Slow Dynamics in Supercooled Water
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FRANCESCO SCIORTINO (a), PIERO TARTAGLIA,(*), PAOLA GALLO (**), SOW-HSIN CHEN (**) (*) Dipartimento di Fisica and Istituto Nazionale per la Fisica della Materia, Universitd di Roma La Sapienza, P.le Aldo Moro 2, 1-00185, Roma, Italy, fsldectar.romal.infn.it (**) Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT We review some recent results on the self-dynamics in deep supercooled (simulated) water, obtained by analyzing very long Molecular Dynamics simulations. We discuss the possibility of interpreting the observed slowing down of the dynamics in terms of Mode Coupling Theory for supercooled liquids and, at the same time, of associating the experimentally observed anomalies of the transport coefficients in water on lowering the temperature to the formation of long living cages. The so-called critical Angell temperature TA in supercooled water could be interpreted as kinetic glass transition temperature, relaxing the need of a thermodynamic singularity for the explanation of the dynamic anomalies of liquid water. In the end we discuss the possibility that TA acts as cross-over temperature from fragile to strong liquid behavior. INTRODUCTION Supercooled water is characterized by an anomalous increase in specific heat and in compressibility on cooling[l]. This thermodynamic anomalous behavior, which originates from the presence of a line of temperatures of maximum density[2], has been associated with the presence of an experimentally inaccessible thermodynamic instability in the region of phase space where the crystal is the stable phase[3, 4]. The temperature dependence of transport coefficients in water is also anomalous. It is strongly non-Arrhenius and it is rather well represented by power-law[5] in IT - TA(P)I"Y. Here TA(P) is the so-called (pressure dependent) Angell temperature and y is the corresponding exponent. The impressive power-law behavior in the transport coefficient has been one argument in favor of a thermodynamic instability, notwithstanding the observed pressure dependence of the
exponent -[5]. A theoretical framework to interpret the divergences of transport coefficient in supercooled water, which does not require the presence of a thermodynamic instability, would be very valuable in the discussion of the thermodynamics behavior of supercooled water. This topic, recently addressed in Refs. [6, 7, 8, 9], is the main point of this paper. We also present some new results on the relation between translational and rotational correlation functions, which strengthen the interpretation of the slow dynamics in SPC/E water in terms of Mode Coupling Theory (MCT) [11, 12, 13]. DISCUSSION The study we review here[7, 10] is based on the analysis of very long (up to 100 ns) molecular dynamics simulations, performed modelling the water-water interaction with a well known effective pair-additive rigid potential, the SPC/E potential[14]. Very deep supercooled states have been simulated along an isobar and results for the self-molecule motion have been careful
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