Ajay S. Panwar

Brownian dynamics simulations of polymer stretching and transport in a complex electroosmotic flow

Brownian dynamics simulations are performed to study the stretching and transport of flexible polymers in complex electroosmotic flows. The flows are generated by prescribing a spatially periodic charge distribution on the walls of a parallel-plate channel and applying an electric field parallel to the direction of charge modulation. The polymer molecules are modeled as bead–spring chains and the model parameters are chosen to be representative of DNA. Simulations are performed for the cases of uncharged and charged polymers. For the uncharged case, it is found that the stagnation point in the center of the flow is not effective at stretching the polymers due to the fact that the flow has an inhomogeneous velocity gradient. It is observed that polymers tend to become trapped in the recirculation rolls near that stagnation point, and that the amount of stretching that does occur is directly proportional to the amount of time that the polymer spends near the stagnation point at the wall. For the charged cas...

Brownian dynamics simulations of polyelectrolyte adsorption in shear flow

Brownian dynamics simulations are used to study the adsorption of an isolated polyelectrolyte molecule onto an oppositely charged flat surface in the absence and the presence of an imposed shear flow. The polyelectrolyte is modeled as a freely jointed bead-rod chain where excluded volume interactions are incorporated by using a hard-sphere potential. The total charge along the backbone is distributed uniformly among all the beads, and the beads are allowed to interact with one another and the charged surface through screened Coulombic interactions. The simulations are performed by placing the molecule a fixed distance above the surface, and the adsorption behavior is then studied as a function of screening length. In the absence of an imposed flow, the chain is found to lie flat and extended on the adsorbing surface in the limit of weak screening, whereas in the limit of strong screening it desorbs from the surface and attains free-solution behavior. For intermediate screening, only a small portion of the chain adsorbs and it becomes highly extended in the direction normal to the surface. An imposed shear flow tends to orient the chain in the direction of flow and also leads to increased contact of the chain with the surface.

Electrophoresis of a bead-rod chain through a narrow slit: A Brownian dynamics study

We use two-dimensional Brownian dynamics simulations to study the electrophoresis of a bead-rod chain through a narrow slit. A constant electric field is assumed to act inside and outside of the slit, and each bead on the chain is assigned a constant uniform charge. We calculate the dependence of the polymer transit velocity on chain length, slit dimensions (width-to-length ratio), and electric-field strength. For sufficiently narrow slits, the transit velocity increases nonlinearly with the applied field for low-field strengths, whereas it increases linearly for high-field strengths. In the low-field strength region and for sufficiently narrow slits, the transit velocity decreases rapidly for small chain lengths and then decreases slowly beyond a critical chain length. As the slit width increases, the transit velocity decreases with chain length in more continuous manner, and for sufficiently large slits the transit velocity becomes independent of chain length as expected. Distributions of the chain end-to-end distances and the translocation times depend strongly on the relative size of the chain to the slit. These results show the sensitivity of the transit velocity vs chain length relationship to the slit dimensions and applied electric-field strength, and suggest that there may be an optimal slit width for a given field strength and vice versa. The results may be useful for microfluidic separations and for understanding the motion of biological polymers through narrow constrictions.

Effect of Reaction Temperature on Structural and Optical Properties of Reduced Graphene Oxide

Present study is aimed to see the influence of reaction temperature on the structural and optical properties of reduced graphene oxide powder samples. In the present work, aqueous solutions of graphite oxide (GO) were reduced at two different temperatures (80 and 95° C) using hydrazine hydrate. XRD and Raman studies showed that sample reduced at higher temperature has lesser defects. UV-visible spectral studies showed a blue shift in the absorption peak of GO after reduction due to the increased structural ordering because of restoration of sp 2 carbon with temperature. More shift is observed for the sample reduced at higher temperature. Our study establishes a correlation between structural and optical properties of reduced graphene oxide powder samples with temperature.

Exoelectrogens Leading to Precise Reduction of Graphene Oxide by Flexibly Switching Their Environment during Respiration

Reduced graphene oxide (RGO) has been prepared by a simple, cost-effective, and green route. In this work, graphene oxide (GO) has been reduced using Gram-negative facultative anaerobe S. dysenteriae, having exogenic properties of electron transfer via electron shuttling. Apparently, different concentrations of GO were successfully reduced with almost complete mass recovery. An effective role of lipopolysaccharide has been observed while comparing RGO reduced by S. dysenteriae and S. aureus. It was observed that the absence of lipopolysaccharide in Gram-positive S. aureus leads to a disrupted cell wall and that S.aureus could not survive in the presence of GO, leading to poor and inefficient reduction of GO, as shown in our results. However, S. dysenteriae having an outer lipopolysaccharide layer on its cell membrane reduced GO efficiently and the reduction process was extracellular for it. RGO prepared in our work has been characterized by X-ray diffraction, ζ potential, X-ray photoelectron spectroscopy, and Raman spectroscopy techniques, and the results were found to be in good agreement with those of chemically reduced GO. As agglomeration of RGO is the major issue to overcome while chemically reducing GO, we observed that RGO prepared by a bacterial route in our work has ζ potential value of -26.62 mV, good enough to avoid restacking of RGO. The role of exoelectrogens in electron transfer in the extracellular space has been depicted. Toxin released extracellularly during the process paves the way for reduction of GO due to its affinity towards oxygen.

Deagglomeration of multi-walled carbon nanotubes via an organic modifier: structure and mechanism

We have investigated the agglomeration behaviour of two types of multi-walled carbon nanotubes (MWNTs; N-MWNTs and D-MWNTs), which have different chemical functionalities, average diameter, varying extent of agglomeration and agglomerations. The properties were altered by varying the agglomerated structure. The strength of the MWNT agglomerates was estimated via nanoindentation. The work done to indent D-MWNT agglomerates (3910.3 × 10(-8) erg) was higher than for N-MWNTs agglomerates (2316.4 × 10(-8) erg). An organic modifier, the Li salt of 6-aminohexanoic acid (Li-AHA), was used to deagglomerate the MWNTs in an aqueous medium. The stability of the aqueous dispersion of Li-AHA-modified MWNTs was analyzed by UV-vis spectroscopy and zeta potential measurements. An increase in Li-AHA concentration increased the dispersion of MWNTs in the aqueous medium. Furthermore, the mechanism of dispersion of the two types of MWNTs in the aqueous medium in the presence of Li-AHA was determined based on the electrostatic charge repulsion between the negatively charged species. A fluorescence-activated cell sorting technique was used to assess the debundling of MWNT agglomerates in the aqueous medium. We examined the morphology-property relationship in Li-AHA-modified MWNTs.

Dispersion of non-covalently modified graphene in aqueous medium: a molecular dynamics simulation approach

Molecular dynamics were used to simulate the dispersion of graphene in aqueous medium in the presence of a novel organic modifier, sodium salt of 6-amino hexanoic acid (Na-AHA), which non-covalently modifies the graphene surfaces. The modifier molecule contains an ionizable carboxylate head group and an aliphatic tail. The extent of dispersion was estimated by calculating the potential of mean force (PMF) as a function of increasing concentration of the modifier using the thermodynamic perturbation method in conjunction with molecular dynamics simulations. With increasing concentration of the modifier, the PMF changed from a short-range strong attraction to a long-range repulsion at higher modifier concentrations. The simulation results clearly show the adsorption of modifier molecules at the graphene–water interface, which in turn causes the graphene surfaces to acquire a negative charge. Further, the development of a negative electric potential at the graphene surfaces induces a long-range electrostatic repulsion between the graphene sheets, clearly pointing to an electrostatic stabilization of Na-AHA modified-graphene in aqueous medium.