Chemical Engineering

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Shiv Nadar University

The self-assembly and deposition mechanisms of nanoparticles in droplets on a substrate are significant in many inkjet printing-based industrial applications such as microelectronics, display systems, and paint manufacturing. In this study, we describe the underlying lack mechanisms of the self-assembly and deposition behavior of nanoparticles in inkjet-printed. We also demonstrate the development of flexible and photonic crystals structural color based on self-assembled silica photonic crystals. © 2017 IEEE.

(SoE) School of Engineering

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TRANSDUCERS 2017 - 19th International Conference on Solid-State Sensors, Actuators and Microsystems

Permanent encapsulation of nanoparticle patterns formed by inkjet printer for transparent and flexible anti-counterfeit applications

The linear stability of a bilayer fiber spinning process is analyzed for axisymmetric disturbances under isothermal conditions. The co-extruded fiber consists of two concentric layers of rheologically stratified polymeric fluids. While the core fluid is an entangled polymer melt described using the eXtended Pom-Pom (XPP) model, the sheath layer is a low molecular weight unentangled polymer modeled as an Upper Convected Maxwell (UCM) fluid. The analysis indicates that, for fast extensional flows, such an arrangement enhances the critical draw ratio of the process over that found from the spinning of XPP fluid alone, thus delaying the onset of draw resonance and stabilizing the system. Further, the range of flow Deborah number for stable spinning is found to be much broader than that for the UCM fluid alone. For low to moderately elastic flows, the stability behavior is governed mainly by the rheology of the more-elastic core layer (XPP fluid), whereas for the highly elastic flows, the stability behavior is dominated by the less-elastic sheath layer (UCM fluid). The sensitivity of the stability diagram with respect to various spinning flow parameters, like the relative fraction of core and sheath layers, the rheological stratification of two fluids, and the spine-line dimensions, has also been examined in order to identify the region of enhanced stability such that the draw resonance is suppressed. © 2017 Elsevier Ltd

Chemical Engineering Science

Stability analysis of bilayer polymer fiber spinning process

The self-assembly and deposition mechanisms of nanoparticles in droplets on a substrate are of significant importance in many inkjet printing-based industrial applications such as microelectronics, display systems, and paint manufacturing. However, a comprehensive investigation into the velocity field of fluid and its accompanying particle transport behavior in injected droplets undergoing immediate evaporation has not been conducted. In this study, we describe the underlying mechanisms of the self-assembly and deposition behavior of nanoparticles in inkjet-printed, evaporating droplets by visualizing the internal fluid flows. We additionally characterize the relationship between the internal fluid flows and nanoparticle patterns by changing not only the wettability and temperature of the substrate, but also the chemical composition of nanoparticle suspensions. We verify that Marangoni flow generated on a hydrophobic PDMS substrate with a contact angle (CA) of >90° helps the formation of dome-shaped nanoparticle structures, while radially outward flow generated on a hydrophilic glass substrate with a CA of <10° produces either mono-layered and flat, or ring-shaped nanoparticle structures, depending on the number density of the suspension. The presented characterization results provide not only valuable mechanistic insights, but also practical guidelines for inkjet printing-based nanoparticle applications. © 2017 Elsevier B.V.

Sensors and Actuators, B: Chemical

Characterizing self-assembly and deposition behavior of nanoparticles in inkjet-printed evaporating droplets

In this study, we propose an ultra-fast responsive PC based volatile organic compounds (VOCs) sensor using a nanoscale easy tear process inspired by commercially available 'easy tear package'. Colloidal crystal-polydimethylsiloxane (PDMS) composite can be realized through nanoscale tear propagation along the interface between the outer surface of crystallized nanoparticles and bulk PDMS. Not only cuboid but also dome shaped colloidal crystal-PDMS composite is successfully obtained using the assembled nanoparticles constructed by wettability contrast patterning or inkjet printing. The response time for VOCs detection is dramatically improved by allowing the direct contact with VOCs because of removing the residual on colloidal crystals perfectly. © 2017 IEEE.

Nanoscale easy tear process for ultra-fast responsive colloidal crystal-PDMS composite VOCs sensors

Experiments and numerical simulations were carried out for an evaporating sessile droplet. The droplet was confined in the narrow gap between two glass plates, making it a "Hele-Shaw" droplet and particle image velocimetry technique was used. In case of the evaporating droplet with pinned contact line and exposed to ambient condition, two symmetric but counterrotating convection cells were observed. After complete evaporation, the particles deposited on the substrate near the contact line. The direction of the flow was reversed for a droplet placed on uniformly heated substrate, and the final deposition pattern was a large spot at the center with a thin line at the periphery. For asymmetrically heated substrate a single convection cell appeared, and the final deposition was also asymmetric. When the liquid was subjected to localized heating, the contact line no longer remains pinned and a relatively uniform deposition was obtained after complete drying. © 2015 American Institute of Chemical Engineers.

AIChE Journal

Internal fluid motion and particle transport in externally heated sessile droplets

The role of die-swell in the stability of fiber spinning flow of entangled polymer melts is analyzed using the linear as well as finite amplitude stability analyses. The nonlinear extensional rheology of polymer is described by using the eXtended Pom-Pom (XPP) model which captures the strain hardening effect present in the realistic flows. The stability of the slender body fiber profile to axisymmetric disturbances is studied under the isothermal conditions. At the top end of the spinning flow near die-exit, the swelling in fiber cross-section area is introduced whose magnitude is governed both by the flow within the die and the outside spinning force. Introducing die-swell in the analysis significantly influences the stability of the spinning flow. The flexibility in the fiber profile at the upstream end of the flow, a feature absent in the previous analyses ignoring die-swell, plays a stabilizing role. However, for a given flow, die-swell implies stronger effective extension than indicated by the apparent draw ratio, defined based on capillary area. This higher effective extension tends to decrease the critical draw ratio for the onset of draw resonance, suggesting destabilizing role of die-swell. The destabilizing role is found to dominate in the overall stability behavior. The stability diagram is constructed by plotting the critical draw ratio for the onset of draw resonance against the flow Deborah number. The sensitivity of the stability behavior on the rheology parameters and the die-geometry has been studied. The finite amplitude disturbances are also considered using the weakly nonlinear stability analysis in order to understand the nature of flow in the proximity of the transition point. The flow exhibits supercritical Hopf bifurcation for low to moderate Deborah numbers, indicating the presence of an oscillatory steady state in the unstable regime. The amplitude of the oscillations in the fiber cross-section area is estimated. The finite amplitude bifurcation crosses over to the subcritical manifold as Deborah number is increased beyond a certain crossover value. The subcritical state for fast flows manifests in the form of unbounded growth of transient disturbances of amplitude greater than a certain threshold value. The die-swell effect tends to lower the Deborah number for the crossover to the subcritically unstable state. © 2016 Elsevier B.V.

Journal of Non-Newtonian Fluid Mechanics

Die-swell effect in draw resonance of polymeric spin-line

A new yet simple transformation is proposed to significantly improve the accuracy of computational fluid dynamics (CFD) modeling and simulations of free radical polymerization (FRP) reactions carried out especially in flow microreactors. The new transformation makes the kinetic rate coefficients dimensionless in terms of concentration. To that extent, the chemical data (chemical species concentration and kinetic rate coefficients values) can be fed in original molar form instead of usual mass form to CFD software package while simulating chemical species as passive scalars. The normalization of various variables (passive scalars) helps in reducing the numerical stiffness as well as numerical errors during simulations. Another advantage of this new transformation is that the expression for transformed reaction rate equations remains unchanged thus enabling an easy coding and debugging process. The new transformation was first validated through numerical simulation against theoretical analytical solution of FRP for homogeneous batch reactor. It was then validated through CFD simulation against published experimental data for FRP in coiled tube microreactor under steady-state flow condition. It has also been demonstrated that in CFD simulations of FRP in flow microreactors, significant error arises for the prediction of number-average chain length (and thus $${\text{MW}}_{n}$$MWn, number-average molecular weight) from the use of chemical data in mass form instead of original molar form. This new transformation is thus found to be more suitable for CFD simulations in flow reactors compared to previous Zhu’s transformation. © 2014, Springer-Verlag Berlin Heidelberg.

Microfluidics and Nanofluidics

New transformation proposed for improving CFD simulation of free radical polymerization reactions in microreactors

This work is focused on intermediate pyrolysis of sawdust in a 100. g batch reactor at 500. ±. 10. °C. The pyrolysis vapors coming from reactor were directly quenched with chilled biodiesel from jatropha oil at 4. ±. 2. °C to obtain a suitable blend for stationary engine application. Total bio-oil yield was found to be 37\% (wt.). After layer separation from the resultant mixture, biodiesel miscible bio-oil phase calorific value was found to be 39. ±. 0.5. MJ/kg, which indicated that the bio-oil/biodiesel mixture was stable within the conditions tested and hence suitable for engine applications. The overall process provides a platform for blended biofuel, biochar as substitute of sub-bituminous coal and pyro-gas for combined heating and power application. © 2015 Elsevier B.V.