Ravi Kumar Pujala

Ergodicity breaking and aging dynamics in Laponite–Montmorillonite mixed clay dispersions

Ergodicity breaking and aging dynamics in a mixture of colloidal clays, Laponite (L) and Montmorillonite (MMT), were investigated. The intensity of the light scattered off L-MMT dispersions scaled with solid volume fraction (ϕ) as, I ∼ ϕα, with α = 1.49, for Laponite α = 0.95, for 2 × 10−4 τEB), the slow mode relaxation was no more diffusive and revealed τ2 ∼ q−1, indicating dynamic arrest of the platelets in the glass phase. The temporal evolution of the slow mode relaxation time τ2 in full aging regime i.e. above τEB, followed a power law growth. The τEB was found to decrease with concentration as τEB ∼ exp(−ϕ/ϕ0). A sharp rise in anisotropy, Dp in the L-MMT glass system occurring precisely at the ergodicity breaking time was observed with the overall aging dependence given by Dp ∼ t0.46w. Heterogeneity in these samples was probed from the Cole–Cole plot of rheology data. It is concluded that the aging dynamics was governed by the Laponite platelets with the MMT providing a suitable matrix for appropriate interparticle interactions.

Slow dynamics, hydration and heterogeneity in Laponite dispersions

Age dependent hydration, heterogeneity and network rigidity of aqueous Laponite dispersions in their sol, gel and glass phases have been studied systematically in the clay concentration range c = 0.3–3.5 % (w/v) which revealed the following interesting features: (i) both in the sol and gel regime (c < 1.8% (w/v)), the intensity of light scattered I(q,c) scaled with concentration c as, I(q,c) ∼ cα with α = 0.95 at t = 0, and 0.63 after t = 6 months implying that this temporal growth resulted from the formation of a colloidal gel; (ii) in the glass phase (c ≥ 2% (w/v)), scattered intensity from samples remained constant (α = 0), meaning that there was disorder in the system at higher solid contents, and the clay particles were randomly arranged, and this intensity profile did not change with the aging; (iii) the dynamic structure factor data showed low concentration dispersions (gels) having a single mode relaxation time (fast mode) which shifted to longer relaxation time with aging. In the glass phase, in addition to the fast mode that remained invariant of aging, a slow mode which shifted to longer relaxation time with aging was noticed; (iv) rigidity modulus (G′) and yield stress (σ) measurements showed that there are two universal power-law relations governing their behavior as a function of clay concentration; G′, σ ∼ cδ with δ = 2.3 for gels and 2.8 for glass. Both these parameters revealed similar logarithmic time dependence with aging as σ, G′ = βln(tw/tm), where β is a constant and tm is the characteristic time scale for the development of micro structures; (v) Raman spectroscopy data revealed three distinct OH bands characteristic of the structure of water which revealed change in the structured water content with the aging, and the heterogeneity of the samples was established using a Cole–Cole plot. We have proposed a time-dependent phase diagram for the salt free suspensions of Laponite clay using the abovementioned results.

Unified scaling behavior of physical properties of clays in alcohol solutions

This paper reports observation of universal scaling of physical properties of clay particles, Laponite (aspect ratio=30) (L) and Na Montmorillonite (MMT, aspect ratio=200), in aqueous alcohol solutions (methanol, ethanol and 1-propanol) with solvent polarity, defined through reaction field factor f(OH)(ɛ(0),n)=[(ɛ(0) - 1/ɛ(0) + 2) - (n(2) - 1/n(2) + 2)], at room temperature (20°C). Here, ɛ(0) and n are the static dielectric constant and refractive index of the solvent concerned. Physical properties (Z) such as zeta potential, effective aggregate size, viscosity and surface tension scaled with the relative solvent polarity as Z∼δf(α); δf=(f(w)(ɛ(0),n) - f(OH)(ɛ(0),n)), where f(w)(ɛ(0),n) is the reaction field factor for water, Z is the normalized physical property, and α is its characteristic scaling exponent. The value of this exponent was found to be invariant of aspect ratio of the clay but dependent on the solvent polarity only.

Materials and Characterization Techniques

This chapter reviews characterization techniques with emphasis on their introduction and principle of operation for the experiments to achieve the goal of this thesis. Lights scattering techniques like dynamic light scattering, static light scattering, depolarized light scattering; deformation technique like rheology, microscopic technique such as transmission electron microscopy were used to probe the samples. Other techniques such as Raman spectroscopy, viscometry and tensiometry were also used. In addition, we also give brief accounts of materials such as clays and their characterization and solvents used in this thesis work.

In-situ Observation of Hierarchical Self-Assembly Driven by Bicontinuous Gelation in Mixed Nanodisc Dispersions

The search for new functional soft materials with precise and reconfigurable structures at the nano and meso-scale is a major challenge as well as objective of the current science. Patchy colloids of different shapes and functionalities are considered important new building blocks of a bottom-up approach towards rational design of new soft materials largely governed by anisotropic interactions. Herein, we investigate the self-assembly, growth of hierarchical microstructures and aging dynamics of 2D nano-platelets of two different aspect ratios (Laponite ~25 and Montmorillonite ~250) which form gels with different porosity that is achieved by tuning their mixing ratios. Qualitative in situ real-space studies are carried out, including fluorescent confocal microscopy imaging of the bicontinuous gelation process or final states, which provides dynamic visualization of the self-organization. The bicontinuous gels exhibit a foam-like morphology having pores of a few micrometers in size that can be tuned by varying the mixing ratio of nanoplatelets. It is shown that this new class of clay gels has unique and tunable physical properties that will find potential applications in the development of low cost lithium ion batteries, nanocomposites and nuclear waste management.

Anisotropic Ordering in Nanoclay Dispersions Induced by Water–Air Interface

This chapter reports the kinetics of interface induced generation and propagation of arrested phase caused anisotropy in Laponite dispersions probed by depolarized light scattering experiments. Growth of anisotropy as function of waiting time and temperature have been explored in detail. The relaxation dynamics of the dispersion induced a water–air interface has been studied systematically.

Phase Diagram of Aging Laponite Dispersions

This chapter presents the age dependent hydration, heterogeneity and network rigidity of aqueous Laponite dispersions in its sol, gel and glass phase. Time dependent 3-D phase diagram of Laponite dispersions was proposed. The 3-D phase diagram is based on the hydration of the extensively aged Laponite dispersions, scattering techniques and the rheological studies.

Thermal Ordering in Mixed Nanoclay Dispersions

This chapter investigates the temperature induced orientational ordering in 1:1 Laponite-MMT dispersion. This dispersion showed thermally activated irreversible conformational phase transition from mostly isotropic (disordered) to strongly anisotropic (ordered) phase at temperature ≈60 °C. This phase transition was modeled through Landau formalism.

Phase Diagram of Aging Montmorillonite Dispersions

This chapter investigates the phase diagram of extensively aged MMT dispersions. Formation of gel through different routes is reported. Distinctive phase separation, equilibrium fluid and equilibrium gels in the t w -c phase space are observed for the first time ever in MMT dispersions (Fig. 5.1).

Aging Dynamics in Mixed Nanoclay Dispersions

This chapter investigates the ergodicity breaking and aging dynamics in mixture of colloidal clays, Laponite (L) and Montmorillonite (MMT). The relaxation dynamics has been studied systematically through the light scattering experiments. Spontaneously evolved phases like gel and glass phases have been identified. The growth of anisotropy of the system with age of the sample is observed in the mixed clay dispersions for the first time ever in the literature.