Dynamical crossover in supercritical core-softened fluids

Abstract

It is well known that some liquids can demonstrate anomalous behavior. Interestingly, this behavior can be qualitatively reproduced with simple core-softened isotropic pair-potential systems. Although anomalous properties of liquids usually take place at low and moderate temperatures it was recently recognized that many important phenomena can appear in supercritical fluids. However, no studies of supercritical behavior of core-softened fluids is reported. This paper reports a study of dynamical crossover in supercritical core-softened systems. The crossover line is calculated from three different criteria and good agreement between them is observed. It is found that the behavior of the dynamical crossover line of core-softened systems is quite complex due to its quasi-binary nature. The most fundamental approach to the behavior of matter relies on the interactions between the particles of the substance. One can use an approach based on quantum mechanical treatment of interactions which is the most accurate one. However, such ab-initio methods require a lot of computational resources and cannot be applied to the systems larger then several hundreds of atoms. A great advantage was made by application of so called effective potentials. These potentials are constructed in such a way that they allow to obtain some principal properties of interest with much smaller efforts. In the simplest case the interaction energy is approximated by pair interactions only. Among the most studied in the literature is Lennard-Jones (LJ) system which has the potential U (r) = ε This system demonstrates a generic view of phase diagram of a substance containing gas, liquid and crystal phases and well describes the behavior of noble gases and some molecular substances.In our recent works it was shown that supercritical region of the phase diagram can be divided into two parts: rigid liquid and dense gas [1][2][3]. These regions differ by the microscopic dynamics of particles and are separated by a crossover line called Frenkel line. Later on the phenomenon of dynamical crossover was studied for a number of other fluids [4][5][6][7][8].Another topic attracting wide attention of researchers is related to anomalous behavior of liquids (see, for example, [9] for the list of anomalies of water). It was found that models with isotropic pair core-softened potentials can demonstrate anomalous behavior [10][11][12][13][14][15][16][17]. Diffusion, density and structural anomalies are widely discussed in the literature [18]. Such systems can also demonstrate numerous structural phase transitions in solid region. This kind of behavior cannot be obtained in systems like LJ. These observations allow to suppose that the behavior of the Frenkel line can also be more complex in the systems with core-softened potentials.A particular form of core-softened system studied in our previous works is characterized by the following interaction potential:where d and ε set the length and energy scales and λ i and σ i are varied. A large set of parameters was cons...

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