Chemical Engineering

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The extensional flow of a polymeric fluid during the fibre spinning process is studied for finite-amplitude stability behaviour. The spinning flow is assumed to be inertialess and isothermal. The nonlinear extensional rheology of the polymer is described with the help of the eXtended Pom-Pom (XXP) model, which is known to exhibit a significant strain hardening effect necessary for fibre spinning applications. The linear stability analysis predicts an instability known as draw resonance when the draw ratio, DR, defined as the ratio of the velocities at the two ends of the fibre in the air gap, exceeds a certain critical value, DRc. The critical draw ratio DRc depends on the fluid elasticity represented by the Deborah number, De = λυ0/L, the ratio of the polymer relaxation time to the flow time scale, thus constructing a stability diagram in the DRc-De plane. Here, λ is the characteristic relaxation time of the polymer, υ0 is the extrudate velocity through the die exit and L is the length of the air gap for the spinning flow. In the present study, we carry out a weakly nonlinear stability analysis to examine the dynamics of the disturbance amplitude in the vicinity of the transition point. The analysis reveals the nature of the bifurcation at the transition point and constructs a finite-amplitude manifold providing insight into the draw resonance phenomena. The effect of the fluid elasticity on the nature of the bifurcation and the finite-amplitude branch is examined, and the findings are correlated to the extensional rheological behaviour of the polymer fluid. For flows at small Deborah number, the Landau constant, which captures the role of nonlinearities, is found to be negative, indicating supercritical Hopf bifurcation at the transition point. In the linearly unstable region, the equilibrium amplitude of the disturbance is estimated and shows a limit cycle behaviour. As the fluid elasticity is increased, initially the equilibrium amplitude is found to decrease below its Newtonian value, reaching the lowest value for De when the strain hardening effect is maximum. With further increase in elasticity, the material undergoes strain softening behaviour which leads to an increase in the equilibrium amplitude of the oscillations in the fibre cross-section area, indicating a destabilizing effect of elasticity in this regime. Interestingly, at a certain high Deborah number, the bifurcation crosses over from supercritical to subcritical nature. In the subcritical regime, a threshold amplitude branch is constructed from the amplitude equation. © 2015 Cambridge University Press.

(SoE) School of Engineering

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Journal of Fluid Mechanics

Weakly nonlinear stability analysis of polymer fibre spinning

Several amino acid anion (aminate) based ionic liquids (ILs) were synthesized, characterized and tested for CO2 absorption capacity. Butylmethyl imidazolium ([bmim]+) was chosen as the cation for all the ILs studied. Among all the aminate-ILs studied, arginate anion ([ARG]) IL has shown more CO2 absorption capacity followed by lysinate ([LYS]) and histidinate ([HIS]) in terms of mol CO2/mol IL. The high CO2 absorption capacity of [ARG] anion was attributed to the availability of more accessible amine (N) groups. The CO2 molar uptake in aminate-ILs studied is in the order of [ARG] (4N)&gt;[LYS] (2N)&gt;[HIS] (3N)&gt;[MET] (1N)&gt;[LEU] (1N)&gt;[GLY] (1N)&gt;[VAL] (1N)&gt;[ALA] (1N)&gt;[PRO] (1N). The FTIR of the CO2 absorbed aminate-ILs confirms the chemical absorption of CO2 through the formation of carbamate species. The aminate-ILs showed higher CO2 solubility compared to amine functionalized cation ILs and non-functionalized ILs. On mass basis (gCO2/kg IL), aminate-ILs showed 70% more CO2 absorption capacity than primary amine-cation ILs, 150% more than tertiary amine-cation ILs and 600% more than non-functionalized ILs. © 2014 Elsevier B.V.

Chemical Engineering Journal

CO2 absorption studies in amino acid-anion based ionic liquids

We demonstrate that the coffee ring effect in drying microdroplets is strongly influenced by the topography of the substrate. The fluid motion and particle distribution inside the evaporating droplet were investigated through Particle image Velocimetry (PIV) experiments. Numerical simulations were also performed and the results were compared with the experiments. It was observed that the patterning in the self-assembly can be controlled and is expected to contribute significantly in the fabrication of devices associated with many applications such as sensor, soft lithography and touch screen in smart phones. © 15CBMS-0001.

MicroTAS 2015 - 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences

Microflow and particle transport in sessile evaporating droplet on curved surfaces

Computing and Systems Technology Division 2015 - Core Programming Area at the 2015 AIChE Annual Meeting

Algorithm to deal with very large and very small numbers to enable utilization of analytical solution of free radical polymerization during gel effect

This article presents the implementation of the Chiu, Carratt, and Soong (CCS) gel/glass model in an analytical solution (AS) derived for the free radical polymerization under isothermal condition. This implantation allows AS to be applicable for the whole range of conversion thus making it more useful for practical applications. The results were compared with numerical solution (NS) as well as with experimental data for two different monomers: styrene (St) and methyl methacrylate (MMA). MMA with chain transfer agent was also used for this purpose. The results were found to be in good agreement with both NS and experimental data. NS with and without quasi-steady state assumption were also found to be in good agreement with each other for the entire range of conversion. As constant time step was used, the effect of stiffness on AS during gel effect was visible at low temperature compared to higher temperature. © 2014 American Chemical Society.

Macromolecules

Analytical solution of free radical polymerization: Applications- implementing gel effect using CCS model

Continuous-flow atom transfer radical polymerization of 2-(dimethylamino)ethyl methacrylate in tubular microreactors of different diameters and geometries is studied. Scale-up of tubular reactors from micro (876μm ID) to milliscales (1753 and 4083μm IDs) is investigated. Coil flow inverter (CFI) reactors having 3 and 6m length (three and seven bends, respectively) are also considered for this study. Positive effects of flow inversion are visible in all three types of reactors expressed by an increase in molecular weight and monomer conversion as well as a decrease in the PDI for the same operating parameters. An increase in reactor diameter results in an increase in the throughput. It is worthy to mention that a CFI reactor having 4083μm ID and 3m (three bends) is found to increase the throughput by ten times without significantly increasing the PDI compared to a coiled tube microreactor with 876μm ID, 6m and no flow inversion pattern. However, pressure drops are higher (+0.1 bar) in case of larger diameter tubes. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Macromolecular Reaction Engineering

Flow inversion: An effective means to scale-up controlled radical polymerization tubular microreactors

Several amine-functionalized cation ILs were successfully synthesized, characterized and tested for CO2 absorption studies. Non-functionalized ILs were studied for comparison purpose. The primary amine-ILs have shown higher CO2 solubilities both on molar basis (mol of CO2/mol of IL) and on volume basis (g of CO2/L of IL), compared to non-functionalized and tertiary-amine ILs. Besides amine-functionalization of the cation, anion of the IL has strong influence on CO2 solubility due to its contribution by physical absorption. As a result, the [TF2N]-anion amine-ILs have higher CO2 solubility than [BF4]-anion amine-ILs. Among non-amine functionalized ILs, [P(14)666][TF2N] has the highest CO2 solubility. © 2013 The Korean Society of Industrial and Engineering Chemistry.

Journal of Industrial and Engineering Chemistry

Carbon dioxide absorption studies using amine-functionalized ionic liquids

In the present study, catalytic wet peroxidation (CWPO) process has been applied for the degradation of nitrobenzene bearing wastewater using copper impregnated granular activated carbon with hydrogen peroxide as oxidant. Various catalysts were prepared by varying the Cu loading on activated carbon and these were characterized by N2 adsorption-desorption, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FE-SEM), thermogravimetric analysis (TGA), temperature programmed reduction (TPR) and UV-visible diffuse reflectance spectra (UV-DRS). Effect of parameters such as copper loading, pH, catalyst dose, hydrogen peroxide dose and temperature were explored on nitrobenzene and total organic carbon (TOC) removal. TOC removal of 89% was achieved after 4 h treatment at optimum condition of Cu loading = 2.5 wt%, pH ≈ 3.0, catalyst dosage = 0.25 g/l, oxidant dosage = 4.5 (moles of oxidant/moles of NB) and temperature = 55 C. © 2014 Elsevier B.V. All rights reserved.

Separation and Purification Technology

Catalytic oxidation of nitrobenzene by copper loaded activated carbon

Evaporation-induced particle deposition patterns like coffee rings provide easy visual identification that is beneficial for developing inexpensive and simple diagnostic devices for detecting pathogens. In this study, the effect of chemotaxis on such pattern formation has been realized experimentally in drying droplets of bacterial suspensions. We have investigated the velocity field, concentration profile, and deposition pattern in the evaporating droplet of Escherichia coli suspension in the presence and absence of nutrients. Flow visualization experiments using particle image velocimetry (PIV) were carried out with E. coli bacteria as biological tracer particles. Experiments were conducted for suspensions of motile (live) as well as nonmotile (dead) bacteria. In the absence of any nutrient gradient like sugar on the substrate, both types of bacterial suspension showed two symmetric convection cells and a ring like deposition of particles after complete evaporation. Interestingly, the droplet containing live bacterial suspension showed a different velocity field when the sugar was placed at the base of the droplet. This can be attributed to the chemoattractant nature of the sugar, which induced chemotaxis among live bacteria targeted toward the nutrient site. Deposition of the suspended bacteria was also displaced toward the nutrient site as the evaporation proceeded. Our experiments demonstrate that both velocity fields and concentration patterns can be altered by chemotaxis to modify the pattern formation in evaporating droplet containing live bacteria. These results highlight the role of bacterial chemotaxis in modifying coffee ring patterns.

Langmuir : the ACS journal of surfaces and colloids

Fluid flow and particle dynamics inside an evaporating droplet containing live bacteria displaying chemotaxis

Analysis of fluid flow and particle transport inside evaporating droplets exposed to external radiation was carried out by experiments and numerical simulations. In this study, we have shown that by altering the free surface temperature we can modify the fluid flow profile inside the droplet and hence the deposition pattern of solute particles on the substrate. The fluid velocity and particle concentration profiles inside the evaporating droplet were measured by Particle Image Velocimetry (PIV) technique. Experiments were carried out on a small sessile water droplet containing dispersed polystyrene particles. To avoid problem of image correction encountered in PIV measurements with 3D droplets, our experiments were performed on an equivalent disc shaped 2D drop sandwiched between two non-wetting surfaces, while the base of the droplet was pinned to a wetting surface. The top surface of the droplet was heated by Infrared (IR) light. The temperature of droplet surface was measured by thermocouples. The velocity field, particle concentration profile and particle deposition patterns were studied during evaporation process. We have also performed numerical simulations by solving continuity, momentum and energy transport equations. The computed velocity profiles resulting from buoyancy and Marangoni convection are in qualitative agreement with the experiments. © 2013 Elsevier Ltd. All rights reserved.