665 research outputs found
Transition from Icosahedral to Decahedral Structure in a Coexisting Solid-Liquid Nickel Cluster
We have used molecular dynamics simulations to construct a microcanonical
caloric curve for a 1415-atom Ni icosahedron. Prior to melting the Ni cluster
exhibits static solid-liquid phase coexistence. Initially a partial icosahedral
structure coexists with a non-wetting melt. However at energies very close to
the melting point the icosahedral structure is replaced by a truncated
decahedral structure which is almost fully wet by the melt. This structure
remains until the cluster fully melts. The transition appears to be driven by a
preference for the melt to wet the decahedral structure.Comment: 7 pages, 6 figure
Reentrant Adhesion Behavior in Nanocluster Deposition
We simulate the collision of atomic clusters with a weakly attractive surface
using molecular dynamics in a regime between soft-landing and fragmentation,
where the cluster undergoes large deformation but remains intact. As a function
of incident kinetic energy, we find a transition from adhesion to reflection at
low kinetic energies. We also identify a second adhesive regime at intermediate
kinetic energies, where strong deformation of the cluster leads to an increase
in contact area and adhesive energy.Comment: 7 pages, 6 figure
Solid-liquid phase coexistence and structural transitions in palladium clusters
We use molecular dynamics with an embedded atom potential to study the
behavior of palladium nanoclusters near the melting point in the microcanonical
ensemble. We see transitions from both fcc and decahedral ground state
structures to icosahedral structures prior to melting over a range of cluster
sizes. In all cases this transition occurs during solid-liquid phase
coexistence and the mechanism for the transition appears to be fluctuations in
the molten fraction of the cluster and subsequent recrystallization into the
icosahedral structure.Comment: 8 pages, 6 figure
Molecular dynamics simulations of reflection and adhesion behavior in Lennard-Jones cluster deposition
We conduct molecular dynamics simulations of the collision of atomic clusters
with a weakly-attractive surface. We focus on an intermediate regime, between
soft-landing and fragmentation, where the cluster undergoes deformation on
impact but remains largely intact, and will either adhere to the surface (and
possibly slide), or be reflected. We find that the outcome of the collision is
determined by the Weber number, We i.e. the ratio of the kinetic energy to the
adhesion energy, with a transition between adhesion and reflection occurring as
We passes through unity. We also identify two distinct collision regimes: in
one regime the collision is largely elastic and deformation of the cluster is
relatively small but in the second regime the deformation is large and the
adhesion energy starts to depend on the kinetic energy. If the transition
between these two regimes occurs at a similar kinetic energy to that of the
transition between reflection and adhesion, then we find that the probability
of adhesion for a cluster can be bimodal. In addition we investigate the
effects of the angle of incidence on adhesion and reflection. Finally we
compare our findings both with recent experimental results and with macroscopic
theories of particle collisions.Comment: 18 pages, 13 figure
Effective slip boundary conditions for flows over nanoscale chemical heterogeneities
We study slip boundary conditions for simple fluids at surfaces with
nanoscale chemical heterogeneities. Using a perturbative approach, we examine
the flow of a Newtonian fluid far from a surface described by a heterogeneous
Navier slip boundary condition. In the far-field, we obtain expressions for an
effective slip boundary condition in certain limiting cases. These expressions
are compared to numerical solutions which show they work well when applied in
the appropriate limits. The implications for experimental measurements and for
the design of surfaces that exhibit large slip lengths are discussed.Comment: 14 pages, 3 figure
Effect of Patterned Slip on Micro and Nanofluidic Flows
We consider the flow of a Newtonian fluid in a nano or microchannel with
walls that have patterned variations in slip length. We formulate a set of
equations to describe the effects on an incompressible Newtonian flow of small
variations in slip, and solve these equations for slow flows. We test these
equations using molecular dynamics simulations of flow between two walls which
have patterned variations in wettability. Good qualitative agreement and a
reasonable degree of quantitative agreement is found between the theory and the
molecular dynamics simulations. The results of both analyses show that
patterned wettability can be used to induce complex variations in flow. Finally
we discuss the implications of our results for the design of microfluidic
mixers using slip.Comment: 13 pages, 12 figures, final version for publicatio
Surface Melting and Breakup of Metal Nanowires: Theory and Molecular Dynamics Simulations
We consider the surface melting of metal nanowires by solving a
phenomenological two-parabola Landau model and by conducting molecular dynamics
simulations of nickel and aluminium nanowires. The model suggests that surface
melting will precede bulk melting when the spreading parameter
for the melt in contact with the solid surface is positive (i.e. if the melt
wets or partially wets the surface) and the wire is sufficiently thick, as is
the case for planar surfaces and sufficiently large nanoparticles. Surface
melting does not occur if is negative. We test this model,
which assumes the surface energies of the wire are isotropic, using molecular
dynamics simulations. For nickel, we observe the onset of anisotropic surface
melting associated with each of the two surface facets present, but this gives
way to uniform surface melting and the solid melts radially until the solid
core eventually breaks up. For aluminium, while we observe complete surface
melting of one facet, the lowest energy surface remains partially dry even up
to the point where the melt completely penetrates the solid core.Comment: 11 pages, 13 figures, article submission preprin
Surface-reconstructed Icosahedral Structures for Lead Clusters
We describe a new family of icosahedral structures for lead clusters. In
general, structures in this family contain a Mackay icosahedral core with a
reconstructed two-shell outer-layer. This family includes the anti-Mackay
icosahedra, which have have a Mackay icosahedral core but with most of the
surface atoms in hexagonal close-packed positions. Using a many-body glue
potential for lead, we identify two icosahedral structures in this family which
have the lowest energies of any known structure in the size range from 900 to
15000 lead atoms. We show that these structures are stabilized by a feature of
the many-body glue part of the interatomic potential.Comment: 9 pages, 8 figure
Superheating and solid-liquid phase coexistence in nanoparticles with non-melting surfaces
We present a phenomenological model of melting in nanoparticles with facets
that are only partially wet by their liquid phase. We show that in this model,
as the solid nanoparticle seeks to avoid coexistence with the liquid, the
microcanonical melting temperature can exceed the bulk melting point, and that
the onset of coexistence is a first-order transition. We show that these
results are consistent with molecular dynamics simulations of aluminum
nanoparticles which remain solid above the bulk melting temperature.Comment: 8 pages, 5 figure
Pengaruh Penggunaan Asbuton Butir Pada Campuran Laston
Kebutuhan aspal di Indonesia semakin lama semakin meningkat, untuk pembuatan jalan baru maupun perbaikan jalan. Konstruksi dengan bahan dasar menggunakan aspal pen 60 di Indonesia seringkali menggunakan bahan impor, sehingga digunakan pemanfaatan aspal modifikasi menggunakan aspal pen 60 dengan asbuton butir. Penelitian ini bertujuan untuk mengetahui pengaruh nilai karakteristik marshall dari campuran laston dengan asbuton butir Lawele yang diharapkan mampu menggantikan sebagian penggunaan aspal pen 60 sebagai perkerasan jalan raya. Penelitian ini menggunakan aspal minyak pen 60 dengan kadar 5,5%, 6%, 6,5%, 7% sesuai dengan ketentuan Binamarga dan campuran laston dengan asbuton butir dan aspal pen 60. Kadar aspal pen 60 dan asbuton butir yang digunakan adalah 65%–35%, 70%–30%, 75%–25%, 80%–20%. Hasil penelitian menunjukkan bahwa campuran laston dengan asbuton butir dapat digunakan sebagai perkerasan jalan, mempunyai nilai stabilitas yang memenuhi syarat campuran aspal modifikasi campuran laston dengan KAO 6%
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