Department of Chemical Engineering

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It is now well-accepted that hydrophilic nanoparticles (NPs) lightly grafted with polymer chains self-assemble into a variety of superstructures when placed in a hydrophobic homopolymer matrix or in a small molecule solvent. Currently, it is thought that a given NP sample should only assemble into one kind of superstructure depending on the relative balance between favorable NP core-core attractions and steric repulsion between grafted polymer chains. Surprisingly, we find that each sample shows the simultaneous formation of a variety of NP-assemblies, e.g., well-dispersed particles, strings, and aggregates. We show through the generalization of a simple geometric model that accounting for the distributions of the NP core size and the number of grafted chains on each NP (which is especially important at low coverages) allows us to quantitatively model the aggregate shape distribution. We conclude that, in contrast to molecular surfactants with well-defined chemistries, the self-assembly of these NP analogues is dominated by such fluctuation effects. © 2017 American Chemical Society.

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Macromolecules

Impact of the Distributions of Core Size and Grafting Density on the Self-Assembly of Polymer Grafted Nanoparticles

Two-dimensional base state solutions for rimming flows and their stability analysis to small axial perturbations are analyzed numerically. A thin liquid film which is uniformly covered with an insoluble surfactant flows inside a counterclockwise rotating horizontal cylinder. In the present work, a mathematical model is obtained which consists of coupled thin film thickness and surfactant concentration evolution equations. The governing equations are obtained by simplifying the momentum and species transport equations using the thin-film approximation. The model equations include the effect of gravity, viscosity, capillarity, inertia, and Marangoni stress. The concentration gradients generated due to flow result in the surface tension gradient that generates the Marangoni stress near the interface region. The oscillations in the flow due to inertia are damped out by the Marangoni stress. It is observed that the Marangoni stress has stabilizing effect, whereas inertia and surface tension enhance the instability growth rate. In the presence of low diffusion of the surfactant or large value of the Péclet number, the Marangoni stress becomes more effective. The analytically obtained eigenvalues match well with the numerically computed eigenvalues in the absence of gravity. © 2017 Author(s).

Physics of Fluids

Stability analysis of rimming flow inside a horizontally rotating cylinder in the presence of an insoluble surfactant

The present numerical work explores the mixed convective flow and heat transfer analysis in top lid driven square cavity containing heated triangular block with constant heat flux (CHF) thermal condition. The fluid flow in the cavity is driven by top moving wall in +x direction, while all other walls are stationary. Governing equations are solved by using finite volume method (FVM) and SIMPLE scheme for range of pertaining parameters such as, Reynolds number (Re = 1, 10, 50, 100, 500 and 1000), Prandtl number (Pr = 1, 50, 100), and Grashof number (Gr = 0, 102, 103, 104 and 105). The influence of size of blockage (b = 10%, 20% and 30%) on momentum and heat transfer characteristics has also been explored. The physical insights of systems are delineated by analyzing variation of streamlines and isotherm contours; while the heat transfer characteristics are explored by using dimensionless heat transfer coefficient or Nusselt number (local as well as averaged values). It is observed that increase in Reynolds number enhances the heat transfer rate (Nusselt number) only up to Re = 180–220, later, deterioration of heat transfer rate is observed. The critical (Recr) as well as stagnant (Restag) Reynolds number of heat transfer for present case are identified. © 2016 Elsevier Ltd

International Journal of Heat and Mass Transfer

Computational analysis of mixed convection heat transfer characteristics in lid-driven cavity containing triangular block with constant heat flux: Effect of Prandtl and Grashof numbers

Self-association (i.e. interchain aggregation) behavior of atactic poly(ethacrylic acid) PEA in dilute aqueous solution as function of degree-of-neutralization by Na+ counter-ions (i.e. charge fraction f) was investigated by molecular dynamics simulations. Aggregation is found to occur in the range 0 ≤ f ≤0.7 in agreement with experimental results compared at specified polymer concentration Cp = 0.36 mol/l in dilute solution. The macromolecular solution was characterized and analysed for radius-of-gyration, torsion angle distribution, inter and intra-molecular hydrogen bonds, radial distribution functions of intermolecular and inter-atomic pairs, inter-chain contacts and solvation enthalpy. The PEA chains form aggregate through attractive inter-chain interaction via hydrogen bonding, in the range f &lt; 0.7, in agreement with experimental observation. The numbers of inter-chain contacts decreases with f. A critical structural transition occurs at f = 0.7, observed via simulations for the first time, in Rg as well as inter-chain H-bonds. The inter-chain distance increases with f due to repulsive interactions between COO− groups on the chains. PEA-PEA electrostatic interactions dominant solvation enthalpy. The PEA solvation enthalpy becomes increasingly favorable with increase in f. The transition enthalpy change, in going from uncharged (acid) state to fully charged state (f = 1) is unfavorable towards aggregate formation. © 2017 Elsevier Inc.

Journal of Molecular Graphics and Modelling

Factors responsible for the aggregation behavior of hydrophobic polyelectrolyte PEA in aqueous solution studied by molecular dynamics simulations

In this study, ultrafiltration membranes are prepared in outside skin tubular configurations using polysulfone polymer with and without mesoporous silica and aluminium oxide powder (~3.8–4.0 nm pore size) independently. Prepared membranes are characterized in terms of pure water flux, separation of single uncharged solutes like polyethylene oxide (PEO), water contact angle and average surface roughness. After filtration of turbid seawater (50–60 NTU) to remove the turbidity, pure water flux recoveries of all the membranes were evaluated after cleaning by backwashing with deionized (DI) water. It was found that flux recovery is better in nanocomposite membranes than in purely polysulfone coated membranes. The extent of turbidity removal was studied as a function of turbidity load and temperature of seawater. Incorporation of the porous nanoparticles in pure polymer matrixes not only enhanced the water flux without sacrificing selectivity but it also increased the fouling resistance of the UF membranes. After 2 month operation, no performance deterioration was observed for all the tubular membranes. © 2017, Desalination Publications. All rights reserved.

Desalination and Water Treatment

Polysulfone-mesoporous metal oxide based tubular nanocomposite ultrafiltration membranes for turbidity removal of seawater

Abstract.: The stability of a thin electrolyte liquid film driven by gravity over a vertical substrate is presented. A film thickness evolution equation is derived and solved numerically. The substrate is non-uniformly heated from below which is modeled by imposing a temperature profile at the liquid-solid interface. The electrohydrodynamics is included in the model with Maxwell’s stress tensor. The governing flow and energy equations are simplified using the lubrication approximation. The Poisson-Boltzmann equation with Debye-Hückel approximation is used for the potential which is generated inside the film due to a charged layer at the liquid-solid interface. The positive temperature gradient at the substrate leads to the formation of a thermocapillary ridge due to an opposing Marangoni stress. This thermocapillary ridge becomes unstable beyond critical parameters related to Marangoni stress and convective energy loss at the free surface. The electroosmotic flow has no effect on the base profile of the film, but enhances its instability. A parameter space is presented delineating the stable and unstable regimes for the film dynamics. Graphical abstract: Figure not available: see fulltext. © 2017, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

The European Physical Journal E

Stability of a gravity-driven thin electrolyte film flowing over a heated substrate

Response to "Comment on 'Stability analysis of the rimming flow inside a uniformly heated rotating horizontal cylinder'" [Phys. Fluids 29, 089101 (2017)]

This work illustrates the steady state, two dimensional natural convective flow and heat transfer features in square enclosure containing heated hexagonal block maintained either at constant wall temperature (CWT) or uniform heat flux (UHF) thermal conditions. Governing equations (mass, momentum and energy) are solved by using finite volume method (FVM) with 3rd order accurate QUICK discretization scheme and SIMPLE algorithm for range of field pertinent parameters such as, Grashof number (103 ≤ Gr ≤ 106), Prandtl number (1 ≤ Pr ≤ 100) and power law index (0.5 ≤ n ≤ 1.5). The analysis of momentum and heat transfer characteristics are delineated by evolution of streamlines, isotherms, variation of average Nusselt number value and Colburn factor for natural convection (jnH). A remarkable change is observed on fluid flow and thermal distribution pattern in cavity for both thermal conditions. Nusselt number shows linear variation with Grashof and Prandtl numbers; while rate of heat transfer by convection decreases for power law index value. Higher heat transfer rate can be achieved by using uniform heat flux condition. A Nusselt number correlation is developed for possible utilization in engineering/scientific design purpose. © 2016 Elsevier B.V.

Chinese Journal of Chemical Engineering

Laminar natural convection characteristics in an enclosure with heated hexagonal block for non-Newtonian power law fluids

Binary transition metal oxides have attracted wide attention in Li-ion batteries due to their multivalent electronic states and improved electrical conductivity. In this context, we have, for the first time, successfully synthesized reduced graphene oxide- nickel tungstate (RGO/NiWO4) nanocomposites by one pot hydrothermal method and employed them as anode for Li-ion battery. Scanning and transmission electron microscopic images show complete dispersion of NiWO4 nanoparticles (NPs) of size 20-50 nm on reduced graphene oxide sheets. The synthesized nanocomposites RGO/NiWO4 (1-3) (RGO:NiWO4 wt ratios of 50:50, 25:75 and 15:85) exhibit enhance electrochemical performance and cycling stability as compared to bare NiWO4 NPs. The electrodes made with bare NiWO4, RGO/ NiWO4-1, RGO/ NiWO4-2 and RGO/ NiWO4-3 nanocomposites deliver specific capacities of 67, 578, 647 and 498 mAh g-1 respectively at 100 mA g-1 current density after 100 cycles achieving 98% coulombic efficiency. The improved cycling performance of nanocomposite is attributed to the incorporation of conducting graphene nanosheets, which not only act as flexible confinements but also prevent the detachment, agglomeration and pulverization of NiWO4 NPs. The excellent electrochemical performance of nanocomposite electrode in terms of specific capacity, cycling stability and rate capability makes it an ideal anode material for lithium ion battery. © 2017 The Electrochemical Society. All rights reserved.

Journal of the Electrochemical Society

Hydrothermally synthesized reduced graphene oxide-NiWO4 nanocomposite for lithium-ion battery anode

Affordable silver-polymer core-shell nanoparticles are prepared using the layer-by-layer (LbL) technique. The metallic silver core is encapsulated with an ultra-thin protective shell that prevents oxidation and clustering without compromising the plasmonic properties. The core-shell nanoparticles retain their plasmonic near field enhancement effect, as studied from finite element numerical simulations. Control over the shell thickness up to the sub-nanometer level is there for key. The particles are used to prepare a plasmonic Ag-TiO2 photocatalyst of which the gas phase photocatalytic activity is monitored over a period of four months. The described system outperforms pristine TiO2 and retains its plasmonic enhancement in contrast to TiO2 modified with bare silver nanoparticles. With this an important step is made toward the development of long-term stable plasmonic (photocatalytic) applications. © 2016 Elsevier B.V.