Kajari Kargupta

Templating of Thin Films Induced by Dewetting on Patterned Surfaces

The instability, dynamics, and morphological transitions of patterns in thin liquid films on chemically heterogeneous striped surfaces are investigated based on 3D nonlinear simulations. The film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic length scale of the instability, lambda(h). The thin film pattern replicates the substrate surface energy pattern closely only when (i) the periodicity of substrate pattern lies between lambda(h) and 2lambda(h), and (ii) the stripe width is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length above which complex morphological features unlike the substrate pattern are formed.

Morphological self-organization by dewetting in thin films on chemically patterned substrates

The spontaneous pattern formation and morphological transitions in thin liquid films on chemically heterogeneous, periodic patterned surfaces are studied based on nonlinear simulations. Conditions are identified for the creation of desired mesostructures in soft materials by spontaneous dewetting on patterned substrates. On a surface consisting of alternating less and more wettable stripes, dewetting is suppressed on some less wettable stripes when their spacing is below a characteristic length scale of instability (λh), which is smaller than the spinodal length scale of instability. Ideal templating, i.e., replication of the substrate surface energy pattern in the thin film morphology occurs only when (a) the periodicity of substrate pattern is greater than λh, (b) width of the less wettable stripe is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length above which complex morphological features bearing little resemblance to the su...

Analysis of the performance of a continuous membrane bioreactor with cell recycling during ethanol fermentation

Abstract The effects of the introduction of cell recycling on ethanol productivity were examined for a continuous membrane fermentor-separator (CMFS) with continuous removal of ethanol by pervaporation. Modifications of the Ghose-Tyagi specific growth rate model and the Luedeking-Piret production model were used to formulate and simulate the CMFS model. The effects of pervaporation for systems with and without a cell separator were compared in terms of yeast cell density, substrate utilization, ethanol concentration in the fermentation broth and productivity. The results demonstrate that the elimination of cell wash-out allows the CMFS with cell separator unit to operate at a very high value of dilution rate and increases the productivity of ethanol at the same value of the pervaporation factor (PF) compared with a system without cell separator. An increase in the value of PF always results in an increase in ethanol productivity.

Reduced graphene oxide-polyaniline composites—synthesis, characterization and optimization for thermoelectric applications

Reduced graphene oxide (rGO) can improve the thermoelectric properties of polyaniline (PANI) by varying its concentration in composites of rGO nanosheets and PANI. The figure of merit (ZT) of rGO–PANI composites is increased with an increasing percentage of rGO (up to 50%), which is 7.5 times higher as compared to pure PANI. High resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) analyses show a uniform growth of PANI over the surface of rGO as a template, leading to a more ordered structure with high crystallinity during polymerization. Compared to pure PANI, both the electrical conductivity and thermoelectric power of the rGO–PANI composite is higher due to the increased carrier mobility as confirmed by a Hall effect measurement. Fourier transform infrared spectroscopy (FTIR), ultra-violet visible range spectroscopy (UV-Vis) and Raman spectroscopy analyses reveal that strong π–π interactions assisted the uniform distribution of PANI on the rGO nanosheets. Other strong interactions include electrostatic forces and hydrogen bonding between rGO and PANI, which provide a route for constructing highly ordered chain structures with improved thermoelectric performance of PANI. There is no significant change in the thermal conductivity of the rGO–PANI composite as compared to pure PANI, which improves the thermoelectric performance of composite.

Synthesis, characterization and enhanced thermoelectric performance of structurally ordered cable-like novel polyaniline–bismuth telluride nanocomposite

Bismuth telluride (Bi₂Te₃) nanorods and polyaniline (PANI) nanoparticles have been synthesized by employing solvothermal and chemical oxidative processes, respectively. Nanocomposites, comprising structurally ordered PANI preferentially grown along the surface of a Bi₂Te₃ nanorods template, are synthesized using in situ polymerization. X-ray powder diffraction, UV-vis and Raman spectral analysis confirm the highly ordered chain structure of PANI on Bi₂Te₃ nanorods, leading to a higher extent of doping, higher chain mobility and enhancement of the thermoelectric performance. Above 380 K, the PANI-Bi₂Te₃ nanocomposite with a core-shell/cable-like structure exhibits a higher thermoelectric power factor than either pure PANI or Bi₂Te₃. At room temperature the thermal conductivity of the composite is lower than that of its pure constituents, due to selective phonon scattering by the nanointerfaces designed in the PANI-Bi₂Te₃ nanocable structures. The figure of merit of the nanocomposite at room temperature is comparable to the values reported in the literature for bulk polymer-based composite thermoelectric materials.

Instability and dynamics of thin slipping films

The linear stability analysis of the full Navier–Stokes equations shows that the surface instability and dynamics of thin liquid films are profoundly altered by the presence of slippage on the substrate. For example, the exponents for the length scale (λm∝h0n; h0 is film thickness) and time scale of instability (tr∝h0m) change nonmonotonically with slippage [for van der Waals force induced instability, n∈(1.25,2), m∈(3,6)]. Slippage always encourages faster rupture and can greatly reduce the number density of holes for moderate to strong slip. Thus, any interpretation of thin film experiments, including determination of intermolecular forces from the length and time scales, needs to account for the possibility of slippage.

Self-organized structures in thin liquid films on chemically heterogeneous substrates: Effect of antagonistic short and long range interactions

Surface instability, dynamics, and morphology in spontaneous dewetting of a thin liquid film on a chemically heterogeneous substrate are studied based on nonlinear simulations for a system subjected to a long range van der Waals attraction and soft short-range repulsion. Characteristics of dewetting by a heterogeneity are clearly contrasted with the spinodal dewetting on a homogeneous surface. In the presence of a chemical heterogeneity, the instability is engendered by the gradient of intermolecular interactions that lead to a microscale wettability contrast. The time scale of instability can be substantially less than the spinodal time scale, especially for thinner films close to the critical thickness, and it varies inversely with the potential difference induced by the heterogeneity. Heterogeneity, on a very small length scale, can even destabilize a spinodally stable film. A local ordering of the structure (droplets and holes) around the heterogeneity produces “castle-moat,” “ripples,” and “flower” l...

Thin liquid films on chemically heterogeneous substrates: self-organization, dynamics and patterns in systems displaying a secondary minimum

Surface instability in a thin liquid film engendered by a micro-scale wettability contrast, resulting from the spatial gradients in the intermolecular interactions, is investigated based on 3D simulations for a system displaying both the primary and secondary minima in its force vs. distance curve. Characteristic dynamical and morphological features of the evolution on a chemically heterogeneous substrate are identified for two different cases: (a) a single patch or step of heterogeneity (b) multiple periodically arranged stripes of heterogeneity. The presence of heterogeneity can cause true rupture at the primary minimum by surmounting the energy barrier between the two minima. The breakup time on heterogeneous surfaces varies inversely with the gradient in the potential induced by the heterogeneity and can be several orders of magnitude smaller than the spinodal dewetting time on homogeneous surfaces. Heterogeneity can also cause rupture in spinodally stable films and produce complex and locally ordered morphological features (e.g., ripples) absent in the spinodal dewetting. On a chemically patterned surface composed of alternating more and less wettable stripes, film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic lengthscale of instability, λh which is close to the spinodal lengthscale. Thin film pattern ideally replicates the surface energy pattern only when, (a) the periodicity of substrate pattern is greater than λh, (b) width of the less wettable stripe is lower than a transition length above which complex morphological features are formed.

Cryogenic carbon dioxide separation from natural gas: a review based on conventional and novel emerging technologies

Abstract Fossil fuels are the major contributors to the emission of anthropogenic carbon dioxide (CO2) to the atmosphere, rendering global warming a challenging issue to the researchers and industries. Although natural gas has been recommended as a clean fuel compared to other fossil fuels, geological sources of natural gas are not free of impurities. Economical commercialization of natural gas with high sour gas contents as well as facilitating the geosequestration of sour gases for enhanced oil recovery (EOR) need several environmentally sound and cost-effective gas separation methods. Moreover, stringent restrictions should be drawn to mitigate the unfettered greenhouse gas emissions to the atmosphere. In the present study, existing low-temperature conventional CO2 capture methods, namely, cryogenic distillation process along with emerging nonconventional and hybrid methods, have been demonstrated. Also, the limitations and operational conditions during the application of these processes have been mentioned. The future prospects of the emerging technologies have been compared with conventional methods. Hybrid cryogenic distillation networks for multiproduct industrial production of different hydrocarbons and CO2 products at higher pressures of 40 bar and above showed promising potentials. A concise classification and summary of innovative emerging technologies along with conventional methods has been presented in this paper for possible future commercial exploitation.

Quantitative approach for the prediction of preferential sorption in the case of pervaporation of a physico-chemically similar binary mixture

Selective transport by pervaporation of physico-chemically similar and polar components through a polar membrane is complicated due to competitive sorption and diffusion phenomena. In the present study the mechanism of preferential sorption has been predicted for such a system, methanol-ethylene glycol-cellophane. The mechanism has been explained in terms of intercrystalline swelling of the polymer matrix in presence of increasing methanol concentration in the feed. The reduction in preferential sorption at increasing wt% of methanol in the feed may be due to the increasing accessibility of the membrane towards ethylene glycol. This phenomenon has been quantitatively explained by considering a non-linear dependence on concentration of the binary liquid-polymer interaction parameter. Theoretical sorption data have been derived from the Flory-Huggins thermodynamics by using the swelling equilibrium condition. The coupling and plasticization phenomena in sorption are explained in terms of liquid-polymer interaction parameters. The theoretical results show good agreement with previously published experimental data.