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Nucleation at large metastability is still largely an unsolved problem, although is a problem of tremendous current interest, with wide practical value. It is well-accepted that the classical nucleation theory (CNT) fails to provide a qualitative picture and gives incorrect quantitative values for such quantities as activation free energy barrier and supersaturation dependence of nucleation rate, especially at large metastability. In this article, we present a powerful alternative formalism to treat nucleation at large supersaturation. This formalism goes over to the classical picture at small supersaturation where CNT is expected to be valid. The new theory is based on an extended set of order parameters in terms of k-th largest liquid-like clusters where k=1 is the largest cluster in the system, k=2 is the second largest cluster and so on. We derive an analytic expression for the free energy of formation of the k-th largest cluster which shows that at large metastability the barrier of growth for the few largest liquid-like clusters disappear, the nucleation becomes collective and the approach to the critical size occurs by barrierless diffusion in the cluster size space. The expression for the rate of barrier crossing predicts a weaker supersaturation dependence than that of CNT at large metastability. Such a cross-over behavior has indeed been observed in recent experiments but eluded an explanation till now. In order to understand the large numerical difference between simulation predictions and experimental results, we carried out a study of the dependence on the range of intermolecular interaction of both the surface tension of an equilibrium planar gas-liquid interface and the free energy barrier of nucleation. Both are found to depend significantly on the range of interaction for a Lennard-Jones potential, both in two and three dimensions.
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We find an interesting interplay between the range of the attractive part of the interaction potential and the extent of metastability (as measured by supersaturation) in gas-liquid nucleation. We explore and exploit this interplay to obtain new insight into nucleation phenomena. Just like its dependence on supersaturation (S), the free energy barrier of nucleation is found to depend strongly on the range of the interaction potential. Actually, the entire free energy surface, F(n), where n is the size of the liquid-like cluster, shows this dependence. The evidences and the reasons for this strong dependence are as follows. (i) The surface tension increases dramatically on increasing the range of interaction potential. In three dimensional Lennard-Jones system, the value of the surface tension increases from 0.494 for a cut-off of 2.5 {sigma} to 1.09 when the full range of the potential is involved. In two dimensional LJ system, the value of the line tension increases from 0.05 to 0.18, under the same variation of the potential range. (ii) The density of the gas phase at coexistence decreases while that of the liquid phase increases substantially on increasing the range of the interaction potential. (iii) As a result of the above, at a given supersaturation S, the size of the critical nucleus and the free energy barrier both increase with increase in the range of interaction potential. (iv) Surprisingly, however, we find that the functional form predicted by the classical nucleation theory (CNT) for the dependence of the free energy barrier on the size of the nucleus to remain valid except at the largest value of S studied. (v) The agreement between CNT prediction and simulated values of the barrier is supersaturation dependent and worsens with increase in the range of interaction potential, and increases above 10 kBT at the largest supersaturation that could be studied.
We develop a theory in order to describe the effect of relaxation in a condensed medium upon the quantum decay of a metastable liquid near the spinodal at low temperatures. We find that both the regime and the rate of quantum nucleation strongly depend on the relaxation time and its temperature behavior. The quantum nucleation rate slows down with the decrease of the relaxation time. We also discuss the low temperature experiments on cavitation in normal $^3$He and superfluid $^4$He at negative pressures. It is the sharp distinctions in the high frequency sound mode and in the temperature behavior of the relaxation time that make the quantum cavitation kinetics in $^3$He and $^4$He completely different in kind.
We study the metastable equilibrium properties of the Potts model with heat-bath transition rates using a novel expansion. The method is especially powerful for large number of state spin variables and it is notably accurate in a rather wide range of temperatures around the phase transition.
We investigate the evolution of a particle in a Lorentz gas where the background scatters move and collide with each other. As in the standard Lorentz gas, we assume that the particle is negligibly light in comparison with scatters. We show that the average particle speed grows in time as t^{lambda/(4+lambda)} in three dimensions when the particle-scatter potential diverges as r^{-lambda} in the small separation limit. The typical displacement of the particle exhibits a universal linear growth in time independently on the density of the background gas and the particle-scatter interaction. The velocity and position distributions approach universal scaling forms. We determine the former, while for the position distribution we establish conjecturally exact scaling forms for the one and two-dimensional Lorentz gas.
Fusion energy stands out as a promising alternative for a future decarbonised energy system. To be sustainable, future fusion nuclear reactors will have to produce their own tritium. In the so-called breeding blanket of a reactor, the neutron bombardment of lithium will produce the desired tritium, but also helium, which can trigger nucleation mechanisms owing to the very low solubility of helium in liquid metals. An understanding of the underlying microscopic processes is important for improving the efficiency, sustainability and reliability of the fusion energy conversion process. A spontaneous creation of helium drops or bubbles in the liquid metal used as breeding material in some designs may be a serious issue for the performance of the breeding blankets. This phenomenon has yet to be fully studied and understood. This work aims to provide some insight on the behavior of lithium and helium mixtures at experimentally corresponding operating conditions (843 K and pressures between 0.1 and 7 GPa). We report a microscopic study of the thermodynamic, structural and dynamical properties of lithium-helium mixtures, as a first step to the simulation of the environment in a nuclear fusion power plant. We introduce a microscopic model devised to describe the formation of helium drops in the thermodynamic range considered. A transition from a miscible homogeneous mixture to a phase-separated one, in which helium drops are nucleated, is observed as the pressure is increased above 0.175 GPa. The diffusion coefficient of lithium (2 {AA} 2 /ps) is in excellent agreement with reference experimental data, whereas the diffusion coefficient of helium is in the range of 1 {AA} 2 /ps and tends to decrease as pressure increases. The radii of helium drops have been found to be between 1 and 2 {AA}.
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