4,852 research outputs found
Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow
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 n-pentacontahectane(C150), i.e. one
system below the entanglement length and one above. 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. We show that the viscosity of
the system is not affected by the crystalline nuclei.Comment: 9 page
The Early Crystal Nucleation Process in Hard Spheres shows Synchronised Ordering and Densification
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
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