Europhys. Lett.
Volume 76, Number 1, October 2006
Page(s) 74 - 80
Section Condensed matter: structural, mechanical and thermal properties
Published online 30 August 2006
Europhys. Lett., 76 (1), pp. 74-80 (2006)
DOI: 10.1209/epl/i2006-10234-2

Linear viscoelastic properties of aging suspensions

E. H. Purnomo, D. van den Ende, J. Mellema and F. Mugele

Physics of Complex Fluids, Department of Science and Technology, University of Twente P.O. Box 217, 7500 AE Enschede, The Netherlands

received 2 May 2006; accepted in final form 8 August 2006
published online 30 August 2006

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 densely packed suspensions at various temperatures below the volume transition temperature. The system is found to display a strong aging behavior. The viscoelastic moduli are compared to the predictions of the soft glassy rheology model. The model predicts quantitatively the loss modulus $\ensuremath G^{\prime \prime }(\omega ,t)$, the elastic modulus $\ensuremath G^{\prime }(\omega ,t)$ and their ratio $\ensuremath G^{\prime
\prime }(\omega ,t)/G^{\prime }(\omega ,t)$. A relative noise temperature of $\ensuremath (\mathcal{X}/\mathcal{X}_{g})\approx 0.62$ is obtained. Despite the fact that the microgel particles are more compressed and hence less mobile at a lower thermodynamic temperature T, $\ensuremath (\mathcal{X}/\mathcal{X}_{g})$ is found to be essentially independent of T within the range of $5\,^{\circ}$C to $10\,^{\circ}$C below the volume transition temperature.

61.43.Fs - Glasses.
83.60.Bc - Linear viscoelasticity.
83.60.-a - Material behavior.

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