Ageing under oscillatory stress: Role of energy barrier distribution in thixotropic materials

In this work the ageing dynamics of soft solids of aqueous suspension of laponite has been investigated under the oscillatory stress field. We observed that, at small stresses elastic and viscous moduli showed a steady rise with the elastic modulus increasing at a faster rate than the viscous modulus. However, at higher stresses both the moduli underwent a sudden rise by several orders of magnitude with the onset of rise getting shifted to a higher age for a larger shear stress. We believe that the observed behavior is due to interaction of barrier height distribution of the potential energy wells in which the particle is trapped and strain induced potential energy enhancement of the particles. Strain induced in the material causes yielding of the particles that are trapped in the shallower wells. Those trapped in the deeper wells continue to age enhancing the cage diffusion timescale and consequently the viscosity, which lowers the magnitude of strain allowing more particles to age. This coupled dependence of strain, viscosity and ageing causes forward feedback for a given magnitude of stress leading to sudden rise in both the moduli. Changing the microstructure of the laponite suspension by adding salt affected the barrier heights distribution that showed a profound influence on the ageing behavior. Interestingly, this study suggests a possibility that any apparently yielded material with negligible elastic modulus may get jammed at a very large waiting time. (C) 2009 Elsevier Ltd. All rights reserved.