Supercritical Fluids – Not Quite So Indistinguishable
While the gaseous and solid states of matter are relatively well understood, and clearly distinguished by their lack of or presence of long-range order, a complete characterisation of the liquid state is still being debated. Generally, liquids have been thought of by analogy to gases, as an extreme case of non-ideal gases. The problem with this approach is that one can compress a fluid to reach densities comparable to those of the solid, which cannot be tackled by analogy to gases. Midway through the last century, Yakov Frenkel proposed a ‘Kinetic Theory of Liquids’, where he aimed to describe liquids by analogy to solids, leading to a more realistic but also significantly conceptually denser theory than traditional gas analogies. Due to the latter fact his theory was largely ignored by researchers without a particular interest in very high densities, i.e. the canonical liquids community. However, in 2012, a crossover was proposed to occur in supercritical and near-supercritical fluids (Brazhkin, PRL) between a non-rigid ‘gas’-like state and a rigid ‘liquid’-like state. This crossover, called a ‘Frenkel line’, was originally characterised by the onset of oscillations in the velocity autocorrelation function of fluids as density is increased. In this talk I will present our efforts concerning this topic, first finding a structural marker associated with the Frenkel line crossover, identifying correlated changes in the evolution of the nearest neighbour coordination number and diffusion constant and the use of a machine-learned interatomic forcefield positing this crossover to originate at the triple point and formulate an analytic criterion for it. I will talk about how a fluid state crossover can be coherently identified in both traditional fluids (Krypton, Nitrogen) as well as colloidal/micellar systems, and discuss whether this is related to the Frenkel line, the recently found ‘c’-transition (Brazhkin, PRE 2021) or if we can even call a continuous crossover a ‘line’.
