Abstract
It has been reported that at temperatures above the critical there is no “continuity of liquid and gas”, as originally hypothesized by van der Waals [1]. Rather, both gas and liquid phases, with characteristic properties as such, extend to supercritical temperatures [2]-[4]. Each phase is bounded by the locus of a percolation transition, i.e. a higher-order thermodynamic phase change associated with percolation of gas clusters in a large void, or liquid interstitial vacancies in a large cluster. Between these two-phase bounds, it is reported there exists a mesophase that resembles an otherwise homogeneous dispersion of gas micro-bubbles in liquid (foam) and a dispersion of liquid micro-droplets in gas (mist). Such a colloidal-like state of a pure one-component fluid represents a hitherto unchartered equilibrium state of matter besides pure solid, liquid or gas. Here we provide compelling evidence, from molecular dynamics (MD) simulations, for the existence of this supercritical mesophase and its colloidal nature. We report preliminary results of computer simulations for a model fluid using a simplistic representation of atoms or molecules, i.e. a hard-core repulsion with an attraction so short that the atoms are referred to as “adhesive spheres”. Molecular clusters, and hence percolation transitions, are unambiguously defined. Graphics of color-coded clusters show colloidal characteristics of the supercritical mesophase. We append this Letter to Natural Science with a debate on the scientific merits of its content courtesy of correspondence with Nature (Appendix).
Highlights
Along supercritical isotherms, the densities at which molecular clusters of occupied sites in the gas phase, or clusters of unoccupied voids in the liquid phase, first become macroscopic, are referred to percolation transitions
For a gas of attractive atoms, rigidity decreases with density because fluctuations increase with polymerisation, whereas for a liquid, rigidity increases with density because the voids become fewer as density increases. This condition holds true for the subcritical isotherms of liquid and gas when T < Tc. To observe this intermediate mesophase that is neither pure liquid nor gas, we have investigated a model fluid of cohesive atomic spheres that gives rise to well-defined molecular clusters
Recent discoveries of an alternative description of liquid-gas criticality have been extended for adhesive-sphere fluids
Summary
The densities at which molecular clusters of occupied sites in the gas phase, or clusters of unoccupied voids in the liquid phase, first become macroscopic, are referred to percolation transitions. The percolation transitions of the hard-sphere fluid, as defined by Kratky [7], have been computed [8] and found to be weak higher-order thermodynamic phase, transitions that occurs as the fluctuations in either number density, or equivalently available volume, change at the percolation. With sufficient cooling, at a “critical temperature”, the percolation loci intersect in the Gibbs pressure-temperature (p-T) plane At this point, a liquid state at the density of the available-volume percolation transition coexists in thermodynamic equilibrium with a vapor state at lower density of a percolation transition associated with bonded clusters of gas molecules
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