Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Electrically excited liquid water : lessons from floating water bridge
    Wexler, A.D. - \ 2016
    Wageningen University. Promotor(en): Cees Buisman, co-promotor(en): J. Woisetschläger; E.C. Fuchs. - Wageningen : Wageningen University - ISBN 9789462577039 - 223
    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.

    Electrodynamics of colloids
    Minor, M. - \ 1998
    Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Minor - ISBN 9789054858010 - 145
    colloïden - adsorptie - oppervlakten - oppervlaktechemie - elektrodynamica - colloids - adsorption - surfaces - surface chemistry - electrodynamics - cum laude

    The goal of the present study is to deepen the insight into the non-equilibrium properties of the electric double layer of colloidal systems. Of basic interest are the ionic mobilities in the different regions of the electric double layer as well as the potential at the plane of shear, i.e., the electrokinetic potential (ζ-potential). These parameters determine the colloidal behaviour under non-equilibrium conditions when the double layer is perturbed, for instance if external fields are applied and in particle-particle interaction during coagulation.

    One of the experimental methods utilized in this study is the measurement of the conductivity and the streaming potential of close-packed plugs of particles. From the resulting data we retrieved the dzeta.gif -potential, the surface conductivity, and the mobility of the counterions behind the plane of shear. The results are well comparable to those from the experimental low-frequency (LF) dielectric response of dilute dispersions of latex particles.

    The electrodynamic parameters can be influenced by adsorbing neutral polymer onto the surface

    It is shown that the ζ-potential as well as the mobilities of the ions behind the plane of shear are decreased by the polymer film.

    The data in the above studies were successfully interpreted under the assumption of local equilibrium between the (complete) electric double layer and the adjacent electrolyte. However, there are double-layer conditions where this assumption is violated. In order to study these, we theoretically investigated the influence of relaxation of the compact part of the double layer (occupied inner-Helmholtz Stern layer) on the LF dielectric response and electrophoretic mobility. Possible relaxation mechanisms are retarded adsorption/desorption and ion migration along the surface. Along the same lines, the stability of the sol against coagulation was expressed in terms of the relaxation characteristics of the Stem layer.

    Chapter 2 dealt with the determination of plug conductivities and streaming potentials of a close-packed porous plug of latex particles for a number of indifferent electrolytes and ionic strengths. From these, the dzeta.gif -potentials and surface conductivities were computed. Monodisperse sulphate latex is an ideal model system since the surface charge consists of strong acidic groups so that a constant surface charge density is maintained throughout all the experiments. It was shown that the surface conductivity is insensitive to the ionic strength and that a large part of the countercharge is situated behind the shear plane. Furthermore, it was demonstrated that the ions in the double layer have a mobility close to the bulk mobility.<

    In chapter 3 practical expressions were developed for the low-frequency (LF) dielectric response of dilute dispersions of spherical particles suspended in a binary electrolyte. The LF dielectric response of dilute sulphate latex dispersions was experimentally determined in the frequency range of 500 Hz to 500 kHz as a function of the ionic strength of suspending KCI. The resulting surface conductivities are insensitive to the ionic strength and practically identical to the values obtained by steady state methods (chapter 2). It was proposed that counterion motion can be retarded by specific interaction with the surface and by neutral polymer hairs present on the surface. In order to test the latter effect, the influence of the adsorption of uncharged polymer poly(ethylene) oxide onto the latex surface was investigated by means of LF spectroscopy, plug conductivities and streaming potentials of plugs in chapter 4. It was found that the polymer film on the surface reduces the surface conductivity. The drag on the ions in the polymer film can be described by considering the polymer layer as an inhomogeneous Brinkman fluid, characterised by a Darcy permeability which depends on the local polymer volume fraction. The polymer and counterion distributions were calculated from statistical self-consistent field lattice models.<

    In order to investigate the influence of the surface charge density on the streaming potential and static conductivity, plugs of monodisperse spherical Stöber-silica particles were studied in chapter 5. Contrary to the latex, the surface charge density of silica can be controlled by pH. The high-charge silica plug showed more surface conduction than the low-charge plug since more mobile counterions are present in the double layer of the former. Stöber-silica particles are highly porous. For the relatively large particles under consideration, the major part of the countercharge is situated in the micropores of the particles. It was shown that these counterions do not contribute to the plug conductivity because of their low mobility.

    Chapter 6 analysed the dynamic aspects of particle electrophoresis. It was shown theoretically as well as experimentally that colloidal particles respond to an applied electric field much faster than does the liquid inside a measuring capillary. Therefore, it is possible to apply an alternating electric field with such a frequency that unwanted electroosmosis, induced by charge on the capillary wall, is suppressed, whereas the particles are still able to follow the field according to their dc mobility. This study illustrates that knowledge of the dynamics and the corresponding relaxation times is not only of purely scientific interest, but that it also offers solutions to very practical problems.

    In chapter 7 the influence of polarization of surface charge (or charge in an inner-Helmholtz layer) on the particle mobility, static conductivity, and low-frequency dielectric response was studied within the framework of the thin double-layer theory. It was shown that the characteristic times of relaxation processes in the Stern layer are accessible from dielectric spectroscopy. The relaxation phenomena under consideration are Stern-layer polarization via retarded adsorption/desorption and polarization via lateral transport in the Stem layer. The two processes may occur simultaneously. Since these relaxation processes are also relevant for particle-particle interaction, chapter 8 considered the implications for colloidal stability. In the situation of small transient disequilibrations of the surface charge, the stability could be expressed in terms of the characteristic times of surface charge relaxation. This allows the use of electrodynamic data obtained by dielectric spectroscopy in the interpretation of colloidal stability. On an even more rigorous level, the free energy of particle-particle interaction was also considered in the space of the two variables surface charge and separation. This formalism opens the way to investigate coagulation far from equilibrium.

    The electrical potential as a gauge of photosynthetic performance in plant chloroplasts : a patch-clamp study
    Voorthuysen, T. van - \ 1997
    Agricultural University. Promotor(en): W.J. Vredenberg; J.H.F. Snel. - S.l. : Van Voorthuysen - ISBN 9789054856696 - 155
    fotosynthese - chloroplasten - elektromagnetisch veld - elektrodynamica - elektromagnetisme - photosynthesis - chloroplasts - electromagnetic field - electrodynamics - electromagnetism

    The earliest events in the energisation of the photosynthetic membrane upon light capture are the formation of a transmembrane electrical potential (AV) and a transmembrane proton gradient (ΔpH). In this thesis ΔΨis employed for the study of the bioenergetics of chloroplast photosynthesis and its regulation by ΔpH in the shade adapted plant peperomia ( Peperomia metallic a) and the high-light adapted plant spinach ( Spinacia oleracea ). Electrochromism (P515) was used and a patch-clamp method was developed yielding two complementing tools for the detection of ΔΨ. The patch-clamp method enables the detection of relatively large light- induced currents (photocurrents) or potentials (photopotentials) of a single P. metallica chloroplast. An electrical equivalent scheme is introduced incorporating amongst others the thylakoid membrane resistance and capacitance and an access resistance which, at least partly, is supposed to be associated with low (lateral) conductance phases of thylakoid lamellae. The light-induced electrical responses reflect the operation of the photosynthetic current-generators and the way generated current is distributed throughout the chloroplast conductance network. Simultaneous measurements of light- and current-induced responses allow the separation of electrogenic events from changes in chloroplast conductances. A kinetically well defined slow secondary phase (R1 /Q) could be distilled from the flash- induced photocurrent/-potential which is related to the turnover of the cyt. b 6 f complex (Q-cycle). Generally, the rise of R1/Q was sigmoidal. This biphasic rise is modelled by a consecutive reaction scheme with two relaxation times of 13 and 28 ms which likely reflect the oxidation of plastoquinol and reduction of plastoquinone at the lumen and stroma membrane/water interface of the b 6 f complex, respectively. A P515 fraction (Rl /RC f ) of about 20 % is inadequately stabilised in dark-adapted spinach chloroplasts and decays rapidly with a relaxation time of 1 - 2 ms. A fast dissipation of ΔΨas generated by photosystern (PS) II is suggested to cause R1/RC f . It is hypothesised that adequate charge stabilisation depends on efficient energy coupling between PS II and the cyt. b 6 f complex which is only guaranteed in superclusters composed of both protein complexes. Energisation causes a suppression of about 50 % of PS II-dependent charge separation which is dark reversible with a relaxation time of about 20 s and is likely induced by the low lumenal pH created by lightdriven proton pumping. The results are best explained by a reaction center quenching model in which a fraction of PS II centers exhibits a rapid charge recombination. Flash-induced changes in chloroplast conductances are first demonstrated. The seal conductance decreases transiently upon a brief flash with a minimum of 0.3 - 5 % at 50 - 200 ms after the flash and a slow relaxation in 1 - 10 s. It is proposed that an important part of the conductance changes is intimately associated with changes in the lateral conductances of thylakoids, in particular those of the narrow spaced grana thylakoids.

    Increasing the shelf life of food by using microwave energy (in Dutch)
    Bartels, P.V. - \ 1996
    Voedingsmiddelentechnologie 29 (1996)22. - ISSN 0042-7934 - p. 35 - 38.
    dierlijke producten - effecten - elektromagnetisch veld - elektromagnetische straling - voedselbewaring - voedingsmiddelen - houdbaarheid (kwaliteit) - magnetisch veld - vee- en vleesindustrie - vleeswaren - elektrodynamica - elektromagnetisme - animal products - effects - electromagnetic field - electromagnetic radiation - food preservation - foods - keeping quality - magnetic field - meat and livestock industry - meat products - electrodynamics - electromagnetism
    De voordelen van het toepassen van een conventionele verhitting met daaraan toegevoegd een elektromagnetische verhitting
    Electrodynamics of the AgI/solution interface
    Polder, R.B. - \ 1984
    Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch, co-promotor(en): H.P. van Leeuwen. - Wageningen : Polder - 77
    vloeistofmechanica - capillairen - oppervlaktespanning - elektromagnetisch veld - zilver - jodide - analytische methoden - elektrochemie - grenslaag - oppervlakteverschijnselen - elektroanalytische analyse - elektrodynamica - elektromagnetisme - fluid mechanics - capillaries - surface tension - electromagnetic field - silver - iodide - analytical methods - electrochemistry - boundary layer - surface phenomena - electroanalytical analysis - electrodynamics - electromagnetism

    The purpose of this study is to gain insight in electrodynamic processes in colloidal systems, that is, in the electrical currents that flow because of the movement of charged particles. There is a need for such insight, because the DUO theory describing the stability of electrocratic colloids cannot answer the following question: can interacting particles in the short time of Brownian encounter adjust their charge to the disequilibration resulting from the overlap of the double layers and thus keep their potential constant ? Or will the particles keep their charge constant during the interaction? The answer depends on the rate of various possible charge transfer processes. We have chosen for electrodes to investigate the dynamic phenomena of interest, and for AgI as the model substance.
    Chapter I offers a general introduction to the theme, and describes the outline of this study.
    The experimental technique used (the coulostatic impulse method) is based on the following experiment. A small departure from the equilibrium potential is instantaneously imposed on two identical electrodes. The overpotential relaxes and the decay, of which the precise shape contains information on the various processes, is recorded. In chapter II we describe the preparation of the AgI electrodes and the other materials, the setup and the procedure to convert the decay signal into an impedance spectrum.
    In chapter III an analysis of the impedance spectrum is made, and the possible components of the electrical equivalent circuit are discussed. It is shown that surface roughness of the electrode seriously complicates the mass transport impedance and this may ruin the analysis of the impedance spectrum. It is concluded that the combined analysis of two admittance functions, employing both the real and imaginary components, provides the best method: it allows to clearly recognize the effects of surface roughness, and the analysis can easily be automated. It is also shown that ion transfer through the interface is a rapid process, and that diffusion in the solution (mass transport) is the rate limiting step.
    In chapter IV experimental results are presented in terms of capacitances and Warburg (diffusional) coefficients under various conditions of potential and electrolyte concentration. The data refer to 'clean' electrodes, as well as to electrodes with an adsorbed polymer layer. The polymers used were PVA (M ca. 9 * 10 4 ) and PVP (M ca. 9 * 10 5 ). The results are compared with literature data. Generally, the agreement was satisfactory, and a tentative explanation for the trends in the capacitances was forwarded. The Warburg coefficients showed some deviation from the theoretically expected behaviour, the more so when polymer was adsorbed.
    Chapter V summarizes the literature on relaxation processes during interaction in colloids, and resumes the experimental information from the previous chapters. Particularly the assessment of ion transfer as a fast process calls for a reconsideration of the hitherto existing picture. Three possible relaxation routes (after ion transfer) of the excess charge on the particle are described and relaxation times for each are estimated. It is concluded that silver iodide particles, uncovered or covered with a polymer layer, can adjust their surface charge on the time scale of a collision by at least one of the transport processes described. The interaction thus takes place under conditions of constant potential .

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