Diversity oriented microwave-assisted synthesis of (-)-steganacin aza-analogues

A novel microwave-assisted, highly efficient protocol for the synthesis of hitherto unknown aza-analogues of (-)-Steganacin, a naturally occurring bisbenzocyclooctadiene lignan lactone with potent antileukemic and tubulin polymerization inhibitory activity, has been developed. Focused microwave irradiation is demonstrated to be highly beneficial in promoting the three crucial steps of the sequence to effect the final ring closure: the Suzuki-Miyaura reaction, Cu-mediated A3-coupling, as well as the intramolecular Huisgen 1,3-dipolar cycloaddition.     

A novel and versatile entry to asymmetrically substituted pyrazines

A novel and convenient procedure for the synthesis of asymmetrically tri- and tetrasubstituted pyrazines starting from para-methoxybenzyl-protected 3,5-dichloro-2(1H)-pyrazinones was elaborated. The key step is the conversion of the intermediate para-methoxybenzyl-protected thiopyrazinone upon treatment with MeI/I(2), into a pyrazine, rendering the chlorine in the C5-position susceptible to substitution. This approach entails the orthogonal introduction of the four substituents of the pyrazine scaffold. The application of microwave irradiation during different steps of the sequence has been shown to be highly valuable for speeding up reactions.     

Electrowetting-based control of droplet transition and morphology on artificially microstructured surfaces

Electrowetting (EW) has recently been demonstrated as a powerful tool for controlling droplet morphology on smooth and artificially structured surfaces. The present work involves a systematic experimental investigation of the influence of electrowetting in determining and altering the state of a static droplet resting on an artificially microstructured surface. Extensive experimentation is carried out to benchmark a previously developed energy-minimization-based model that analyzed the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The EW voltage required to trigger a transition from the Cassie state to the Wenzel state is experimentally determined for surfaces having a wide range of surface parameters (surface roughness and fraction of surface area covered with pillars). The reversibility of the Cassie-Wenzel transition upon the removal of the EW voltage is also quantified and analyzed. The experimental results from the present work form the basis for the design of surfaces that enable dynamic control of droplet morphology. A significant finding from the present work is that nonconservative dissipative forces have a significant influence in opposing fluid flow inside the microstructured surface that inhibits reversibility of the Cassie-Wenzel transition. The artificially structured surfaces considered in this work have microscale roughness feature sizes that permits direct visual observation of EW-induced Cassie-Wenzel droplet transition; this is the first reported visual confirmation of EW-induced droplet state transition.     

Palladium-catalyzed copper(I)-mediated cross-coupling of arylboronic acids and 2(1H)-pyrazinones facilitated by microwave irradiation with simultaneous cooling

The application of a palladium-catalyzed Cu(I)-mediated Liebeskind-Srogl protocol for the decoration of the 2(1H)-pyrazinone scaffold resulted in significantly improved yields and rates when performed under microwave irradiation with simultaneous cooling.     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

Dynamics of ice nucleation on water repellent surfaces

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.     

A finer modeling approach for numerically predicting single track geometry in two dimensions during Laser Rapid Manufacturing

We simulated the temperature distribution and single track geometry in Laser Rapid Manufacturing (LRM) of metal structures using two dimensional FEM with a finer modeling approach. This approach involved the calculation of excessive enthalpies above melting point for all nodal points in the process domain and using those for the computation of local track height at every node along the track width on the substrate. Laser power, laser beam size with its spatial profile, scan speed, powder feed rate and powder stream diameter with its flow distribution were taken as user-defined input parameters to simulate the single track geometry during LRM. The simulated results were experimentally verified. The percentage errors in simulated and corresponding experimental track heights along the normalized track width were calculated and compared with those of other existing models. With our modeling approach, the localized errors in predicting track geometries were found to be the least. This model is capable of dynamically predicting the temperatures and track geometry at various user-defined input parameters.     

Modelling of displacement washing of pulp fibers using orthogonal collocation on finite elements

Flow of fluid through packed bed of porous particles is modelled with the help of Peclet number (Pe) and Biot number (Bi). Packed bed is divided into three zones, flowing liquor, intrapore solute present in pores of particles and solute adsorbed on particle surface. Langmuir isotherm is used to describe the relationship between intrapore solute concentration and concentration of solute adsorbed on particle surface, whereas the bulk fluid concentration and the intrapore solute concentration are related by linear adsorption isotherm. Model predicted values are also compared with the experimental values. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Additive-free Aqueous Dispersions of Two-Dimensional Materials with Glial Cell Compatibility and Enzymatic Degradability

Water-dispersible two-dimensional (2D) materials are desirable for diverse applications. Aqueous dispersions make processing safer and greener and enable evaluation of these materials on biological and environmental fronts. To evaluate the effects of 2D materials with biological systems, obtaining dispersions without additives is critical and has been a challenge. Herein, a method was developed for obtaining additive-free aqueous dispersions of 2D materials like transition metal dichalcogenides and hexagonal boron nitride (h-BN). The nanosheet dispersions were investigated through spectroscopic and microscopic methods, along with the role of size on stability. The aqueous media enabled investigations on cytocompatibility and enzymatic degradation of molybdenum disulphide (MoS₂) and h-BN. Cytocompatibility with mixed glial cells was observed up to concentrations of 100 μg mL⁻¹, suggesting their plausible usage in bioelectronics. Besides, biodegradation using human myeloperoxidase (hMPO) mediated catalysis was investigated through Raman spectroscopy and electron microscopy. The findings suggested that additive-free 2H-MoS₂ and h-BN were degradable by hMPO, with 2H-phase exhibiting better resistance to degradation than the 1T-phase, while h-BN exhibited slower degradation. The findings pave a path for incorporating 2D materials in the burgeoning field of transient bioelectronics.