|Title||Electrically excited liquid water : lessons from floating water bridge|
|Source||Wageningen University. Promotor(en): Cees Buisman, co-promotor(en): J. Woisetschläger; E.C. Fuchs. - Wageningen : Wageningen University - ISBN 9789462577039 - 223|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||water - liquids - electric field - thermodynamics - bridges - equilibrium - disequilibrium - electrodynamics - nuclear magnetic resonance - neutron scattering - infrared spectroscopy - spectroscopy - gas chromatography - electrical conductivity - interferometry - spectral analysis - physics - water - vloeistoffen (liquids) - elektrisch veld - thermodynamica - bruggen - evenwicht - verstoord evenwicht - elektrodynamica - kernmagnetische resonantie - neutronenverstrooiing - infraroodspectroscopie - spectroscopie - gaschromatografie - elektrische geleidbaarheid - interferometrie - spectraalanalyse - fysica|
Water is essential to a healthy and secure world. Developing new technologies which can take full advantage of the unique attributes of water is important for meeting the ever increasing global demand while reducing the production footprint. Water exhibits unexpected departures in more than 70 physical and chemical properties compared to other molecular species of similar chemical composition. The principal cause for this behavior is ascribed to the large number of hydrogen bonds which form between neighboring water molecules. Hydrogen bonds are moderately strong in water and exhibit both electrostatic as well as covalent character. When examining the liquid state these interactions play a significantly role in moderating the interchange between dynamics and structure. In disordered materials such as a liquid there are three length scales of importance: 1) at the microscopic molecular level interactions dominate, 2) the macroscopic level where classical forces act upon the statistically isotropic medium, and 3) the mesoscopic level where heterogeneous interactions dominate through evolving transient structures each with unique dynamical behaviors. The mesoscale is important to most environmental and biological processes and is even more poorly understood than the liquid state in general. The aim of this thesis is to explore the extended molecular behavior of liquid water excited by strong electric field gradients.
The floating water bridge belongs to a larger class of phenomena called electrohydrodynamic (EHD) liquid bridges. These self-suspending liquid catenaries can occur in a number of polar liquids provided the conductivity is low. They exhibit elevated temperatures and bidirectional flow patterns, as well as sub-Hz diameter oscillations. The flow behavior and dynamics of these bridges is complex but can be addressed by continuum level EHD theory. The polarizing effect of the electric field gradient accelerates the fluid tangentially along the surface resulting in a Taylor pump which supplies the bridge with liquid. The free hanging section is stable against gravity within a band of operational parameters whereby the electric field strength is sufficient confine fluid elements within the bridge. A standardized protocol for operating stable EHD bridges in multiple configurations is developed and presented. This is the primary tool used throughout the thesis and provides a macroscopic object for the experimental examination of how forces which typically only occur over a few nanometers in nature affect the organization of polar liquids, notably water. In order to examine the role that the electric field gradient plays in the observed molecular changes found in EHD bridges a simple point-plane electrode system was also employed.
There are available a number of tools which provide complimentary information on chemical and physical processes occurring in the liquid state. A brief introduction is given on the interaction between electromagnetic waves and matter with respect to field theory and methods from atomic physics. The basis for interaction over different length scales is established. Electrochemical characterization includes the quantification and identification of the charge carrying species present, the relative proton concentration, and the complex dielectric response. The vibrational and rotational motion of molecules is measured with a combination of infrared emission spectroscopy and imaging and permits the detection of both the thermal bath and non-equilibrium molecular excited states. The local structure of the liquid matter contained in the bridge can be elucidated through the methods discussed here. X-rays provide information on the electron density whereas neutrons reveal nuclear positions. Together with isotope substitution a complete picture of the motionally averaged local structure of the liquid in the bridge can be recovered. QENS is a special case of inelastic scattering which permits the measurement of diffusion, relaxation, and other slow energy or mass transfer modes in materials using a time-of-flight spectrometer. This data compliments the NMR methods used herein specifically to probe the environment of protons in the system; and to provide clues about the strength of both intra- intermolecular coupling in the system. Very small perturbations in the optical properties of a liquid can be detected using interferometry; these ultimately reflect changes in the polarizability of the liquid which can arise from changes in physical properties. Raman scattering is an inelastic method which can probe changes to the polarizability of a liquid that reflect shifts in the local molecular environment and can be used to determine both local and non-local vibrational coupling.
Magnetic resonance imaging was used to track the flow field present in the bridge without the use of tracer particles; revealing that the bridge has a layered structure, with distinct flow regimes lying one on top of the other. Investigation of the electrochemistry in the water bridge found that protons account for 87% of the charge transport in the bridge. Impedance spectroscopy and pH measurement corroborate the finding that a proton gradient forms across the entire system. The results from elastic neutron and X-ray scattering reveal that the static structure is unchanged within the given accuracy of the employed measurements. However, the systematic analysis of the data using a reverse Monte Carlo computer simulation revealed significant dynamical changes that are reliable above the limited instrument precision. The imposed electric field of an EHD bridge distorts the local Coulombic interactions between molecules altering the dielectric relaxation pathway so that it becomes more favorable for the absorbed energy to become trapped locally for a longer period of time. The electric field in the bridge system is not uniform. Strong field gradients are present which stimulate changes in the molecular polarizability, generating gradients of physical properties, and restricting the allowed rotational-vibrational relaxation transitions. These trends are comparable to those from ultrafast relaxation measurements where the vibrational lifetime of the OH stretch of HDO was found to be significantly shorter in the bridge than in the neat liquid. This absorbed energy, however, remained trapped in a local intermediate state longer in the bridge before being released as a thermal perturbation. The nuclear relaxation dynamics in a glycerol bridge showed similar behavior where the transverse and longitudinal magnetization lifetimes diverged from the expectation values given the systems temperature.
From the experimental observations several features of electrically excited water appear. At the gross continuum level the operation of a floating water bridge results in the production of a charge imbalance between anolyte and catholyte. This is in part due to the enhanced proton mobility in the bridge. Protons no longer are confined to the hydrogen bond mediated Grotthuss mechanism but can travel even faster through a delocalized state. This means that charge can be pumped faster than it can be neutralized resulting in the observed electrochemical differences. The energy level of protons in the conduction channel is the difference between the ground and excited state levels observed as a non-thermal emission feature in the mid-infrared. The proton channel will be active over relatively short distances and will experience interruptions due to fluctuations in molecular position driven by local force gradients. These channels are localized and discontinuous providing the physical basis for the onset of mesoscale dynamic heterogeneity in the excited liquid. The picture begins to emerge whereby local trapping states and long-range cooperative coupling modes dynamically exchange energy. The energy exchange is far from equilibrium and supports multiple transfer mechanisms. At the mesoscale the excited state exhibits traits of a chaotic dynamical system and provides a varied energetic landscape whereby rotational-vibrational transition dipoles, nuclear spin states, and thermodynamic potentials, such as the configurational entropy, non-adiabatically – that is there is a pumping of heat in response to the induced fluctuating gradient fields. The transfer of perturbations from local to collective modes and vice versa requires that the chemical, thermal, and electromagnetic potentials present in the molecular milieu be linked to the entropy production.
This early foray into the non-equilibrium dynamics and mesoscale organization of electrically excited liquid water opens an opportunity to develop technologies which better mimic nature. Taking biological systems as the exemplary standard by which to compare it is necessary to develop soft matter based technical systems which take advantage of the link between electric, magnetic, and thermal fields to drive chemical and physical processes with higher efficiency. Water, as well as other polar liquids, can be locally controlled so as to induce spatial variation in the chemical potential whereby one can imagine a reactor where disparate physical or chemical process can occur in close proximity without the need for rigid segregating structures. Furthermore, this level of control is dynamical such that the organization of the partitioning in the liquid can be changed in time so that the total energy requirement of the intended process is optimized. With such an approach it is conceivable that the size, complexity, and energetic costs of performing many industrial and municipal processes can be reduced. Rather than treating liquids as bulk fluids the opportunity presents itself to use the internal structure and dynamics of liquids to build fluid technologies.