Rheological Properties of Aging Suspensions

Eko H. Purnomo, Dirk van den Ende, Frieder Mugele and Jorrit Mellema

Physics of Complex Fluids Department of Science and Technology, University of Twente

P.O. Box 217, 7500 AE Enschede, The Netherlands

The slow dynamics of soft glassy materials have attracted quite some attention in the last decades and have been found in amorphous polymers, spin glasses, foams, densely packed suspensions, and living cells. Aging of the materials is one of the consequences of the slow dynamics: the relaxation processes slow down with the age of the material but it never achieves an equilibrium state.

We have examined the linear viscoelastic behavior of poly-N-isopropylacrylamide (PNIPAM) microgel suspensions in order to obtain insight in the aging processes in these densily packed suspensions. The viscoelastic moduli are compared to the prediction of the soft glassy rheology model of Sollich et al. [1]. The model predicts quantitavely the loss modulus G"(ω,t), the elastic modulus G′(ω,t), and their ratio G"(ω,t)/G′(ω,t). The relative noise-temperature (x/x_g) of the PNIPAM suspension obtained from the analysis is smaller at lower temperature, when the microgel particles are more swollen, indicating a lower mobility of the microgel particles in a denser suspension.

[1] P. Sollich: Rheological constitutive equation for a model of soft glassy materials. Phys. Rev. E 58, 738-759 (1998).