The physics of empty liquids: from patchy particles to water
Not all simple liquids have simple microscopic interactions, and not all simple microscopic interactions give rise to simple liquids.
Liquid metals are an example of simple liquids with complicated interactions, and water is the best example of a complicated liquid with simple interactions. It is thus perhaps not surprising that the study of liquids is a very active research field, encompassing systems with widely different length and energy scales. In addition to simple liquids (i.e. van der Waals liquids) and complex liquids (e.g. polymer melts), in recent years a new liquid category has been identified, called “empty liquids”.
Empty liquids represent a wide class of materials whose constituents arrange in a random network through reversible bonds. These bonds can be physically realized with a variety of interactions, such as hydrogen bonding, lock-and-key interactions, DNA base pairing, hydrophobic, dipolar, and even entropic interactions. Materials that fall in the category of empty liquids go from patterned colloidal particles, clays, DNA wireframe origamis, all the way down to simple molecules, like water, or even atoms, like silicon.
In a recent work recently published on Reports on Progress in Physics, a group of researchers of the Physics Department of Sapienza University of Rome (J. Russo, F. Leoni, F. Sciortino), in collaboration with IBM Research UK (F. Martelli), have made an extensive account of how the simple interactions in simple liquids give rise to very complex behaviour, including the factors that control the appearance of thermodynamic and dynamic anomalies, the possibility of liquid–liquid phase transitions, and the crystallization of open crystalline structures. Without forgetting why empty liquids are empty in the first place.
J. Russo, F. Leoni, F. Martelli, F. Sciortino
Reports on Progress in Physics