higher concentration of particles led to stronger repulsive interaction. The shear-unjamming transition occurred as the shear stress exceeded the yield stress σy. A jamming phase diagram for the microgel suspension was established in a [1/w, σRh3/(kBθ), T] coordinate. The nonlinear viscoelasticity was observed by the large amplitude oscillatory shear (LAOS) analysis within the shear-unjamming region, which suggested that the shear-unjamming state under high strain would not be equivalent in rheology to the unjamming state appeared at high temperature or low concentration without shearing.