1,390 research outputs found
Zero-temperature phase diagram of the second layer of He adsorbed on graphene
The phase diagram at zero temperature of He adsorbed on an helium
incommensurate triangular solid on top of a single graphene sheet has been
obtained using the diffusion Monte Carlo method. We have found that, in
accordance with previous experimental and simulation results for graphite, the
ground state of He on this setup is a liquid that, upon compression,
transforms into a triangular solid. To define the stability limits of both
liquid and solid phases, we considered not only the adsorption energies of the
atoms located on the second layer but the average energy of the atoms in both
layers. Our results show that the lower density limit for a stable liquid in
the second layer is 0.163 0.005 \AA and that the lower limit for
the existence of an incommensurate solid on the second layer is 0.186
0.003 \AA. Both values are in overall agreement with the results of
torsional oscillator experiments and heat capacity measurements on graphite.
The 4/7 and 7/12 registered solids are found to be metastable with respect to
triangular incommensurate arrangements of the same density.Comment: 7 pages, accepted for publication in Phys. Rev.
4He adsorbed outside a single carbon nanotube
The phase diagrams of He adsorbed on the external surfaces of single
armchair carbon nanotubes with radii in the range 3.42 -- 10.85 \AA \ are
calculated using the diffusion Monte Carlo method. For nanotubes narrower than
a (10,10) one, the ground state is an incommensurate solid similar to the one
found for H on the same substrates. For wider nanotubes, the phase with the
minimum energy per particle is a liquid layer. Curved
registered solids similar to the ones found on graphene and graphite were
unstable for all the tubes considered.Comment: 6 pages, accepted for publication in Phys. Rev.
Liquid and solid phases of 3He on graphite
Recent heat-capacity experiments show quite unambiguously the existence of a
liquid He phase adsorbed on graphite. This liquid is stable at an extremely
low density, possibly one of the lowest found in Nature. Previous theoretical
calculations of the same system, and in strictly two dimensions, agree with the
result that this liquid phase is not stable and the system is in the gas phase.
We calculated the phase diagram of normal He adsorbed on graphite at
using quantum Monte Carlo methods. Considering a fully corrugated substrate we
observe that at densities lower that 0.006 \AA the system is a very
dilute gas, that at that density is in equilibrium with a liquid of density
0.014 \AA. Our prediction matches very well the recent experimental
findings on the same system. On the contrary, when a flat substrate is
considered, no gas-liquid coexistence is found, in agreement with previous
calculations. We also report results on the different solid structures, and the
corresponding phase transitions that appear at higher densities.Comment: 5 page
Isotopic effects of hydrogen adsorption in carbon nanotubes
We present diffusion Monte Carlo calculations of D adsorbed inside a
narrow carbon nanotube. The 1D D equation of state is reported, and the
one-dimensional character of the adsorbed D is analyzed. The isotopic
dependence of the constitutive properties of the quantum fluid are studied by
comparing D and H. Quantum effects due to their different masses are
observed both in the energetic and the structural properties. The influence of
the interatomic potential in one-dimensional systems is also studied by
comparing the properties of D and He which have nearly the same mass
but a sizeably different potential. The physics of molecular hydrogen adsorbed
in the interstitial channels of a bundle of nanotubes is analyzed by means of
both a diffusion Monte Carlo calculation and an approximate mean field method.Comment: 17 pages, revtex, 9 ps figures, to be appear in Phys. Rev.
Phase diagram of H2 adsorbed on graphene
The phase diagram of the first layer of H adsorbed on top of a single
graphene sheet has been calculated by means of a series of diffusion Monte
Carlo (DMC) simulations. We have found that, as in the case of He, the
ground state of molecular hydrogen is a commensurate
structure, followed, upon a pressure increase, by an incommensurate triangular
solid. A striped phase of intermediate density was also considered, and found
lying on top of the equilibrium curve separating both commensurate and
incommensurate solids.Comment: 5 pages, 3 figure
Generation and Breakup of Worthington Jets After Cavity Collapse
Helped by the careful analysis of their experimental data, Worthington (1897)
described roughly the mechanism underlying the formation of high-speed jets
ejected after the impact of an axisymmetric solid on a liquid-air interface. In
this work we combine detailed boundary-integral simulations with analytical
modeling to describe the formation and break-up of such Worthington jets in two
common physical systems: the impact of a circular disc on a liquid surface and
the release of air bubbles from an underwater nozzle. We first show that the
jet base dynamics can be predicted for both systems using our earlier model in
Gekle, Gordillo, van der Meer and Lohse. Phys. Rev. Lett. 102 (2009).
Nevertheless, our main point here is to present a model which allows us to
accurately predict the shape of the entire jet. Good agreement with numerics
and some experimental data is found. Moreover, we find that, contrarily to the
capillary breakup of liquid cylinders in vacuum studied by Rayleigh, the
breakup of stretched liquid jets at high values of both Weber and Reynolds
numbers is not triggered by the growth of perturbations coming from an external
source of noise. Instead, the jet breaks up due to the capillary deceleration
of the liquid at the tip which produces a corrugation to the jet shape. This
perturbation, which is self-induced by the flow, will grow in time promoted by
a capillary mechanism. We are able to predict the exact shape evolution of
Worthington jets ejected after the impact of a solid object - including the
size of small droplets ejected from the tip due to a surface-tension driven
instability - using as the single input parameters the minimum radius of the
cavity and the flow field before the jet emerges
4He adsorbed inside (10,10) single walled carbon nanotubes
Diffusion Monte Carlo calculations on the adsorption of He in open-ended
single walled (10,10) nanotubes are presented. We have found a first order
phase transition separating a low density liquid phase in which all He
atoms are adsorbed close to the tube wall and a high density arrangement
characterized by two helium concentric layers. The energy correction due to the
presence of neighboring tubes in a bundle has also been calculated, finding it
negligible in the density range considered.Comment: 5 pages, accepted for publication in Phys. Rev.
Splash wave and crown breakup after disc impact on a liquid surface
In this paper we analyze the impact of a circular disc on a free surface
using experiments, potential flow numerical simulations and theory. We focus
our attention both on the study of the generation and possible breakup of the
splash wave created after the impact and on the calculation of the force on the
disc. We have experimentally found that drops are only ejected from the rim
located at the top part of the splash --giving rise to what is known as the
crown splash-- if the impact Weber number exceeds a threshold value
\Weber_{crit}\simeq 140. We explain this threshold by defining a local Bond
number based on the rim deceleration and its radius of curvature,
with which we show using both numerical simulations and experiments that a
crown splash only occurs when , revealing that the rim
disrupts due to a Rayleigh-Taylor instability. Neglecting the effect of air, we
show that the flow in the region close to the disc edge possesses a
Weber-number-dependent self-similar structure for every Weber number. From this
we demonstrate that \Bond_{tip}\propto\Weber, explaining both why the
transition to crown splash can be characterized in terms of the impact Weber
number and why this transition occurs for .
Next, including the effect of air, we have developed a theory which predicts
the time-varying thickness of the very thin air cushion that is entrapped
between the impacting solid and the liquid. Our analysis reveals that gas
critically affect the velocity of propagation of the splash wave as well as the
time-varying force on the disc, . The existence of the air layer also
limits the range of times in which the self-similar solution is valid and,
accordingly, the maximum deceleration experienced by the liquid rim, what sets
the length scale of the splash drops ejected when We>\Weber_{crit}
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