484 research outputs found
Heterogeneous critical nucleation on a completely-wettable substrate
Heterogeneous nucleation of a new bulk phase on a flat substrate can be
associated with the surface phase transition called wetting transition. When
this bulk heterogeneous nucleation occurs on a completely-wettable flat
substrate with a zero contact angle, the classical nucleation theory predicts
that the free energy barrier of nucleation vanishes. In fact, there always
exist a critical nucleus and a free energy barrier as the first-order
pre-wetting transition will occur even when the contact angle is zero.
Furthermore, the critical nucleus changes its character from the critical
nucleus of surface phase transition below bulk coexistence (undersaturation) to
the critical nucleus of bulk heterogeneous nucleation above the coexistence
(oversaturation) when it crosses the coexistence. Recently, Sear [J.Chem.Phys
{\bf 129}, 164510 (2008)] has shown by a direct numerical calculation of
nucleation rate that the nucleus does not notice this change when it crosses
the coexistence. In our work the morphology and the work of formation of
critical nucleus on a completely-wettable substrate are re-examined across the
coexistence using the interface-displacement model. Indeed, the morphology and
the work of formation changes continuously at the coexistence. Our results
support the prediction of Sear and will rekindle the interest on heterogeneous
nucleation on a completely-wettable substrate.Comment: 11pages, 9 figures, Journal of Chemical Physics to be publishe
Local structure of liquid carbon controls diamond nucleation
Diamonds melt at temperatures above 4000 K. There are no measurements of the
steady-state rate of the reverse process: diamond nucleation from the melt,
because experiments are difficult at these extreme temperatures and pressures.
Using numerical simulations, we estimate the diamond nucleation rate and find
that it increases by many orders of magnitude when the pressure is increased at
constant supersaturation. The reason is that an increase in pressure changes
the local coordination of carbon atoms from three-fold to four-fold. It turns
out to be much easier to nucleate diamond in a four-fold coordinated liquid
than in a liquid with three-fold coordination, because in the latter case the
free-energy cost to create a diamond-liquid interface is higher. We speculate
that this mechanism for nucleation control is relevant for crystallization in
many network-forming liquids. On the basis of our calculations, we conclude
that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely
in gaseous planets
Transient nucleation in glasses
Nucleation rates in condensed systems are frequently not at their steady state values. Such time dependent (or transient) nucleation is most clearly observed in devitrification studies of metallic and silicate glasses. The origin of transient nucleation and its role in the formation and stability of desired phases and microstructures are discussed. Numerical models of nucleation in isothermal and nonisothermal situations, based on the coupled differential equations describing cluster evolution within the classical theory, are presented. The importance of transient nucleation in glass formation and crystallization is discussed
Scaling properties of critical bubble of homogeneous nucleation in stretched fluid of square-gradient density-functional model with triple-parabolic free energy
The square-gradient density-functional model with triple-parabolic free
energy is used to study homogeneous bubble nucleation in a stretched liquid to
check the scaling rule for the work of formation of the critical bubble as a
function of scaled undersaturation , the
difference in chemical potential between the bulk undersaturated
and saturated liquid divided by between the liquid
spinodal and saturated liquid. In contrast to our study, a similar
density-functional study for a Lennard-Jones liquid by Shen and Debenedetti [J.
Chem. Phys. {\bf 114}, 4149 (2001)] found that not only the work of formation
but other various quantities related to the critical bubble show the scaling
rule, however, we found virtually no scaling relationships in our model near
the coexistence. Although some quantities show almost perfect scaling relations
near the spinodal, the work of formation divided by the value deduced from the
classical nucleation theory shows no scaling in this model even though it
correctly vanishes at the spinodal. Furthermore, the critical bubble does not
show any anomaly near the spinodal as predicted many years ago. In particular,
our model does not show diverging interfacial width at the spinodal, which is
due to the fact that compressibility remains finite until the spinodal is
reached in our parabolic models.Comment: 10 pages, 10 figures, Journal of Chemical Physics accepted for
publicatio
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