Abstract:The thickness W and the surface energy A at the free interface of superfluid 4 He are studied. Results of calculations carried out using density functionals for cylindrical and spherical systems are presented in a unified way, including a comparison with the behavior of planar slabs. It is found that for large species W is independent of the geometry. The obtained values of W are compared with prior theoretical results and experimental data. Experimental data favor results evaluated by adopting finite range ap… Show more
“…We find that the interface thickness of n -C 12 F 26 is larger than that of n -C 13 H 28 by about 30% at the same temperature of 375 K. The plot of surface tension vs interfacial thickness for n -C 12 F 26 and n -C 13 H 28 melts in Figure shows that there is an appreciable correlation between the surface tension and the interface thickness of these liquids despite the difference in their chemical constitution. Several studies on liquid/vapor interface or liquid/liquid interface of various melts theoretically suggested that a thicker interface is correlated with a lower interfacial tension. − Therefore, the inherently low cohesive energy density of PTFE melts results in a large interface thickness as well as a low surface tension. 2 Density profile of n -C 12 F 26 melt film along the z -axis normal to the film surface, obtained from MD simulation at 375 K. z = 0 corresponds to the film center.…”
The surface characteristics of thin films of poly(tetrafluoroethylene) (PTFE) melts were investigated
by molecular dynamics simulations, employing the recent explicit atomic force field, including partial charges,
which was fine-tuned to the experimental PVT data, the chain conformation and the crystal structure
(Macromolecules
2003, 36, 5331). Surface tension was calculated from the virial equation of the pressure tensor,
taking into full account of the long-range correction terms, in good agreement with experimental data. Compared
with polymethylenes of the same chain length, PTFEs have larger vacuum/liquid interface thicknesses concomitant
with lower surface tensions. In the surface region, the chain conformations remain unperturbed, but the chain
backbone segments tend to be oriented parallel to the surface whereas the chain-end segments tend to be
perpendicular to the surface. As compared with polymethylenes, the orientation of PTFE segments in the surface
is found to persist deeper into the film due to the larger intermolecular orientational correlation length in PTFE
melts than in polymethylene melts.
“…We find that the interface thickness of n -C 12 F 26 is larger than that of n -C 13 H 28 by about 30% at the same temperature of 375 K. The plot of surface tension vs interfacial thickness for n -C 12 F 26 and n -C 13 H 28 melts in Figure shows that there is an appreciable correlation between the surface tension and the interface thickness of these liquids despite the difference in their chemical constitution. Several studies on liquid/vapor interface or liquid/liquid interface of various melts theoretically suggested that a thicker interface is correlated with a lower interfacial tension. − Therefore, the inherently low cohesive energy density of PTFE melts results in a large interface thickness as well as a low surface tension. 2 Density profile of n -C 12 F 26 melt film along the z -axis normal to the film surface, obtained from MD simulation at 375 K. z = 0 corresponds to the film center.…”
The surface characteristics of thin films of poly(tetrafluoroethylene) (PTFE) melts were investigated
by molecular dynamics simulations, employing the recent explicit atomic force field, including partial charges,
which was fine-tuned to the experimental PVT data, the chain conformation and the crystal structure
(Macromolecules
2003, 36, 5331). Surface tension was calculated from the virial equation of the pressure tensor,
taking into full account of the long-range correction terms, in good agreement with experimental data. Compared
with polymethylenes of the same chain length, PTFEs have larger vacuum/liquid interface thicknesses concomitant
with lower surface tensions. In the surface region, the chain conformations remain unperturbed, but the chain
backbone segments tend to be oriented parallel to the surface whereas the chain-end segments tend to be
perpendicular to the surface. As compared with polymethylenes, the orientation of PTFE segments in the surface
is found to persist deeper into the film due to the larger intermolecular orientational correlation length in PTFE
melts than in polymethylene melts.
“…The main inaccuracies of CNT concern the surface tension as well as the droplet surface area. For the surface tension, deviations from the capillarity approximation γ ≈ γ 0 are known to occur for nanoscopically curved interfaces (Moody and Attard, 2003, Szybisz and Urrutia, 2003, Horsch et al, 2008b. The Tolman (1949) approach implies huge deviations from γ 0 for droplets on the molecular length scale corresponding to the Tolman length δ.…”
Vapour-liquid equilibria (VLE) and the influence of an inert carrier gas on homogeneous vapour to liquid nucleation are investigated by molecular simulation for quaternary mixtures of carbon dioxide, nitrogen, oxygen, and argon. Canonical ensemble molecular dynamics simulation using the Yasuoka-Matsumoto method is applied to nucleation in supersaturated vapours that contain more carbon dioxide than in the saturated state at the dew line. Established molecular models are employed that are known to accurately reproduce the VLE of the pure fluids as well as their binary and ternary mixtures. On the basis of these models, also the quaternary VLE properties of the bulk fluid are determined with the Grand Equilibrium method.Simulation results for the carrier gas influence on the nucleation rate are compared with the classical nucleation theory (CNT) considering the 'pressure effect ' [Phys. Rev. Lett. 101: 125703 (2008)]. It is found that the presence of air as a carrier gas decreases the nucleation rate only slightly and, in particular, to a significantly lower extent than predicted by CNT. The nucleation rate of carbon dioxide is generally underestimated by CNT, leading to a deviation between one and two orders of magnitude for pure carbon dioxide in the vicinity of the spinodal line and up to three orders of magnitude in presence of air as a carrier gas. Furthermore, CNT predicts a temperature dependence of the nucleation rate in the spinodal limit, which cannot be confirmed by molecular simulation.
It is shown that 3 He impurities in sufficiently large 4 He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid-vapor interface. We report the analysis performed for superfluid 4 He adsorbed on the external surface of the nano-fullerene C 60 and on cylindrical nano-wires of Au. It is found that a single 3 He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure 4 He clusters.
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