title: Water: A Tale of Two Liquids.
abstract: Water is the most abundant liquid on earth and also the substance with the largest number of anomalies in its properties. It is a prerequisite for life and as such a most important subject of current research in chemical physics and physical chemistry. In spite of its simplicity as a liquid, it has an enormously rich phase diagram where different types of ices, amorphous phases, and anomalies disclose a path that points to unique thermodynamics of its supercooled liquid state that still hides many unraveled secrets. In this review we describe the behavior of water in the regime from ambient conditions to the deeply supercooled region. The review describes simulations and experiments on this anomalous liquid. Several scenarios have been proposed to explain the anomalous properties that become strongly enhanced in the supercooled region. Among those, the second critical-point scenario has been investigated extensively, and at present most experimental evidence point to this scenario. Starting from very low temperatures, a coexistence line between a high-density amorphous phase and a low-density amorphous phase would continue in a coexistence line between a high-density and a low-density liquid phase terminating in a liquid-liquid critical point, LLCP. On approaching this LLCP from the one-phase region, a crossover in thermodynamics and dynamics can be found. This is discussed based on a picture of a temperature-dependent balance between a high-density liquid and a low-density liquid favored by, respectively, entropy and enthalpy, leading to a consistent picture of the thermodynamics of bulk water. Ice nucleation is also discussed, since this is what severely impedes experimental investigation of the vicinity of the proposed LLCP. Experimental investigation of stretched water, i.e., water at negative pressure, gives access to a different regime of the complex water diagram. Different ways to inhibit crystallization through confinement and aqueous solutions are discussed through results from experiments and simulations using the most sophisticated and advanced techniques. These findings represent tiles of a global picture that still needs to be completed. Some of the possible experimental lines of research that are essential to complete this picture are explored.
authors.affiliation: Dipartimento di Matematica e Fisica, Università Roma Tre , Via della Vasca Navale 84, 00146 Rome, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Gallo
authors.affiliation: Department of Physics, AlbaNova University Center, Stockholm University , SE-106 91 Stockholm, Sweden.
authors.forename: Katrin
authors.initials: K
authors.lastname: Amann-Winkel
authors.affiliation: Department of Chemistry and Biochemistry, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Charles Austen
authors.initials: CA
authors.lastname: Angell
authors.affiliation: Institute for Physical Science and Technology and Department of Chemical and Biomolecular Engineering, University of Maryland , College Park, Maryland 20742, United States.
authors.forename: Mikhail Alexeevich
authors.initials: MA
authors.lastname: Anisimov
authors.affiliation: Institut Lumière Matière, UMR5306 Université Claude Bernard Lyon 1-CNRS, Université de Lyon, Institut Universitaire de France , 69622 Villeurbanne, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Caupin
authors.affiliation: Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas , New Delhi 110016, India.
authors.forename: Charusita
authors.initials: C
authors.lastname: Chakravarty
authors.affiliation: Center for Polymer Studies and Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Erik
authors.initials: E
authors.lastname: Lascaris
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck , 6020 Innsbruck, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Loerting
authors.affiliation: Department of Chemical and Biological Engineering, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Athanassios Zois
authors.initials: AZ
authors.lastname: Panagiotopoulos
authors.affiliation: Institute of Industrial Science, University of Tokyo , 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.|School of Mathematics, University of Bristol , Bristol BS8 1TW, United Kingdom.
authors.forename: John
authors.initials: J
authors.lastname: Russo
authors.affiliation: Biomedical and X-ray Physics, Department of Applied Physics, AlbaNova University Center, KTH Royal Institute of Technology , SE-106 91 Stockholm, Sweden.
authors.forename: Jonas Alexander
authors.initials: JA
authors.lastname: Sellberg
authors.affiliation: Center for Polymer Studies and Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Harry Eugene
authors.initials: HE
authors.lastname: Stanley
authors.affiliation: Institute of Industrial Science, University of Tokyo , 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
authors.forename: Hajime
authors.initials: H
authors.lastname: Tanaka
authors.affiliation: Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid , 28040 Madrid, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Vega
authors.affiliation: International Centre for Quantum Materials and School of Physics, Peking University , Beijing 100871, China.|Collaborative Innovation Center of Quantum Matter, Beijing 100871, China.
authors.forename: Limei
authors.initials: L
authors.lastname: Xu
authors.affiliation: Department of Physics, AlbaNova University Center, Stockholm University , SE-106 91 Stockholm, Sweden.
authors.forename: Lars Gunnar Moody
authors.initials: LG
authors.lastname: Pettersson