• Siddharth Gautam

      Articles written in Pramana – Journal of Physics

    • Molecular motion in restricted geometries

      Siddharth Gautam S Mitra R Mukhopadhyay

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      Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations: An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.

    • Phase transitions in liquid crystal 6O.4 (p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline)

      Siddharth Gautam R R Choudhury L Panicker S Mitra R Mukhopadhyay

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      DSC measurements on p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline (6O.4) showed that the crystalline to liquid crystalline ($K–S_{\text{H}}$) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the $K–S_{\text{H}}$ transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasi-elastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (𝐾 and $S_{\text{H}}$) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the $S_{\text{H}}$ phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.

    • Rotational dynamics of propylene inside Na-Y zeolite cages

      V K Sharma Mala N Rao Siddharth Gautam A K Tripathi V S Kamble S L Chaplot R Mukhopadhyay

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      We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.

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