Experimental evidence of the effect of aging on the yielding and pre-yielding behavior of bentonite and laponite suspensions

Abstract

Thixotropic yield stress materials show a shear-induced solid-liquid transition at the yielding point, characterized by yield stress and yield strain. It is well known in the literature that the elastic modulus and the yield stress of thixotropic materials increase with aging time. In the current work, we propose a discussion on the brittleness of a suspension of swollen bentonite in water, focusing mainly on the role of aging times on the yield strain and on the critical strain at the linear to nonlinear viscoelastic transition of the material. The yield strain was measured in creep and constant shear rate start-up experiments, whereas the linear to nonlinear viscoelastic transition was evaluated from Fourier transforms on transient data in oscillatory shear stress amplitude sweeps. We show that aging increases material brittleness since the yield strain decreases with the resting time. On the other hand, the linear to nonlinear viscoelastic transition strain is surprisingly unaffected by the aging process. Other thixotropic systems were also investigated: 8 and 10 wt. % suspensions of bentonite in water and a 2 wt. % suspension of Laponite® in tap water. These lead to similar observations, showing constant linear to nonlinear viscoelastic strains and decreasing yield strains over increasing aging times. These findings bring relevant information to the intricate open-discussion issue on how to describe the behavior of thixotropic materials below the yield stress.

Studying the aging of Laponite suspensions using extensional rheology

“…This occurred at the same value for all concentrations studied and was not influenced by the aging of the samples. This observation matches those made in a recent study by Wendt et al [ 33 ] who studied suspensions of another thixotropic clay, Bentonite. At strain amplitudes less than γ lve , G′ far exceeds G″ for all concentrations, with values of tan( δ ) ranging from 0.05 for t w = 0 to 0.02 for t w = 320 min.…”

“…It therefore seems correct to say that the Laponite suspensions become increasingly brittle as the samples age. This is consistent with the shear measurements, where reductions in a sample’s yield strain are also sometimes equated with a more brittle behaviour [ 33 , 47 ]. One might therefore expect extensional necking/break-up as a function of sample Hencky strain to be a useful gauge by which a samples ductile–brittle nature could be compared.…”

“…The exponents during both the initial and latter stages decrease with increase in sample concentration. A few studies of thixotropic clays (Bentonite [ 33 ], Na-Montmorillonite [ 14 ] and Laponite [ 14 , 26 ]) have observed power-law growth in the measured value of G′ with waiting time. These studies, however, saw just a single exponent for each sample.…”

“…In glassy thixotropic yield stress fluids, such as Laponite, particles in the quiescent fluid adopt progressively more energetically favourable local configurations with time. This can lead to a stress overshoot, such that the initial measured yield stress (static yield stress) differs from the value measured after the accumulation of significant strain (dynamic yield stress) [ 33 ]. Measurements of the static yield stress σ ys have been defined either as the departure from linearity or the maximum measured stress [ 40 , 42 ].…”

“…In addition to a growing yield stress, there is also a monotonic decrease in the yield strain, the strain at which the stress maximum is reached. Wendt et al studying the thixotropic clay Bentonite in simple shear also observed an increase in the stress overshoot and a decreasing yield strain in samples allowed to age for longer [ 33 ]. They concluded that aging resulted in an increasingly brittle sample.…”