Can a Century Old Experiment Reveal Hidden Properties of Water?
In 1893 Sir William Armstrong placed a cotton thread between two wine glasses filled with chemically pure water. After applying a high voltage, a watery connection formed, and after some time, the cotton thread was pulled into one of the glasses, leaving a rope of water suspended between the two glasses. Although being a very simple experiment, it is of special interest since it comprises a number of phenomena currently tackled in modern water science like electrolysis-less charge transport and
... nanobubbles. This work gives some background information about water research in general and describes the water bridge phenomenon from the viewpoint of different fields such as electrohydrodynamics and quantum field theory. It is shown that the investigation of the floating water bridge led to new discoveries about water, both in the macroscopic and microscopic realm -but these were merely "hidden" in that sense that they only become evident upon application of electric fields. Water is undoubtedly the most important chemical substance of the world. Despite this, and in spite of the fact that it is practically ubiquitous, it still represents one of the best explored [1,2] and yet least understood substances    , as its so-called "anomalies" are famous (e.g.,    ). Many attempts have been made in order to measure or calculate the structure of water beyond the H 2 O molecule. This is a difficult task due to the hydrogen bond (H-bond) network which can be formed by two or more water OPEN ACCESS Water 2010, 2 382 molecules. These bonds are characterized by a binding energy between 1 and 50 kJ/mol, depending on and the type of strength of interaction and the local geometry  , and their network is nowadays held responsible for many of water's weird properties  and is also the reason why water cannot be treated as a "simple liquid"  . From a theoretical point of view, the difficulties in understanding liquid water can probably be attributed to two features: cooperative hydrogen bonding (the fact that the binding energy of two H-bonded molecules is modified by the presence of a third molecule [10,12-16]) and nuclear quantum effects. Such effects occur because the proton is so light that classical mechanics can no longer adequately describe properties like spatial dispersion of the hydrogen positions, nuclear tunneling, zero-point energy and, naturally, quantization of nuclear motions. Much work has been done recently toward the simulation of liquid water [17-25], its intrinsic ions H + and OH -  and other ions in solution  using ab initio electronic-structure methods, sometimes together with quantum dynamics methods [25,28-33], but still more work is called for in order to get a more complete and accurate picture of the liquid. So obviously, the description of this substance by its chemical formula-H 2 O-reflects the reductionism concept of the current scientific paradigm, and in this special case it fails to do justice to the structure and the properties of water. If one considers, e.g., phosphorus pentoxide, whose formula is given as P 4 O 10 according to its dimeric structure, a proposal for the formula of water could be (H 2 O) n . However, as pointed out above, in this case, the situation is much more complex. Numerous structural models of water proposed by many authors can be found in the book of Eisenberg and Kauzman . The X-ray studies of water and ice by are mentioned here as a representative example of the many investigations succeeding that book. For detailed up-to-date structural analyses of water based on neutron scattering and/or X-ray diffraction, reference is made to the works of Teixeira and Luzar  and Soper (e.g., [38, 39] ), exemplary studies on supercooled water as well as a possible liquid-liquid phase transition were published by , Mishima and Stanley  and Yamada et al.  (and references quoted therein). In his recent paper, Soper  explains that when using an empirical potential structure refinement (EPSR) to form a single atomistic structural model which was simultaneously consistent with both X-ray and neutron diffraction data, X-ray data for water provide a powerful constraint on possible structural models. However, it is not possible to rigorously define the precise position and height of the first peak in the OO radial distribution function; and different neutron datasets on water give rise to further small uncertainties in the position of the hydrogen bond peaks. According to Soper, one general conclusion from the combined use of neutron and X-ray data is that many of the classical water potentials may have a core which is strongly repulsive at short distances producing too sharp a peak in r-space at too short a distance. Thus, a softer core potential is proposed  . Leetmaa et al. recently reported on the consistency of such models with infrared/Raman and X-ray absorption spectra  . They claim that the overall agreement of calculated spectra based upon established structural models is still unsatisfactory, and that no water model exists that can equally well describe IR/Raman, X-ray absorption spectroscopy, and diffraction data  . In a subsequent work  , they furthermore show that there is no strict proof of tetrahedral water based on diffraction and IR/Raman data, and that the tetrahedral structure model must, to fit diffraction data, be less structured than most models obtained from molecular dynamics simulations  .