  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 thereby 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.

Soft Condensed Matter