Ravi Kumar Pujala

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  Article: Slow dynamics, hydration and heterogeneity in Laponite dispersions

  Ravi Kumar Pujala
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  ABSTRACT: 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.
  Soft Matter 01/2013; · 4.39 Impact Factor
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  Article: Ergodicity breaking and aging dynamics in Laponite–Montmorillonite mixed clay dispersions

  RAVI KUMAR PUJALA, H.B. BOHIDAR
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  ABSTRACT: 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 < ϕ < 8 × 10−3, and it remained invariant above ϕ = 8 × 10−3. For MMT, α = 1.38 and no such invariance was noticed. Ergodic to non-ergodic transitions were observed in aged samples with a characteristic ergodicity breaking time, τEB ≈ 102 − 105 s. Dynamic structure factor, g1(q,t) exhibited two-mode relaxation, g1(q,t) = a exp(−t/τ1) + (1 − a) exp−(t/τ2)β. The stretching parameter, β was found to decrease linearly with aging time, tw whereas slow mode relaxation time increased as τ2 exp (btw) and τ1 remained invariant of tw. Spatial dependence of τ1 and τ2 showed, τ1,τ2 q−2 in the ergodic phase, considered arising from the diffusive motion. After the ergodicity breaking time (t > 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.
  Soft Matter 05/2012; · 4.39 Impact Factor
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  Article: Unified scaling behavior of physical properties of clays in alcohol solutions.

  Ravi Kumar Pujala, Nisha Pawar, H B Bohidar
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  ABSTRACT: 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.
  Journal of Colloid and Interface Science 09/2011; 364(2):311-6. · 3.07 Impact Factor
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  Article: Landau theory description of observed isotropic to anisotropic phase transition in mixed clay gels.

  Ravi Kumar Pujala, Nisha Pawar, H B Bohidar
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  ABSTRACT: A characteristic new cooperative dehydration transition, in 1:1 Laponite-MMT cogel, was observed at T(c) ≈ 60 °C, a temperature at which the storage modulus (G(')) and depolarization ratio (D(p)) showed sharp increase, and the isotropic cogel turned into an anisotropic one. The dehydration dynamics could be described through power-law relations: G(') ∼ (T(c)-T)(-γ) and D(p) ∼ (T(c)-T)(-β) with γ ≈ β = 0.40 ± 0.05. The x-ray diffraction data revealed that the crystallite size decreased from 17 nm (at 20 °C) to 10 nm (at 80 °C) implying loss of free and inter-planar water. When this cogel was spontaneously cooled below T(c), it exhibited much larger storage modulii values which implied the existence of several metastable states in this system. This phase transition could be modeled through Landau theory, where the depolarization ratio was used as experimental order parameter (ψ). This parameter was found to scale with temperature, as ψ ∼ (T(c)-T)(-α), with power-law exponent α = 0.40 ± 0.05; interestingly, we found α ≈ β ≈ γ.
  The Journal of chemical physics 05/2011; 134(19):194904. · 3.09 Impact Factor
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  Article: Universal sol state behavior and gelation kinetics in mixed clay dispersions.

  Ravi Kumar Pujala, Nisha Pawar, H B Bohidar
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  ABSTRACT: Sol and gel state behavior, in aqueous salt free dispersions, of clays Laponite (L) and Na montmorillonite (MMT) was studied at various mixing ratios (L:MMT = r = 1:0.5, 1:1, and 1:2). In the sol state, the zeta potential and gelation concentration of L-MMT obeyed the universal relation, X(L-MMT) = (rX(L) + X(MMT))/(1 + r), where X is zeta potential or gelation concentration (c(g)), implying that these properties are linear combinations of the same of their individual components. The low frequency storage modulus (G(0)'), relative viscosity (η(r)), and apparent cluster size (R) could be universally described by the power-law, G(0)' ∼ ((c/c(g)) - 1)(t) (c > c(g)), and η(r), R ∼ (1 - (c/c(g)))(-k,ν) (c < c(g)), with t = 1.5, k = 1.1, and υ = 0.8 close to the gelation concentration, for r = 1:1 cogel, consistent with the percolation model description of gelation. Interestingly, the hyperscaling relation δ = t/(k + t) yielded δ = 0.56 not too different from the predicted value ∼0.7, while the experimental value of δ obtained from G''(ω) ∼ ω(δ) close to c ≈ c(g) yielded δ = 1.5, which was at variance with the hyperscaling result. The experimental data, on hand, mostly supported percolation type gelation mechanism. As the cogels were slowly heated, at a characteristic temperature, T(g), a sharp increase in G' value was noticed, implying a transition to gel hardening (a new phase state). The temperature-dependent behavior followed the power-law description, G' ∼ (T(g) - T)(-γ) (T < T(g)), with γ = 0.40 ± 0.05 invariant of composition of the cogel, whereas for MMT and Laponite, γ = 0.25 and 0.55, respectively. It has been shown that the cogel has significantly enhanced mechanical (G(0) increased by 10 times for r = 1:1 cogel) and thermal properties (T(g) increased by 13 °C for 1:1 cogel) that can be exploited to design customized soft materials.
  Langmuir 04/2011; 27(9):5193-203. · 4.19 Impact Factor