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Despite decades of research water’s puzzling properties are not understood and 63 anomalies that distinguish water from other liquids remain unsolved. We introduce some of these unsolved mysteries, and demonstrate recent progress in solving them. We present recent evidence from experiments and computer simulations on bulk and on nanoconfined water supporting the hypothesis that water displays a special transition point (which is not unlike the “tipping point” immortalized by Malcolm Gladwell). The general idea is that when the liquid is near this “tipping point,” it suddenly separates into two distinct liquid phases. This concept of a new critical point is finding application to other liquids as well as water, such as silicon and silica. We also discuss related puzzles, such as the mysterious behavior of water near a protein. Introduction Water is perhaps the most ubiquitous, and the most essential, of any molecule on earth. Indeed, H2O challenges the imagination of even the most creative science fiction writers (such as K. Vonnegut) to picture what life would be like without water, and one often hears the adage “biology cannot be understood until water is understood”). Despite 300 years of research, however, the 63 anomalies (http://www.lsbu.ac.uk/water/anmlies.html) that distinguish water from other liquids lack a coherent explanation so sometimes water is called the prototype “complex fluid”. We will introduce some of the 63 anomalies of water, and will demonstrate some recent progress in solving them using concepts borrowed from various disciplines including chemistry and physics. In particular, we will present evidence from experiments designed to test the hypothesis that water displays a special transition point (which is not unlike the “tipping point” immortalized in Malcolm Gladwell’s book of the same title). The general idea that when water is near this tipping point, it can separate into two distinct liquid phases distinguished by their density. This new concept of a critical point is also proving useful in understanding some of the anomalies of other liquids, such as silicon, silica, and carbon. We will also discuss two other water mysteries, such as the puzzling behavior of water near a protein, and the breakdown of the Stokes-Einstein relation in supercooled water. What is the phenomenon? We start with three thermodynamic functions. The first is the compressibility the response of the volume to an infinitesimal change in pressure. In a typical liquid, this response function decreases when we lower the temperature. I understand this decrease via statistical physics. This thermodynamic response function is proportional to the thermal average of all the fluctuations in specific volume in the system. As we lower the temperature, we imagine that fluctuations of necessity decrease, thus the compressibility decreases. Water is unusual in three respects. First, the average compressibility of water is twice as large as what one would expect were water a typical fluid and were one to plug all the prefactors into the formulas that give compressibility in terms of volume fluctuations. Second, the magnitude of that factor of two actually increases as one lowers the temperature. That being the case, there is ultimately a minimum which occurs at 46°C. Below that temperature, the compressibility increases dramatically. At the lowest attainable temperature (-40°C) the compressibility takes on a value that is twice of that at the minimum. This is not a tiny effect; it is huge (Figure 1 page 12). The second thermodynamic function is the specific heat, and we observe three similar anomalies: it is twice as large as that of a typical liquid, the discrepancy gets bigger as the temperature is lowered, and a minimum occurs at 35°C. The third thermodynamic function is the coefficient of thermal expansion, the response of the volume to an infinitesimal change in temperature. This quantity we assume to always be positive because if there is a local region of the liquid in which the specific volume is larger than the average, then there will be more arrangements of the molecules and hence the entropy will be larger than the average. This is true of almost all liquids, but the magnitude of www.phantomsnet.net E newsletter September 2007 11 Research FIGURE 3 Slip-plane in air or oil Slip-plane under water Slip-plane under water Based on Nature 444, 191-194 (2006) Figure 3