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Yogesh M. Joshi

Department of Chemical Engineering
Indian Institute of Technology Kanpur, India

In this presentation, we will discuss ageing behavior of aqueous suspension of laponite, a model soft glassy material under the deformation field. In general, we observe that viscoelastic behavior of this material is time dependent and is strongly influenced by the deformation field ; the effect is known to arise due to ageing and rejuvenation. We show that irrespective of strength of deformation field (shear stress) and age, when imposed time-scale is normalized with dominating relaxation mode of the system, universal ageing behavior is obtained demonstrating "time-stress superposition ;" the phenomena that may be generic in variety of soft materials. We further discuss the effect of oscillatory stress and temperature on the ageing dynamics of the same. We observe that at the higher magnitude of stress, elastic and viscous moduli of the system undergo a sharp rise with the ageing time. The age at the onset of rise and the sharpness of the same increase with the magnitude of stress. We discuss this dynamics in the context of interplay between the energy barrier distribution of the arrested particles and an enhancement in the potential energy due to the deformation field. With regard to temperature dependence, an increase in temperature is observed to accelerate the ageing process by shifting the ageing dynamics to a lower ageing time. We propose that the microscopic relaxation dynamics, which causes ageing, becomes faster with increase in the temperature and leads to the observed behavior. In stress relaxation experiments on the same system, we observe a strong over-ageing character of the material ; wherein, the relaxation dynamics becomes more sluggish with increase in the magnitude of step strain. We discuss these results in the context of a trap model for jammed soft materials.