Where do you draw the line?

CCMMP Seminars
Dr John Proctor
Andrei Sapelkin
October 31st, 2017 at 14:00
GO Jones Room 610

For most of the 20th century the common theoretical approach to understanding the liquid state was to treat liquids and supercritical fluids as dense non-ideal gases, justified on the basis that liquids and supercritical fluids share important properties with gases; for instance a lack of long-range order. But this approach has some shortcomings; the densities of liquids are close to those of solids, both orders of magnitude larger than for gases. The latent heat of vaporization is usually an order of magnitude larger than the latent heat of fusion, and liquids (unlike gases) can exhibit orientational order similarly to solids.

Therefore, the opposite theoretical approach has also been utilized – treating liquids using similar methods to solids, assuming the atoms (or molecules) are (relatively) closely packed. In this approach, the atoms have definite positions, with occasional vacancies (holes), which can in many respects be treated in the same manner as vacant atomic sites in solids. The solid-like approach was put forward by Frenkel in 1946 in Kinetic Theory of Liquids but it is only in the past 10 years that the power of this alternative approach has been truly appreciated. For instance, it has allowed accurate prediction of the heat capacities of liquids for the first time, and also the prediction that beyond the critical point there exists, after all, a narrow transition between a liquid-like and a gas-like state, christened the Frenkel line (Brazhkin et al., PRE 2012). This line, if its existence is proven, necessitates the revision of textbooks on thermodynamics and planetary science as well as providing knowledge with potential applications to the use of supercritical fluids in industry.

In this seminar I will review the theoretical aspects of the kinetic theory of liquids and then present the experimental evidence for the existence of the Frenkel line produced very recently in my own work (https://arxiv.org/abs/1710.04212) and the work of other researchers.