Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N42: Focus Session: Supercooled and Nanoconfined Water I |
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Sponsoring Units: DCP Chair: Valeria Molinero, University of Utah Room: Hilton Baltimore Holiday Ballroom 3 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N42.00001: Liquid-liquid transition in the ST2 model of water Invited Speaker: Pablo Debenedetti We present clear evidence of the existence of a metastable liquid-liquid phase transition in the ST2 model of water. Using four different techniques (the weighted histogram analysis method with single-particle moves, well-tempered metadynamics with single-particle moves, weighted histograms with parallel tempering and collective particle moves, and conventional molecular dynamics), we calculate the free energy surface over a range of thermodynamic conditions, we perform a finite size scaling analysis for the free energy barrier between the coexisting liquid phases, we demonstrate the attainment of diffusive behavior, and we perform stringent thermodynamic consistency checks. The results provide conclusive evidence of a first-order liquid-liquid transition. We also show that structural equilibration in the sluggish low-density phase is attained over the time scale of our simulations, and that crystallization times are significantly longer than structural equilibration, even under deeply supercooled conditions. We place our results in the context of the theory of metastability. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N42.00002: Entropy-driven liquid-liquid transitions in supercooled water Invited Speaker: Mikhail Anisimov Twenty years ago it was suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of metastable liquid-liquid separation of lower-density and higher-density water. I describe a phenomenological model in which liquid water at low temperatures is viewed as an athermal solution of two hydrogen-bond network structures with different entropies and densities. Alternatively to the lattice-gas/regular solution model, in which fluid phase separation is driven by energy, the phase transition in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the reaction equilibrium constant. The order parameter is associated with the entropy, while the ordering field is a combination of temperature and pressure. The model predicts the location of density maxima at the locus of a near-constant fraction of the lower-density structure. Another example of entropy-driven liquid polyamorphism is the transition between the structurally ordered ``Blue Phase III'' and disordered liquid in some chiral materials; this transition is experimentally accessible. I also discuss the application of the two-state model to binary solutions of supercooled water in which liquid-liquid transition may also become accessible to direct observation. Some atomistic ``water-like'' models such as mW, do not show liquid-liquid separation in the metastable liquid domain. However, even without actual liquid-liquid separation, the anomalies observed in MD simulations of mW can be accurately described by the entropy-driven nonideality of two molecular configurations, the same physics that presumably drives the liquid-liquid transition in real water. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N42.00003: Interplay of the Glass Transition and the Liquid-Liquid Phase Transition in Water Invited Speaker: Nicolas Giovambattista Most liquids can form a single glass or amorphous state when cooled sufficiently fast (in order to prevent crystallization). However, there are a few substances that are relevant to scientific and technological applications which can exist in at least two different amorphous states, a property known as polyamorphism. Examples include silicon, silica, and in particular, water. In the case of water, experiments show the existence of a low-density (LDA) and high-density (HDA) amorphous ice that are separated by a dramatic, first-order like phase transition. It has been argued that the LDA-HDA transformation evolves into a first-order liquid-liquid phase transition (LLPT) at temperatures above the glass transition temperature Tg. However, obtaining direct experimental evidence of the LLPT has been challenging since the LLPT occurs at conditions where water rapidly crystallizes. In this talk, I will (i) discuss the general phenomenology of polyamorphism in water and its implications, and (ii) explore the effects of a LLPT on the pressure dependence of Tg(P) for LDA and HDA. Our study is based on computer simulations of two water models -- one with a LLPT (ST2 model), and one without (SPC/E model). In the absence of a LLPT, Tg(P) for all glasses nearly coincide. Instead, when there is a LLPT, different glasses exhibit dramatically different Tg(P) loci which are directly linked with the LLPT. Available experimental data for Tg(P) are only consistent with the scenario that includes a LLPT (ST2 model) and hence, our results support the view that a LLPT may exist for the case of water. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N42.00004: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N42.00005: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N42.00006: The Putative Liquid-Liquid Transition is a Liquid-Solid Transition in Atomistic Models of Water David Chandler, David Limmer Our detailed and controlled studies of free energy surfaces for models of water find no evidence for reversible polyamorphism, and a general theoretical analysis of the phase behavior of cold water in nano pores shows that measured behaviors of these systems reflect surface modulation and dynamics of ice, not a liquid-liquid critical point. A few workers reach different conclusions, reporting evidence of a liquid-liquid critical point in computer simulations of supercooled water. In some cases, it appears that these contrary results are based upon simulation algorithms that are inconsistent with principles of statistical mechanics, such as using barostats that do not reproduce the correct distribution of volume fluctuations. In other cases, the results appear to be associated with difficulty equilibrating the supercooled material and mistaking metastability for coarsening of the ordered ice phase. In this case, sufficient information is available for us to reproduce the contrary results and to establish that they are artifacts of finite time sampling. This finding leads us to the conclusion that two distinct, reversible liquid phases do not exist in models of supercooled water. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N42.00007: Amorphous ices explained in terms of nonequilibrium phase transitions in supercooled water David Limmer, David Chandler We analyze the phase diagram of supercooled water out-of-equilibrium using concepts from space-time thermodynamics and the dynamic facilitation theory of the glass transition, together with molecular dynamics simulations. We find that when water is driven out-of-equilibrium, it can exist in multiple amorphous states. In contrast, we find that when water is at equilibrium, it can exist in only one liquid state. The amorphous non-equilibrium states are solids, distinguished from the liquid by their lack of mobility, and distinguished from each other by their different densities and local structure. This finding explains the experimentally observed polyamorphism of water as a class of nonequilibrium phenomena involving glasses of different densities. While the amorphous solids can be long lived, they are thermodynamically unstable. When allowed to relax to equilibrium, they crystallize with pathways that pass first through liquid state configurations and then to ordered ice. [Preview Abstract] |
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