What Is a Liquid?

Thermal properties of liquids are important considerations in various applications including thermal storage and power plants where specific heat and thermal conductivity of liquids determine the amount of heat to be stored and transferred [1, 2]. However, our microscopic understanding of thermal properties of liquids lags far behind that of solids and gases [3–5]. In solids (especially crystals), atoms mostly vibrate around their respective mean positions known as equilibrium positions. Therefore, perturbation theory approaches based on these equilibrium positions have been very successful in describing atomic motion in solids and hence, their materials properties. For instance, phonon quasi-particles (quanta of vibrations in crystals) are used to describe heat capacity, thermal expansion, and thermal conductivity in dielectric solids. These can now be predicted with first-principles accuracy. On the other end of the spectrum of matter, atomic interactions are weak in dilute gases and real atomic collisions are used as the basis for theory developments. Some examples include kinetic theory of thermal conductivity and viscosity and the ideal-gas law. However, liquids have neither of these ‘small’ parameters that alleviate challenges in developing rigorous theories. Liquids have dynamically disordered structures that lack spatial periodicity, yet their atomic densities are similar to solids and have strong atomic attractions, first demonstrated by their capillary action recorded by Leonardo da Vinci in the late 15th century [6] in narrow siphons.