The behaviour of a floating water bridge under reduced gravity conditions
Résumé
When high voltage is applied to pure water filled into two beakers close to each other, a connection forms spontaneously, giving the impression of a floating water bridge [ 1]-[8 ]. This phenomenon is of special interest, since it comprises a number of phenomena currently tackled in modern water science. In this work, the behavior of this phenomenon under reduced gravity conditions during a parabolic flight is presented by the means of high speed imaging with fringe projection. An analysis of the behavior is presented and compared to theoretical considerations. Supplementary video clips for the sequences shown in figs. 2, 4, 5&6 as well as 7&8 are available. Introduction 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 between the two glasses, and after some time, the cotton thread was pulled into one of the glasses, leaving, for a few seconds, a rope of water suspended between the lips of the two glasses [ 1 ]. As gimmick from early days of electricity this experiment was handed down through history until the present authors learned about it from W. Uhlig, ETH Zürich [ 2 ]. Although easy to reproduce, this watery 2 connection between the two beakers, which is further referred to as 'floating water bridge ' holds a number of interesting static and dynamic phenomena [ 3]-[8 ]. At macroscopic scale several of these phenomena can be explained by modern electrohydrodynamics, analyzing the motion of fluids in electric fields (see, e.g., the Maxwell pressure tensor considerations by Widom et al. [ 9 ], or the text book on Electrohydodynamics by Castellanos [ 10]). On the molecular scale water can be described by quantum mechanics (e.g. [ 11 ], [ 12]). The gap at mesoscopic scale is bridged by a number of theories including quantum mechanical entanglement and coherent structures in water, theories which are currently discussed (e.g. [ 13]-[17 ] for water in general, and [ 18 ] specifically for the water bridge). Previous experiments [ 3 ] suggested a possible change of the water micro structure inside the water bridge ; first neutron scattering experiments [ 5 ] showed no difference in the microdensity of a D 2 O bridge compared to the bulk ; recent 2D neutron scattering experiments [ 6 ] indicated a preferred molecular orientation within a floating heavy water bridge ; detailed optical investigations [ 7 ] suggested the existence of a mesoscopic bubble network within the water bridge ; and a Raman scattering study on vertical water bridges reported on a polarized water structure induced by the electric field [ 19 ]. A comprehensive review about water bridge research was published recently [ 20 ]. The properties of water at mesoscopic scales have drawn special attention due to their suggested importance to human physiology [ 21 ]. In this work, the first reduced gravity experiments with the floating water bridge are presented. The data gathered with high speed imaging and fringe projection are discussed, the overall behavior is described and compared to theoretical considerations. Experimental The experiments were carried out onboard the PH-NLZ Fairchild Metro II Research Aircraft of the NLR (Nationaal Lucht- en Ruimtevaartlaboratorium - The National Aerospace Laboratory of the Netherlands), Hangar 3, Schiphol, on a specially designed set-up (see Fig. 1). All apparatus were mounted on an aluminum alloy plate provided by the NLR using struts and mountings certified for aviation and safe for accelerations up to 9G. The set-up contained a constant current regulated high voltage power supply (0-20kV) with a 250kΩ/500MΩ voltage divider. Instead of beakers two closed bottles with an additional opening in the sidewall were used. The bottles were 3 mounted on a movable stage in order to manually create water bridges of different length. The water bridge was formed between the sidewall openings of the bottles. The bottles were filled with milli-Q water (conductivity < 1µS/cm) up to ~2mm below these openings. For all experiments, the initial temperature of the water was ~19°C, which was the onboard air temperature. Due to the short duration of the experiments (< 1 min), no significant temperature change was to be expected [ 7 ]. As electrode material, 2 x 2 cm² platinum plates were submerged into the water. One electrode was raised to high potential (+20kV), the other was grounded to the aircraft's general ground. A 42nF ceramic capacitor was connected parallel to the electrodes.
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