Biomedical subjects

A V Anil Kumar

Publications and source records attributed to A V Anil Kumar.

8 recordsLinked to original sources

PubMed2006

Quantum effects on adsorption and diffusion of hydrogen and deuterium in microporous materials.

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.

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PubMed2006

Separation of mixtures at nano length scales: blow torch and levitation effect.

A new conceptual basis for the separation of multicomponent molecular mixtures is proposed. A separation method where different components of the mixtures are driven in opposite directions is realized by a judicious combination of two effects, viz., levitation and blow torch effects. Monte Carlo simulations of two Lennard-Jones binary mixtures with different-sized components are shown to be separated well if at least one of the components lies in the anomalous regime and the others lie in the linear regime. A separation factor of 10(8) is obtained on nano length scales as compared to 10(3), obtainable through conventional methods of separation on macrolength scales.

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PubMed2006

Mechanisms influencing levitation and the scaling laws in nanopores: oscillator model theory.

We provide here a detailed theoretical explanation of the floating molecule or levitation effect, for molecules diffusing through nanopores, using the oscillator model theory (Phys. Rev. Lett. 2003, 91, 126102) recently developed in this laboratory. It is shown that on reduction of pore size the effect occurs due to decrease in frequency of wall collision of diffusing particles at a critical pore size. This effect is, however, absent at high temperatures where the ratio of kinetic energy to the solid-fluid interaction strength is sufficiently large. It is shown that the transport diffusivities scale with this ratio. Scaling of transport diffusivities with respect to mass is also observed, even in the presence of interactions.

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PubMed2005

Quantum effect induced reverse kinetic molecular sieving in microporous materials.

We report kinetic molecular sieving of hydrogen and deuterium in zeolite rho at low temperatures, using atomistic molecular dynamics simulations incorporating quantum effects via the Feynman-Hibbs approach. We find that diffusivities of confined molecules decrease when quantum effects are considered, in contrast with bulk fluids which show an increase. Indeed, at low temperatures, a reverse kinetic sieving effect is demonstrated in which the heavier isotope, deuterium, diffuses faster than hydrogen. At 65 K, the flux selectivity is as high as 46, indicating a good potential for isotope separation.

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PubMed2005

Diffusion maximum as a function of size in dense liquids.

We study the diffusion of small guest particles of different sizes in a host fluid at liquid densities using molecular dynamics simulations. We observe an enhancement of the diffusivity of guest particles for a size related to the structure of the void space of the host fluid, analogous to the "levitation effect" observed for guest diffusion in porous solids. Friction and activation energy are found to be minimum for the guest size with maximum self-diffusivity. Wavelength dependent self-diffusivity indicates a monotonic and oscillatory dependence on wave number k for anomalous and linear regimes, respectively. These are associated with single and bi-exponential decay of the incoherent intermediate scattering function.

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PubMed2002

Rotational dynamics of propane in Na-Y zeolite: a molecular dynamics and quasielastic neutron-scattering study.

We report results from molecular dynamics (MD) simulations and quasielastic neutron-scattering (QENS) measurements on the rotational dynamics of propane in Na-Y zeolite at room temperature with a loading of four molecules per alpha cage. Rotational part of the intermediate scattering function F(Q,t) obtained from the MD simulation suggests that rotational motion is faster relative to the translational motion. Various rotational models fitted to the MD data suggest that rotation is isotropic. It is found that the hydrogen atoms lie, on the average, on a sphere of radius 1.88+/-0.05 A, which is also the average distance of the hydrogen atoms from the center of mass of the propane molecule. Results from QENS measurements are in excellent agreement with those obtained from MD, suggesting that the intermolecular potential employed in the MD simulation provides a realistic description of propane motion within faujasite. The rotational diffusion constant D(R) is 1.05+/-0.09 x 10(12) sec(-1) from the QENS data, which may be compared with that obtained from the MD data (0.82+/-0.05 x 10(12) sec(-1)).

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PubMed2002

Source of reaction-diffusion coupling in confined systems due to temperature inhomogeneities.

Diffusion is often accompanied by a reaction or sorption which can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported with a hot zone presumed to be created by a reaction. Our simulations show that localized hot regions can alter both the kinetic and transport properties. Further, enhancement of the diffusion constant is greater for larger barrier height, a surprising result of considerable significance to many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of hot zone in addition to that which normally exists via concentration.

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PubMed2001

Rotational motion of methane within the confines of zeolite NaCa A: molecular dynamics and ab initio calculations.

Molecular dynamics simulation of a five-site model of methane within zeolite NaCaA and ab initio calculations have been reported. Methane shows a preferential orientation during its passage through the eight-ring window. Partial freezing of certain rotational degrees of freedom is observed during the passage of methane through the eight-ring window, which acts as a bottleneck for diffusion of methane. Both the orientation and the rotational motion of methane and its experimental verification can indicate the accuracy of the intermolecular potential between methane and zeolite employed in this study. Intracage motion of methane shows that methane performs a rolling motion rather than a sliding motion within the supercage.

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