Rheological Behavior of Aqueous Suspensions of Laponite: New Insights into the Ageing Phenomena

TL;DR

This study investigates how aging affects the rheological properties of laponite suspensions, revealing that salt concentration influences the microstructural evolution and dynamic response during aging.

Contribution

It provides new insights into the aging mechanisms of colloidal glasses and gels, highlighting the role of salt concentration on retardation time and microstructural changes.

Findings

Retardation time decreases with age in glass-like suspensions.

Salt concentration weakens the dependence of aging on retardation time.

Aging leads to increased ordering in glass states but not in gel states.

Abstract

In this paper, ageing behavior of suspensions of laponite with varying salt concentration is investigated using rheological tools. It is observed that the ageing is accompanied by an increase in the complex viscosity. The succeeding creep experiments performed at various ages showed damped oscillations in the strain. The characteristic time-scale of the damped oscillations, retardation time, showed a prominent decrease with the age of the system. However, this dependence weakens with an increase in the salt concentration, which is known to change microstructure of the system from glass-like to gel-like. We postulate that a decrease in the retardation time can be represented as a decrease in the viscosity (friction) of the dissipative environment surrounding the arrested entities that opposes elastic deformation of the system. We believe that ageing in colloidal glass leads to a greater ordering that enhances relative spacing between the constituents thereby reducing the frictional resistance. However, since a gel state is inherently different in structure (fractal network) than that of a glass (disordered), ageing in the same does not induce ordering. Consequently, we observe inverse dependence of retardation time on age becoming weaker with an increase in the salt concentration. We analyze these results from a perspective of ageing dynamics of both glass state and gel state of laponite suspensions.