Department of Chemical Engineering

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The phase behaviour of triangle-well (TW) fluids confined inside three types of slit pores: slit pores with or without uniform attraction and slit pores with square-well (SW) type pore-fluid interactions is studied using molecular simulations. The TW potential is chosen to model the fluid-fluid interactions as this model provides a good compromise between realism and simplicity. The simple nature of the models enables us to obtain a qualitative understanding of the effects of confinement on the vapour liquid coexistence curves (VLCCs) of the TW fluids with variable well-width. We note the significant differences in the VLCCs of the confined fluid from the bulk phase behaviour; and also the dependence of the VLCCs on the type of slit pore. These differences have been explained on the basis of the plots of local density along the axis perpendicular to the walls of the slit pore. © 2016 Elsevier B.V.

World University root node

Journal of Molecular Liquids

Fluid phase equilibria of triangle-well fluids confined inside slit pores: A transition matrix Monte Carlo simulation study

In this paper, a sparsity seeking optimization method for estimating the parameters along with the order of output error models of single-input and single-output (SISO), linear time invariant (LTI) system is proposed. It is demonstrated with the help of simulations that the proposed algorithm gives accurate parameter and order estimates on a variety of systems. © 2016

IFAC-PapersOnLine

Parameter Estimation and Model Order Identification of LTI Systems

Photocatalytic degradation of nitrobenzene (NB) and an azo dye (namely azophloxine) was carried out using zinc oxide (ZnO) nanoparticles which were prepared by electrochemical method using zinc electrodes in aqueous oxalic acid solution. Taguchi methodology was applied for studying the effects of four experimental parameters namely catalyst dosage (g/1). time of reaction (h), pH and H2O2 dose. Degradation efficiency of NB was found to increase with an increase in H2O2 concentration and decrease in catalyst dosage and pH. Maximum NB degradation efficiency of 98% was observed at catalyst dosage=l g/1, pH=3, H2O2 to C6H5NO2 ratio of 7 mol/mol after 5 h of treatment. Similarly, maximum dye degradation efficiency of 58% and color removal efficiency of 78% was observed at catalyst dosage=l.25 g/1, pH=4, H2O2 concentration of 8 mmol/1 in 5 h. Overall, prepared ZnO nanoparticles performed better for NB degradation in comparison to azo dye degradation. © 2016 Scientific Publishers. All rights reserved.

Journal of Scientific and Industrial Research

Photocatalytic degradation of nitrobenzene and azo dye using zinc oxide nanoparticles prepared by electrochemical method

The intermolecular structure and solvation enthalpy of anionic polyelectrolyte atactic Na+-polyethacrylate (PEA) in aqueous solution, as a function of added salt concentration Cs (dilute to concentrated) and valency (NaCl versus CaCl2), were investigated via molecular dynamics simulations with explicit-ion-solvent and atomistic polymer description. An increase in Cs leads to a decrease in α, which stabilizes to a constant value beyond critical Cs. A significant reduction in Rg in the presence of CaCl2 salt was observed, due to ion bridging of PEA by Ca2+ ions, in agreement with results available in literature on other similar polycarboxylates. An increase in salt valency reduces the value of critical Cs for the onset of stabilization of the overall size and shape of the polymer chain. The critical Cs ratio for the divalent to monovalent salt case is in excellent agreement with results of Langevin dynamics studies on model systems available in the literature. PEA–water H-bond half-life increases with Cs for CaCl2, but no appreciable effect is seen for NaCl. The hydration of PEA becomes stronger in the presence of divalent salt. The strength of H-bond interaction energy is greater for cations as compared to anions of the salt. The salt cation effect in displacing water molecules from the vicinity of PEA, with increase in Cs, is greater for NaCl solution. The decrease in water coordination to PEA carboxylate groups, due to increased Cs, is more pronounced in NaCl solution. The nature of the behavior of the solvation enthalpy of PEA and the type of intermolecular interactions contributing to it, is in agreement with experimental observations from the literature. The hydration enthalpy of PEA in divalent CaCl2 aqueous salt solution is more exothermic compared to monovalent NaCl salt solution, in agreement with experimental data. The solvation of PEA is thermodynamically more favorable in the case of CaCl2 solution. The exothermic solvation enthalpy, H-bond lifetime, number of H-bonds and H-bond interaction energy are greater in magnitude in CaCl2 aqueous solution. © 2016, Springer-Verlag Berlin Heidelberg.

Journal of Molecular Modeling

Effect of salt valency and concentration on structure and thermodynamic behavior of anionic polyelectrolyte Na+-polyethacrylate aqueous solution

Faraday Discussions

Synthesis of Nanoparticle Assemblies: General discussion

The central governing factors that influence the efficiency of photoelectrochemical (PEC) water splitting reaction are photon absorption, effective charge-carrier separation, and surface electrochemistry. Attempts to improve one of the three factors may debilitate other factors and we explore such issues in hydrogenated TiO2, wherein a significant increase in optical absorption has not resulted in a significant increase in PEC performance, which we attribute to the enhanced recombination rate due to the formation of amorphization/disorderness in the bulk during the hydrogenation process. To this end, we report a methodology to increase the charge-carrier separation with enhanced optical absorption of hydrogenated TiO2. Current methodology involves hydrogenation of non-metal (N and S) doped TiO2 which comprises (1) lowering of the band gap through shifting of the valence band via less electronegative non-metal N, S-doping, (2) lowering of the conduction band level and the band gap via formation of the Ti3+ state and oxygen vacancies by hydrogenation, and (3) material processing to obtain a disordered surface structure which favors higher electrocatalytic (EC) activity. This design strategy yields enhanced PEC activity (%ABPE = 0.38) for the N-S co-doped TiO2 sample hydrogenated at 800 °C for 24 h over possible combinations of N-S co-doped TiO2 samples hydrogenated at 500 °C/24 h, 650 °C/24 h and 800 °C/72 h. This suggests that hydrogenation at lower temperatures does not result in much increase in optical absorption and prolonged hydrogenation results in an increase in optical absorption but a decrease in charge carrier separation by forming disorderness/oxygen vacancies in the bulk. Furthermore, the difference in double layer capacitance (Cdl) calculated from electrochemical impedance spectroscopy (EIS) measurements of these samples reflects the change in the electrochemical surface area (ECSA) and facilitates assessing the key role of surface electrochemistry in PEC water splitting reaction. Additionally, we observed a blue-shift of the absorption spectrum and a decrease in both electrochemical (EC) and photoelectrochemical (PEC) activities after the removal of surface layers through focused ion beam (FIB) sputtering suggesting the importance of surface defects and photon absorption. © the Owner Societies 2016.

Physical Chemistry Chemical Physics

Coupled optical absorption, charge carrier separation, and surface electrochemistry in surface disordered/hydrogenated TiO2 for enhanced PEC water splitting reaction

Iron oxide (Fe3O4) nanoparticle was synthesized by coprecipitation and was modeled and solved using a hybrid (discrete-continuous) model, based on a kinetic Monte Carlo (kMC) simulation scheme. The latter was combined with the constant number MC method, to improve both speed and accuracy of the simulation. Complete particle size distribution (PSD) from simulation matches very well with PSD of both uncoated and coated (with either polyacrylic acid or dextran) Fe3O4 nanoparticles, obtained from our experiments. The model is general, as the time scales of various processes (nucleation, diffusion-growth and coagulation-growth) are incorporated in rate equations, while, input simulation parameters are experimentally measured quantities. With the help of the validated model, effect of coating agent on coagulation-growth was estimated by a single, fitted, coagulation-efficiency parameter. Our simulation shows that, logarithm of coagulation-efficiency scales linearly with logarithm of inverse of the molecular weight of the coating agent. With this scaling law, our model is able to a priori predict the experimental PSD of Fe3O4 nanoparticles, synthesized with an even higher molecular weight of dextran. © 2015 Elsevier Inc..

Journal of Colloid and Interface Science

Role of coating agent in iron oxide nanoparticle formation in an aqueous dispersion: Experiments and simulation

Solution-based synthesis of nanoparticles does not yield monodisperse particles, but rather a well-defined particle size distribution (PSD). There is currently no simple means to anticipate or model these size distributions, which critically affect the properties of the resulting nanomaterials. We simulate the temporal evolution of the PSD in the framework of a nucleation and growth model, with the critical postulate that the coagulation efficiency between two nanoparticles is quantitatively determined by the known, interparticle potential energy. Our simulation based on this ansatz, not only a priori predicts experimentally obtained complete PSDs of uncoated or coated (with poly(acrylic acid)or dextran) iron oxide nanoparticles but also accurately captures the influence of surface coverage of a coating agent on the resulting PSD. © 2016 American Chemical Society.

Journal of Physical Chemistry C

Predicting Complete Size Distribution of Nanoparticles Based on Interparticle Potential: Experiments and Simulation

The dynamics and linear stability of a gravity drive thin film flowing over non-uniformly heated porous substrate are studied. A governing equation for the evolution of film-thickness is derived within the lubrication approximation. Darcy-Brinkman equation is used to model flow in the porous medium along with a tangential stress-jump condition at the interface of the porous layer and the fluid film. A temperature profile is imposed at the solid wall to model an embedded heater beneath the porous layer. At the upstream edge of the heater, an opposing thermocapillary stress at the liquid-air interface leads to the formation of a thermocapillary ridge. The ridge becomes unstable beyond a critical Marangoni number leading to the formation of rivulets that are periodic in the spanwise direction. Increase in the values of parameters such as Darcy number, stress jump coefficient, and porosity is shown to have stabilizing effect on the film dynamics. The critical Marangoni number is shown to increase monotonically with Darcy number for various values of porosity. At large values of stress-jump coefficient, a non-monotonic variation in critical Marangoni number versus Darcy number is shown. A correlation is developed numerically for the ratio of critical Marangoni number at large Darcy number to that for a non-porous substrate as a function of porosity and thickness of the porous substrate. A transient growth analysis is carried out followed by non-linear stability analysis. The non-modal growth is found to be negligible thus indicating that the eigenvalues are physically determinant. © 2016 AIP Publishing LLC.

Physics of Fluids

Stability analysis of non-inertial thin film flow over a heterogeneously heated porous substrate

Natural convection heat transfer in an square enclosure, consisting of a partially heated west wall with east end open to ambient, is investigated numerically by using an in-house computational fluid dynamics solver based on thermal lattice Boltzmann method. In particular, the influences of Rayleigh number (103-106), heating location (bottom, middle, and top) on west wall, and dimensionless heating length (0.25-0.75) on momentum and heat transfer characteristics of air are presented and discussed. The streamline patterns show bifurcation at the lowest Rayleigh number for bottom and middle heating, whereas at the highest Rayleigh number, all heating positions yield bifurcation and elongation of flow patterns with a secondary vortex near the lower side of open end. The middle and bottom heating locations show a linear increase in Nusselt number with heater size, whereas inverse dependence is seen for top heating. The maximum heat transfer is observed in the case of middle heating. As expected, average Nusselt number increased with increasing Rayleigh number. Finally, the functional dependence of the average Nusselt number on flow governing parameters (Rayleigh number and heating length) for different heating locations is presented as a simple predictive empirical relationship. © 2016 Taylor and Francis Group, LLC.