38 research outputs found
Physics of Solid and Liquid Alkali Halide Surfaces Near the Melting Point
This paper presents a broad theoretical and simulation study of the high
temperature behavior of crystalline alkali halide surfaces typified by
NaCl(100), of the liquid NaCl surface near freezing, and of the very unusual
partial wetting of the solid surface by the melt. Simulations are conducted
using two-body rigid ion BMHFT potentials, with full treatment of long-range
Coulomb forces. After a preliminary check of the description of bulk NaCl
provided by these potentials, which seems generally good even at the melting
point, we carry out a new investigation of solid and liquid surfaces. Solid
NaCl(100) is found in this model to be very anharmonic and yet exceptionally
stable when hot. It is predicted by a thermodynamic integration calculation of
the surface free energy that NaCl(100) should be a well ordered, non-melting
surface, metastable even well above the melting point. By contrast, the
simulated liquid NaCl surface is found to exhibit large thermal fluctuations
and no layering order. In spite of that, it is shown to possess a relatively
large surface free energy. The latter is traced to a surface entropy deficit,
reflecting some kind of surface short range order. Finally, the solid-liquid
interface free energy is derived through Young's equation from direct
simulation of partial wetting of NaCl(100) by a liquid droplet. It is concluded
that three elements, namely the exceptional anharmonic stability of the solid
(100) surface, the molecular short range order at the liquid surface, and the
costly solid liquid interface, all conspire to cause the anomalously poor
wetting of the (100) surface by its own melt in the BMHFT model of NaCl -- and
most likely also in real alkali halide surfaces.Comment: modified version of JCP 123, 164701 15 pages, 25 figure
Melting and nonmelting of solid surfaces and nanosystems
We present an extensive but concise review of our present understanding,
largely based on theory and simulation work from our group, on the equilibrium
behavior of solid surfaces and nanosystems close to the bulk melting point. In
the first part we define phenomena, in particular surface melting and
nonmelting, and review some related theoretical approaches, from heuristic
theories to computer simulation. In the second part we describe the surface
melting/nonmelting behavior of several different classes of solids, ranging
from van der Waals crystals, to valence semiconductors, to ionic crystals and
metals. In the third part, we address special cases such as strained solids,
the defreezing of glass surfaces, and rotational surface melting. Next, we
digress briefly to surface layering of a liquid metal, possibly leading to
solid-like or hexatic two dimensional phases floating on the liquid. In the
final part, the relationship of surface melting to the premelting of
nanoclusters and nanowires is reviewed.Comment: 54 pages, 26 figure
Monte Carlo simulations of the solid-liquid transition in hard spheres and colloid-polymer mixtures
Monte Carlo simulations at constant pressure are performed to study
coexistence and interfacial properties of the liquid-solid transition in hard
spheres and in colloid-polymer mixtures. The latter system is described as a
one-component Asakura-Oosawa (AO) model where the polymer's degrees of freedom
are incorporated via an attractive part in the effective potential for the
colloid-colloid interactions. For the considered AO model, the polymer
reservoir packing fraction is eta_p^r=0.1 and the colloid-polymer size ratio is
q=sigma_p/\sigma=0.15 (with sigma_p and sigma the diameter of polymers and
colloids, respectively). Inhomogeneous solid-liquid systems are prepared by
placing the solid fcc phase in the middle of a rectangular simulation box
creating two interfaces with the adjoined bulk liquid. By analyzing the growth
of the crystalline region at various pressures and for different system sizes,
the coexistence pressure p_co is obtained, yielding p_co=11.576 k_BT/sigma^3
for the hard sphere system and p_co=8.0 k_BT/sigma^3 for the AO model (with k_B
the Boltzmann constant and T the temperature). Several order parameters are
introduced to distinguish between solid and liquid phases and to describe the
interfacial properties. From the capillary-wave broadening of the solid-liquid
interface, the interfacial stiffness is obtained for the (100) crystalline
plane, giving the values gamma=0.49 k_BT/sigma^2 for the hard-sphere system and
gamma=0.95 k_BT/sigma^2 for the AO model.Comment: 11 pages, 13 figure
Why Are Alkali Halide Solid Surfaces Not Wetted By Their Own Melt?
Alkali halide (100) crystal surfaces are anomalous, being very poorly wetted
by their own melt at the triple point. We present extensive simulations for
NaCl, followed by calculations of the solid-vapor, solid-liquid, and
liquid-vapor free energies showing that solid NaCl(100) is a nonmelting
surface, and that its full behavior can quantitatively be accounted for within
a simple Born-Meyer-Huggins-Fumi-Tosi model potential. The incomplete wetting
is traced to the conspiracy of three factors: surface anharmonicities
stabilizing the solid surface; a large density jump causing bad liquid-solid
adhesion; incipient NaCl molecular correlations destabilizing the liquid
surface. The latter is pursued in detail, and it is shown that surface
short-range charge order acts to raise the surface tension because incipient
NaCl molecular formation anomalously reduces the surface entropy of liquid NaCl
much below that of solid NaCl(100).Comment: 4 pages, 3 figure
Monte Carlo Methods for Estimating Interfacial Free Energies and Line Tensions
Excess contributions to the free energy due to interfaces occur for many
problems encountered in the statistical physics of condensed matter when
coexistence between different phases is possible (e.g. wetting phenomena,
nucleation, crystal growth, etc.). This article reviews two methods to estimate
both interfacial free energies and line tensions by Monte Carlo simulations of
simple models, (e.g. the Ising model, a symmetrical binary Lennard-Jones fluid
exhibiting a miscibility gap, and a simple Lennard-Jones fluid). One method is
based on thermodynamic integration. This method is useful to study flat and
inclined interfaces for Ising lattices, allowing also the estimation of line
tensions of three-phase contact lines, when the interfaces meet walls (where
"surface fields" may act). A generalization to off-lattice systems is described
as well.
The second method is based on the sampling of the order parameter
distribution of the system throughout the two-phase coexistence region of the
model. Both the interface free energies of flat interfaces and of (spherical or
cylindrical) droplets (or bubbles) can be estimated, including also systems
with walls, where sphere-cap shaped wall-attached droplets occur. The
curvature-dependence of the interfacial free energy is discussed, and estimates
for the line tensions are compared to results from the thermodynamic
integration method. Basic limitations of all these methods are critically
discussed, and an outlook on other approaches is given