Department of Chemical Engineering

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All atom molecular dynamics simulations and experiments were performed to understand the adsorption behavior of gadolinium (Gd3+) ion on the crown ethers grafted polystyrene (PS) surface. Two different types of crown ethers, viz., dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE), were grafted separately on the PS surface to understand the adsorption behavior. We investigate the roles of Gd3+ ion concentration and grafting density (ρs) of the crown ether on the adsorption behavior of Gd3+ ion on the PS surface. The adsorption of Gd3+shows an increasing trend with increasing salt concentration, for all cases of crown ether grafting densities. The adsorption behavior follows the Langmuir isotherm model. The maximum amount of Gd3+ ion adsorption was observed to be 1.83 mg/g for DBCE and 2.02 mg/g for DCHCE at ρs = 2.07 mol/nm2. The maximum amount of Gd3+ ion adsorption on DBCE coated PS beads, in batch experiments, was found to be 1.76 mg/g, which is in good agreement with the theoretical results. The increase in ρs from 0.25 to 2.07 mol/nm2 shows an increase in the qmax value by ∼422% and 329% for DBCE and DCHCE, respectively. The optimum value of the crown ether grafting density is found to be 1.25 mol/nm2, beyond which the qmax saturates. We further investigate the dynamics of the Gd3+ ion by evaluating the diffusion coefficient (D) and mean residence time (τ). It was found that D decreases with increasing salt concentration for both DBCE and DCHCE. On the contrary, as expected, the τ value of Gd3+ increases with an increase in salt concentration. Overall, a 3-fold increase in τ was seen with increasing salt concentration. The potential of mean force analysis using umbrella sampling reveals favorable binding energy for higher grafting density of DCHCE compared to that of DBCE. © 2019 American Chemical Society.

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Journal of Physical Chemistry C

Adsorption of Gadolinium (Gd3+) Ions on the Dibenzo Crown Ether (DBCE) and Dicyclo Hexano Crown Ether (DCHCE) Grafted on the Polystyrene Surface: Insights from All Atom Molecular Dynamics Simulations and Experiments

The rate capability of Li-ion batteries is crucially influenced by Li-ion diffusion within bulk electrodes and electrolyte, and Li-ion transfer across the electrode-electrolyte interface. Relative magnitudes of both bulk and interfacial diffusion energy barriers are essential in order to identify the rate-limiting component in overall Li-ion transport process. Here, we quantitatively estimate migration energy barrier in bulk cathode and at the cathode-electrolyte interface through density functional theory based nudged elastic band calculations. We study a Li-ion transfer pathway wherein a Li-ion solvated by the first coordination shell of the electrolyte changes its coordination structure to that in cathode sub-surface sites, and electrolyte molecules act as ligands for Li-ion. We compute the energy barrier for Li-ion transfer from ethylene carbonate/LiPF6 electrolyte into (010)-surface of LiFePO4 to be 756meV. We explore the effect of partial fluorination of interface due to electrolytic reactions and find a drastic reduction in energy barrier to 410meV. Nevertheless, the energy barrier for interfacial ion-transfer is much higher than that for diffusion through bulk one-dimensional channels of olivine LiFePO4, which is estimated at 290meV. The higher energy barrier for interfacial Li-ion transfer suggests a rate-limiting behavior of interface in overall kinetics of ion transport in Li-ion batteries. © 2019 The Electrochemical Society.

Journal of the Electrochemical Society

Higher energy barrier for interfacial Li-ion transfer from EC/LiPF6 electrolyte into (010) LiFePO4 cathode surface than bulk Li-ion diffusion within both cathode and electrolyte

Manuscript reports on ferrocenium ion entangled in polyelectrolyte matrix as a redox platform for oxidative and reductive sensing of nitric oxide (NO). It was prepared by oxidation of ferrocene in diethyl ether followed by extraction with polystyrene sulfonic acid (PSS) and Nafion® solution. The presence of stable ferrocenium cation was confirmed by FT-IR, UV-visible spectra, 1 HNMR and XRD. It results into a stable cyclic voltammogram in 100 mM Phosphate buffer, pH 7 solution with Ag/AgCl reference and Pt wire counter electrodes. The electrode sensitivity towards NO was confirmed through the increase in oxidative current at +750 mV and reductive current at −800 mV by the addition of 100 μL saturated NO solution. The chronoamperometric response for NO oxidation followed linearity in the concentration range of 200 nM to 3.5 μM. The lower detection limit and sensitivity were found to be 200 nM and be 33.29 μA/μM/cm 2 respectively. The reductive response also followed a linear trend in the concentration range of 100 μM to 1.2 mM with 15.15 μA/μM/cm 2 sensitivity and 100 μM detection limit. It indicates ferrocenium ion entanglement within polyelectrolyte matrix and its viability as a redox platform for oxidative as well as reductive sensing of nitric oxide. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemistrySelect

Ferrocenium Ion Confinement in Polyelectrolyte for Electrochemical Nitric Oxide Sensor

Enhanced electrocatalytic activity of the oxygen evolution reaction (OER) can be achieved through modulation of the electronic structure of the electrocatalytic active sites. This modulation can also be achieved through stabilization of metastable or non-native polymorphs of the electrocatalyst. Non-native (NN) crystal structures differ in their discrete translational symmetry from the bulk native (N) crystal. The variable oxygen evolution reactivity in a basic medium of different polymorphs is demonstrated by synthesizing β/N-, γ/N-NN1-, r/NN1-, α/NN2-, and δ/NN3-MnO2 polymorphs of MnO2 which show different active site densities on the surface, XPS-derived oxidation state of Mn, and bulk electronic conductivity. The specific OER activity [activity per electrochemical surface area (ECSA)] of MnO2 is codependent on the oxidation state of Mn and electronic conductivity. A volcano-based relationship is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor ΔGO∗ - ΔGHO∗, computed from density functional theory (DFT). Both δ/NN3-MnO2 and α/NN2-MnO2 lie closer to the volcano peak but on opposite legs of the volcano. δ/NN3-MnO2 shows higher specific activity due to its low oxidation state (+3.5) of Mn, which is confirmed through the calculated average oxidation states (AOS) from XPS and Bader charge from DFT studies. α/NN2-MnO2 shows higher specific activity due to higher electronic conductivity, which is correlated with its low oxygen vacancy formation energy. Further, the electronic origin of high OER activity in two of the most non-native polymorphs (δ/NN3-MnO2 and α/NN2-MnO2) is ascribed to a shift in the valence Mn-d band closer to the Fermi level leading to stronger O adsorption. The present study demonstrates the efficacy of utilizing non-native polymorphs for OER and unfolds the correlation between OER and variable oxidation states and electronic conductivities, thereby providing directions toward the generalization of these effects to other polymorphic compounds. © 2019 American Chemical Society.

Modulating Oxygen Evolution Reactivity in MnO2 through Polymorphic Engineering

Abstract.: Two-dimensional steady-state solutions and their stability analysis are presented for a gravity-driven thin film of a thermoviscous liquid. The governing equations and boundary conditions are simplified using the lubrication approximation. The analytically obtained film thickness evolution equation consists of various dimensionless parameters such as the Marangoni number, Biot number and thermoviscosity number. The viscosity of the liquid is assumed as an exponential function of temperature. The viscosity decreases within the liquid film as the temperature increases. Due to localized heating interfacial temperature gradients generate surface tension gradient which results into thermocapillary or Marangoni stress. The Marangoni stress opposes the fluid flow at the leading edge of heater leading to an increase in the film thickness locally. This locally thick structure becomes unstable beyond critical values of the parameters that leads to formation of rivulets in the transverse direction. Using the linear stability analysis it is found that the Marangoni stress and the thermoviscous effect have a destabilizing effect on the thin-film flow. At much higher values of the thermoviscosity number another mode of instability appears which is known as thermocapillary instability which leads to oscillating film profiles. For streamwise perturbations, the destabilizing effect of the thermoviscosity number for localized and uniform heating remains consistent. Graphical abstract: [Figure not available: see fulltext.]. © 2019, EDP Sciences, Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature.

The European Physical Journal E

Stability analysis of a gravity-driven thermoviscous liquid film flowing over a heated flat substrate

The present work examines the two-dimensional, steady, laminar mixed convective fluid circulation and heat transfer in a square lid-driven cavity influenced by the change in the location of the triangular block with constant heat flux (CHF) thermal condition. The effect of the changes in the location of block along the vertical centerline (x=0.5,y) of the cavity has been explored. The dependence of various flow governing dimensionless parameters, such as, Reynolds number (Re=1−1000), Richardson number (Ri=0.01,1,10), blockage (B=0.1,0.2,0.3) and location (L y =0.25,0.5,0.75) of triangular block on the streamline, temperature contours, Nusselt numbers, etc. has been illustrated. A normalized Nusselt number is also calculated for studying the difference in heat transfer rate with that of constant wall temperature condition. The results of the study indicates higher heat transfer rates for centered block position (L y =0.5) than remaining two (L y =0.25,0.75). For a given blockage, Nusselt number shows decline with increasing Ri. The summary of research is expressed by using two forms of empirical correlations (Nu¯=f(L y ,Re)),Nu¯=f(B,Re) for possible utilization in engineering design. © 2019 Elsevier Ltd

International Journal of Mechanical Sciences

Mixed convection in a lid-driven cavity containing triangular block with constant heat flux: Effect of location of block

Neuronal network reconstruction is an important problem in neuroscience as it helps understanding neuronal circuit and function. With the advancements of the calcium imaging technique, the dynamic activity of hundreds of neurons can be observed, which also provides a foundation for modelling and inferring network connectivity directly from data. In this paper, the dynamic behavior of each neuron is described using a non-linear autoregressive exogenous input (NARX) model. Since the neurons are inter-connected, the NARX models contain inputs which depend on outputs of the surrounding neurons. The complete model is expressed in a linear-in-parameter form and the network dynamics are identified by obtaining a block-sparse solution given the calcium oscillation data. The solution to the identification problem also provides the network topology simultaneously, i.e., the physical connections between the neurons. To demonstrate the accuracy of the proposed method, two experimental case studies are considered and the results are compared with other network identification methods. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.

IFAC-PapersOnLine

Identification of neuronal networks from calcium oscillation data

The effect of an electric field on the coalescence of two water droplets suspended in an insulating oil (castor oil) in the non-coalescence regime is investigated. Unlike the immediate breakup of the bridge, as reported in earlier studies, e.g. Ristenpart et al. (Nature, vol. 461 (7262), 2009, pp. 377-380), the non-coalescence observed in our experiments indicate that at strong fields the droplets exhibit a tendency to coalesce, the intervening bridge thickens whereafter the bridge dramatically begins to thin, initiating non-coalescence. Numerical simulations using the boundary integral method are able to explain the physical mechanism of thickening of this bridge followed by thinning and non-coalescence. The underlying reason is the competing meridional and azimuthal curvatures which affect the pressure inside the bridge to become either positive or negative under the effect of electric field induced Maxwell stresses. Velocity and pressure profiles confirm this hypothesis and we are able to predict this behaviour of transitory coalescence followed by non-coalescence. © 2019 Cambridge University Press.

Journal of Fluid Mechanics

Breakup and non-coalescence mechanism of aqueous droplets suspended in castor oil under electric field

Electrocoalescence is an industrially relevant process, especially in oil exploration and petroleum refining industries, and is the working principle of the process equipment, an electrocoalescer. In this work, electrocoalescence is modelled using dipolar electrostatic forces which bring the droplets together but are resisted by Stokesian drag forces. We demonstrate that hydrodynamic interactions are important to model electrocoalescence. Similarly, a multi box methodology is necessary for getting improved statistics for drop size distribution, since any coagulation/coalescence process, typically results in the loss of droplets. Moreover, the process of chaining can be modelled by assuming a “wait-time” on contact of the interacting drops, and n-mer formation can be predicted. Our results show a reasonable agreement between model predictions and experimental data. The simulations also show that an increase in volume fraction, average droplet size, and polydispersity enhance the dynamics of the coalescence process. © 2018

Computers and Chemical Engineering

Modelling and particle based simulation of electro-coalescence of a water-in-oil emulsion

In polymer-grafted nanoparticles (PGN), covalent tethering of apolar polymer chains to a polar inorganic nanoparticle core induces the formation of self-assembled aggregates. Since the nature of these aggregates determines bulk mechanical and transport properties, it is of importance to understand the factors that determine the underlying assembly processes. In the literature, the solution assembly of PGNs has been understood in analogy to small-molecule amphiphiles. However, in any experimental realization, PGNs are invariably characterized by additional structural complexity, such as the distributions in the inorganic core size and in the grafted chains (both in their length and grafting density). These strongly influence the assembly of amphiphilic PGNs. We have previously demonstrated that dispersity in core size qualitatively affects the structure of PGN aggregates, and Jayaraman et al. demonstrated the effect of grafted chain-length dispersity. The combined effects of dispersity in the size of the core and grafted chains have not been explored previously. Here, we develop a model that builds on the work of Daoud and Cotton to explore a wide parameter space of PGN with dispersity simultaneously in core size and grafted chain length. We demonstrate that dispersity in core size is the dominant factor affecting the self-assembled solution structure of PGN aggregates. Our work suggests the importance of focusing on synthetic strategies for control of core-size dispersity to control aggregate structure in PGN. © 2019 American Chemical Society.