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Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous media. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. Particularly, aqueous dispersion of Laponite undergoes liquid - to - solid transition at about 1 volume % concentration. The evolution of the physical properties as dispersion transforms to solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behaviors. Furthermore, the continuously developing microstructure that is responsible for eventual formation of soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. Particularly extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or repulsive glass like. We carefully examine and critically analyze the literature and propose a state diagram that suggests aqueous Laponite dispersion to be present in an attractive gel state.
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Investigating microstructure of suspensions with particles having anisotropic shape that share complex interactions is a challenging task leading to competing claims. This work investigates phase behavior of one such system: aqueous Laponite suspension, which is highly contested in the literature, using rheological and microscopic tools. Remarkably, we observe that over a broad range of Laponite (1.4 to 4 weight %) and salt concentrations (0 to 7 mM), the system overwhelmingly demonstrates all the rheological characteristics of the sol-gel transition leading to a percolated network. Analysis of the rheological response leads to fractal dimension that primarily depends on the Laponite concentration. We also obtain the activation energy for gelation, which is observed to decrease with increase in Laponite as well as salt concentration. Significantly, the cryo-TEM images of the post-gel state clearly show presence of a percolated network formed by inter-particle bonds. The present work therefore conclusively establishes the system to be in an attractive gel state resolving a long-standing debate in the literature.
Soft glassy materials are out of thermodynamic equilibrium and show time dependent slowing down of the relaxation dynamics. Under such situation these materials follow Boltzmann superposition principle only in the effective time domain, wherein time dependent relaxation processes are scaled by a constant relaxation time. In this work we extend effective time framework to successfully demonstrate time - temperature superposition of creep and stress relaxation data of a model soft glassy system comprised of clay suspension. Such superposition is possible when average relaxation time of the material changes with time and temperature without affecting shape of the spectrum. We show that variation in relaxation time as a function of temperature facilitates prediction of long and short time rheological behavior through time - temperature superposition from the experiments carried out over experimentally accessible timescales.
Physical properties of out of equilibrium soft materials depend on time as well as deformation history. In this work we propose to transform this major shortcoming into gain by applying controlled deformation field to tailor the rheological properties. We take advantage of the fact that deformation field of a certain magnitude can prevent particles in an aging soft glassy material from occupying energy wells up to a certain depth, thereby populating only the deeper wells. We employ two soft glassy materials with dissimilar microstructures and demonstrate that increase in strength of deformation field while aging leads to narrowing of spectrum of relaxation times. We believe that, in principle, this philosophy can be universally applied to different kinds of glassy materials by changing nature and strength of impetus.
Aqueous dispersion of Laponite, when exposed to carbon dioxide environment leads to in situ inducement of magnesium and lithium ions, which is, however absent when dispersion is exposed to air. Consequently, in the rheological experiments, Laponite dispersion preserved under carbon dioxide shows more spectacular enhancement in the elastic and viscous moduli as a function of time compared to that exposed to air. By measuring concentration of all the ions present in a dispersion as well as change in pH, the evolving inter-particle interactions among the Laponite particles is estimated. DLVO analysis of a limiting case is performed, wherein two particles approach each other in a parallel fashion a situation with maximum repulsive interactions. Interestingly it is observed that DLVO analysis explains the qualitative details of an evolution of elastic and viscous moduli remarkably well thereby successfully relating the macroscopic phenomena to the microscopic interactions.
We study diffusion of heat in an aqueous suspension of disc shaped nanoparticles of Laponite, which has finite elasticity and paste-like consistency, by using the Mach-Zehnder interferometer. We estimate the thermal diffusivity of the suspension by comparing the experimentally obtained temperature distribution to that with analytical solution. We observe that despite highly constrained Brownian diffusivity of particles owing to its soft glassy nature, suspensions at very small concentrations of Laponite demonstrates significant enhancement in thermal diffusivity. We correlate the observed enhancement with the possible microstructures of the Laponite suspension.
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