References of "Anwar, Muhammad 50000433"
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See detailThe early crystal nucleation process in hard spheres shows synchronised ordering and densification
Berryman, Josh UL; Anwar, Muhammad UL; Dorosz, Sven UL et al

in Journal of Chemical Physics (2016), 145

We investigate the early part of the crystal nucleation process in the hard sphere fluid using data produced by computer simulation. We find that hexagonal order manifests continuously in the ... [more ▼]

We investigate the early part of the crystal nucleation process in the hard sphere fluid using data produced by computer simulation. We find that hexagonal order manifests continuously in the overcompressed liquid, beginning approximately one diffusion time before the appearance of the first “solid-like” particle of the nucleating cluster, and that a collective influx of particles towards the nucleation site occurs simultaneously to the ordering process: the density increases leading to nucleation are generated by the same individual particle displacements as the increases in order. We rule out the presence of qualitative differences in the early nucleation process between medium and low overcompressions and also provide evidence against any separation of translational and orientational order on the relevant lengthscales. [less ▲]

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See detailComputer Simulations of Crystallization Mechanism in Polymeric Materials
Anwar, Muhammad UL

Doctoral thesis (2014)

In this work, we have studied crystallization in short polymer chains using molec- ular dynamics simulations. We use a realistic united atom model which is able to reproduce the physical quantities ... [more ▼]

In this work, we have studied crystallization in short polymer chains using molec- ular dynamics simulations. We use a realistic united atom model which is able to reproduce the physical quantities related to phase transitions. We present a study of crystal nucleation from undercooled melts of n-alkanes and identify the molec- ular mechanism of homogeneous crystal nucleation under quiescent conditions and under shear flow. We choose n-eicosane (C20) the length of which is below the en- tanglement length and n-pentacontahectane (C150) the length of which is above the entanglement length so that we can compare results for unentangled and entangled polymer chains. We also provide the crystal growth mechanism of n-eicosane under quiescent conditions. For C150, we present crystal lamellae structure and compare our results with published simulation results. We use a mean first passage time analysis and a committor analysis to determine the critical nucleus size and then to compute the nucleation rate. We observe that the critical nucleus is of cylindrical shape. We report on the effects of shear rate and temperature on the nucleation rates and estimate the critical shear rates, beyond which the nucleation rate in- creases with the shear rate. We show that the critical shear rate corresponds to a Weissenberg number of order unity which is in agreement with previous experimen- tal observation and theoretical work. We also show that the power law behaviour between nucleation rate and shear rate is in agreement with experiments and theory. We compute the viscosity of the system during the formation of crystalline nuclei and we show that the viscosity of the system is not affected by the crystalline nuclei. Finally, we present results of crystallization in the polyethylene (C500) melt under quiescent conditions. [less ▲]

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See detailCrystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow
Anwar, Muhammad UL; Berryman, Josh UL; Schilling, Tanja UL

in Journal of Chemical Physics (2014), (141), 124910

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent ... [more ▼]

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under quiescent conditions and under shear flow. We compare results for n-eicosane (C20) and npentacontahectane (C150), i.e., one system below the entanglement length and one above, at 20%– 30% undercooling. Under quiescent conditions, we observe that entanglement does not have an effect on the nucleation mechanism. For both chain lengths, the chains first align and then straighten locally, then the local density increases and finally positional ordering sets in. At low shear rates the nucleation mechanism is the same as under quiescent conditions, while at high shear rates the chains align and straighten at the same time. We report on the effects of shear rate and temperature on the nucleation rates and estimate the critical shear rates, beyond which the nucleation rates increase with the shear rate. In agreement with previous experimental observation and theoretical work, we find that the critical shear rate corresponds to a Weissenberg number of order 1. Finally, we show that the viscosity of the system is not affected by the crystalline nuclei. [less ▲]

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See detailCrystallization mechanism in melts of short n-alkane chains
Anwar, Muhammad UL; Turci, Francesco UL; Schilling, Tanja UL

in Journal of Chemical Physics (2013), 139(21),

We study crystallization in a model system for eicosane (C20) by means of molecular dynamics simulation and identify the microscopic mechanisms of homogeneous crystal nucleation and growth. For the ... [more ▼]

We study crystallization in a model system for eicosane (C20) by means of molecular dynamics simulation and identify the microscopic mechanisms of homogeneous crystal nucleation and growth. For the nucleation process, we observe that chains first align and then straighten. Then the local density increases and finally the monomer units become ordered positionally. The subsequent crystal growth process is characterized by a sliding-in motion of the chains. Chains preferably attach to the crystalline cluster with one end and then move along the stems of already crystallized chains towards their final position. This process is cooperative, i.e., neighboring chains tend to get attached in clusters rather than independently. [less ▲]

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