On micro-structural effects in dielectric mixtures

The paper presents numerical simulations performed on dielectric properties of two-dimensional binary composites on eleven regular space filling tessellations. First, significant contributions of different parameters, which play an important role in the electrical properties of the composite, are introduced both for designing and analyzing material mixtures. Later, influence of structural differences and intrinsic electrical properties of constituents on the composite's over all electrical properties are investigated. The structural differences are resolved by the spectral density representation approach. The numerical technique, without any {\em a-priori} assumptions, for extracting the spectral density function is also presented.Comment: 24 pages, 8 figure and 7 tables. It is submitted to IEEE Transactions on Dielectrics and Electrical Insulatio

Dielectric mixtures -- electrical properties and modeling

In this paper, a review on dielectric mixtures and the importance of the numerical simulations of dielectric mixtures are presented. It stresses on the interfacial polarization observed in mixtures. It is shown that this polarization can yield different dielectric responses depending on the properties of the constituents and their concentrations. Open question on the subject are also introduced.Comment: 40 pages 12 figures, to be appear in IEEE Trans. on Dielectric

High-K dielectric sulfur-selenium alloys.

Upcoming advancements in flexible technology require mechanically compliant dielectric materials. Current dielectrics have either high dielectric constant, K (e.g., metal oxides) or good flexibility (e.g., polymers). Here, we achieve a golden mean of these properties and obtain a lightweight, viscoelastic, high-K dielectric material by combining two nonpolar, brittle constituents, namely, sulfur (S) and selenium (Se). This S-Se alloy retains polymer-like mechanical flexibility along with a dielectric strength (40 kV/mm) and a high dielectric constant (K = 74 at 1 MHz) similar to those of established metal oxides. Our theoretical model suggests that the principal reason is the strong dipole moment generated due to the unique structural orientation between S and Se atoms. The S-Se alloys can bridge the chasm between mechanically soft and high-K dielectric materials toward several flexible device applications

Hydrobiogeophysics: Linking geo-electrical properties and biogeochemical processes in shallow subsurface environments

Microbially mediated reactions drive (bio)geochemical cycling of nutrients and contaminants in shallow subsurface environments. Environmental forcings exert a strong control on the timing of reactions and the spatial distribution of processes. Spatial and temporal variations in electron acceptor and donor availability may modulate nutrient/contaminant turnover. Characterizing the preferential spatial-zonation of biologically driven reactions, and quantifying turnover rates is hindered by our inability to access the subsurface at the spatial and temporal resolution required to capture reaction kinetics. Typical subsurface sampling methods generate discrete spatial datasets as a function of the prohibitive cost and operational challenges of borehole installation and core sampling, coupled with sparse temporal datasets due to intermittent sampling campaigns. In order to improve our ability to access biogeochemical information within the subsurface, without the need for destructive and intrusive sampling, non-invasive geophysical techniques (a comparatively inexpensive alternative) have been proposed as a means to characterize subsurface reactive compartments and locate zones of enhanced microbial activity and the timing of their development. The challenge lies in linking electrical responses to specific changes in biogeochemical processes. In this thesis, I assess the potential and suitability of spectral induced polarization (SIP) and self-potential (SP) / electrodic potential (EP) derived geo-electrical signals to detect, map, monitor and quantify microbially mediated reactions in partially- and fully-saturated heterogeneous porous media (i.e., soil). I build on existing literature delineating the sensitivity of SIP, SP and EP to biogeochemical processes and both qualitatively and quantitatively link geo-electrical signal dynamics to specific microbial processes at the experimental scale. I address the monitoring of complex, coupled processes in a well-characterized near-natural system, and combine reactive transport models (RTMs) with single-process reactive experiments (reduced complexity), to isolate diagnostic signatures of specific reactions and processes of interest. In Chapter 2, I begin by monitoring biogeochemically modulated geo-electrical signals (SIP and EP), in the variably (and dynamically) saturated reactive zone within the capillary fringe of an artificial soil system. SIP and EP responses show a clear dependence on the depth-distribution of subsurface microbes. Dynamic SIP imaginary conductivity (σ'') responses are only detected in the water table fluctuation zone and, in contrast to real conductivity (σ') data, do not exhibit a direct soil moisture driven dependence. Using multiple lines of evidence, I attribute the observed σ'' dynamics to microbially driven reactions. Chapter 2 highlights that continuous SIP and EP signals, in conjunction with periodic measurements of geochemical indicators, can help determine the location and temporal variability of biogeochemical activity and be used to monitor targeted reaction zones and pathways in complex soil environments. Building on the findings from Chapter 2, that biomass distribution and activation strongly modulate SIP responses, in Chapter 3 and 4, I focus on isolating the geo-electrical contribution of microbes themselves. In Chapter 3, I couple geochemical data, a biomass-explicit diffusion reaction model and SIP spectra from a saturated sand-packed (with alternating layers of ferrihydrite-coated and pure quartz sand) column experiment, inoculated with Shewanella oneidensis, and supplemented with lactate and nitrate. The coupled RTM and geo-electrical data analysis show that imaginary conductivity peaks parallel simulated microbial growth and decay dynamics. I compute effective polarization diameters, from Cole-Cole modeling derived relaxation times, in the range 1 – 3 µm; two orders of magnitude smaller than the smallest quartz grains in the columns, suggesting that polarization of the bacterial cells directly controls the observed chargeability and relaxation dynamics. In Chapter 4, I address the lack of experimental validation of biomass concentrations in Chapter 3. I present a measurement-derived relationship between S. oneidensis abundance and SIP imaginary conductivity, from a microbial growth experiment in fully saturated sand-filled column reactors. Cole-Cole derived relaxation times highlight the changing surface charging properties of cells in response to stress. The addition of concurrent estimates of cell size allow for the first measurement-derived estimation of an apparent Stern layer diffusion coefficient for cells, which validates existing modelled values and helps quantify electrochemical polarization during SIP-based monitoring of microbial dynamics. The relaxation time results from Chapter 4 suggest that bacterial cell surface charge is modified in response to nitrite toxicity-induced stress. In Chapter 5, I present a biomass-explicit reactive transport model, which integrates nitrite-toxicity, as a key modulator of the energy metabolism of S. oneidensis, to predict the rates of nitrate and nitrite reduction. I validate the model with results from two separate experiments (at different experimental scales): (1) a well-mixed batch suspension and (2) the flow-through reactor experiment from Chapter 4. The incorporation of toxicity-induced uncoupling of catabolism and anabolism in the reactive term predicts the observed delay in biomass growth, facilitated by endogenous energy storage when nitrite is present, and consumption of these reserves after its depletion. The model is further validated by the close agreement between the trends in imaginary conductivity and simulated biomass growth and decay dynamics. Finally, in Chapter 6, I apply the RTM-SIP integrative framework from Chapters 3 and 5 to develop quantitative relationships between SIP signals and engineered nanoparticle concentrations. Therein, SIP responses measured during injection of a polymer-coated iron-oxide nanoparticle suspension in columns packed with natural aquifer sand are coupled to output from an advective-dispersive transport model. The results highlight the excellent agreement between simulated nanoparticle concentrations within the columns and SIP signals, suggesting that polarization increases proportional to increasing nanoparticle concentration. The results from Chapter 6, introduce the possibility of quantitative SIP monitoring of coated metal-oxide nanoparticle spatial and temporal distributions. Overall, my results show the applicability of SIP and EP to map and monitor the spatial zonation of biogeochemical hotspots and to detect their temporal activation. By coupling RTMs with geo-electrical datasets, I highlight the direct control that polarization of microbial cells exerts on SIP signals in biotic systems. Furthermore the measurement-derived SIP-biomass quantitative relationship provides a first attempt to directly measure in situ biomass density, using geo-electrical signals as a proxy. I show that geo-electrical signal dynamics (Cole-Cole relaxation time) can be used to inform processes within RTMs. Finally, the implementation of the combined modeling and electrical monitoring approach, to the case of engineered nanoparticles, confirms SIP’s suitability to monitor colloid transport in the environment and highlights considerations for method optimization

PROBING TEMPORAL CHANGES IN MITOCHONDRIAL MEMBRANE POTENTIAL WITH IMPEDANCE SPECTROSCOPY

The electrical properties of mitochondria provide fundamental insights into metabolic processes in health and disease. This research studies electrical impedance spectroscopy as a non-invasive, sensitive, and relatively low cost technique to monitor biological processes, such as those involving changes in mitochondrial membrane potential. Our experimental strategy first involves treating suspensions of live mitochondria with the substrate succinate to stimulate activity of succinate dehydrogenase, or more simply Complex II. This triggers electron flux through Complex II and the remaining complexes of the electron transport chain, enabling them to pump protons across the inner membrane and build up a membrane potential. Subsequent variability is introduced by adding various concentrations of the uncoupler trifluorocarbonylcyanide phenylhydrazone (FCCP) and the neurotransmitter dopamine (DA) to mitochondrial suspensions, and measuring changes in impedance. Our results show that adding succinate decreases impedance, consistent with an increase in dielectric response and membrane potential. Overall, our investigation establishes real-time impedance spectroscopy as a non-destructive, potentially powerful method for membrane potential studies of mitochondria.Physics, Department o

Strategies to overcome interferences during biomass monitoring with dielectric spectroscopy

Dielectric spectroscopy is extensively used to measure the level of viable biomass during fermentations but can suffer from interference by a variety of factors including the presence of dead cells, bubbles, electric and magnetic fields, changes in the medium composition, conductivity changes and the presence of non-cellular particles. Three different approaches were used to overcome these problems. The first involved the separate measurement of the spectra of the interferent and the cells. If the spectra were significantly different then spectra containing the signals of both cells and the interferent could be deconvoluted to separately determine the relative contribution of the cells and the interferent to the spectra. This deconvolution approach was successfully used to estimate the biomass levels of yeast in the presence of spent grains of barley and hardwood in the medium. A similar approach allowed the interference of electrode polarisation on spectra of yeast and microalgae to be compensated for. An attempt to determine the concentration of non-viable cells in a mixture of dead and live cells was less successful because the signal of the non-viable cells was quite small compared to that of viable cells. A second approach involved the use of a filter to keep the interferent away from the probe surface. This was used successfully in the measurement of the yeast concentration in the presence of spent barley grains. A third approach involved the use of a second sensor in addition to the biomass sensor. This allows the signal of the biomass sensor to be compensated for the interferent. In one set of experiments microelectrodes were developed which were able to confine the electric field to a small volume near the electrode surface. Covering the electrode surface with a gel or a membrane stopped cells from entering this volume whilst allowing medium to diffuse through. This allowed the measurement of changes in the electrical properties of the medium without a contribution by the cells. Whilst this approach worked, the response time was too long for practical use. More successful was the simultaneous measurement of the biomass with an infrared optical probe and a dielectric probe. It was found that the signal of the optical probe was independent of the cell viability, whilst the dielectric probe was quite insensitive to non-viable cells. The combined use of the dielectric probe and the optical probe allowed the culture viability to be determined in a straightforward manner

Optical Sensing of Structural Dynamics in Complex Media

Quantifying the structural dynamics of complex media is challenging because of the multiple temporal and spatial scales involved. Thanks to the ability to retrieve collective dynamics noninvasively, light scattering-based approaches are often the methods of choice. This dissertation discusses specific features of dynamic light scattering that utilizes spatio-temporal coherence gating. It is demonstrated that this optical fiber-based approach can operate over a large range of optical regimes and it has a number of unique capabilities such as an effective isolation of single scattering, a large sensitivity, and a high collection efficiency. Moreover, the approach also provides means for proper ensemble averaging, which is necessary when characterizing multi-scale dynamics. A number of applications are reviewed in which these specific characteristics permit recovering dynamic information of complex fluids beyond the capabilities of traditional light scattering-based techniques

Engineered interfaces with polyelectrolyte multilayers, lipid bilayer membranes and virosomes for biomedical applications

213 p.This thesis presents different approaches for the surface engineering by means of polyelectrolyte multilayers (PEMs), alone or in combination with lipid bilayers and influenza virosomes for biomedical applications.In chapter 1, PEMs of poly-L-lysine (PLL) and alginic acid sodium salt (Alg) are fabricated applying the layer by layer (LbL) technique and annealed at constant temperatures; 37, 50 and 80 °C, for 72 hours. Atomic force microscopy (AFM) reveals changes in the topography of the PEM, which is changing from a fibrillar to a smooth surface after annealing. Advancing contact angle in water varies from 36° before annealing to around 93°, 77° and 95° after annealing at 37, 50 and 80 °C respectively. Changes in surface energy after annealing were calculated from advancing and receding contact angle measurements performed with water and with organic solvents. Changes in the physical properties of the PEMs are interpreted as a result of the reorganization of the polyelectrolytes in the PEMs from a layered structure into complexes where the interaction of polycations and polyanions is enhanced. PEMs from biological origin have many potential applications in tissue engineering and regenerative medicine. However, the softness of biocompatible PEMs results in limited cell adhesion. Thermal annealing is suggested as a novel strategy for the enhancement of cellularadhesion on PEMs. The impact of thermal annealing at 37 °C, on the adhesion of human lung cancer A549 and myoblast C2C12 cell lines is studied. Cell adhesion, measured by the projected average cells spreading and focal contact is remarkably improved for the annealed PEMs. Quartz crystal microbalance with dissipation (QCM-D), contact angle and fluorescence spectroscopy measurements show a significant decrease in the adsorption of the bovine serum albumin protein to the PEMs after annealing. Our results provide a simple method to tune the wettability of bio-PEMs, improve cellular adhesion and endow them with antifouling characteristics.In chapter 2, the self-assembly of small unilamellar vesicles (SUVs) of mixed lipids zwitterionic phosphatidylcholine (DOPC, PC) and anionic phosphatidylserine (DOPS, PS) phospholipids on top of PEMs of poly(allylamine hydrochloride) (PAH), as a polycation, and poly(sodium 4-styrenesulfonate) (PSS), as a polyanion, is investigated as a function of the composition of the assembled vesicles by means of QCM-D, cryo-transmission electron microscopy (CryoTEM), AFM and atomic force spectroscopy (AFS). Vesicles with molar percentages of PS between 50 % and 70 % result in the formation of a lipid bilayer on top of the PEMs. AFS studies performed with a PAH-modified cantilever approaching and retracting from the lipid assemblies reveal that the main interaction between PAH and the lipids takes place through hydrogen bonding between the amine groups of PAH and the carboxylate and phosphate groups of PS and with the phosphate groups of PC.The influence of the surface chemistry of PEMs on the formation of lipid bilayers is also studied for PEMs with poly(diallyldimethylammonium chloride) (PDADMAC) aspolycation and top layer, and PSS as polyanion. SUVs composed of DOPC and DOPS at 50:50 molar ratio are deposited on top of PEM films and studied via QCM-D and fluorescence recovery after photobleaching (FRAP). SUVs deposition on PDADMAC/PSS results in vesicles adsorption while on PAH/PSS under the same conditions a bilayer is formed. FRAP measurements confirm that SUVs are not ruptured on top of PDADMAC/PSS. The role of phosphate ions, in solution, on the formation of lipid bilayers is also analysed. ¿-ray photoelectron spectroscopy (XPS) shows the complexation of phosphate salts to the primary amines of PAH and no interaction with the quaternary amines of PDADMAC. ¿ ¿ potential measurements show a potential close to 0 mV for the PAH/PSS multilayers in PBS while PDADMAC/PSS display a potential of 38 mV. A model is presented for the formation of lipid bilayers on PAH/PSS PEMs taking into account the role of phosphate ions in decreasing electrostatic interactions between SUVs and PEMs and the formation of hydrogen bonding between the phospholipids and the primary amines of PAH.QCM-D and FRAP experiments show that when vesicles with a lipid composition of 50:50 DOPC:DOPS are adsorbed on PEMs where PSS is replaced by Alg or poly(acrylic acid) (PAA) the vesicle deposition does not result in a bilayer formation but in bilayer patches together with adsorbed intact vesicles. Therefore, the fusion of the lipid bilayer is not only affected by the last deposited layer that mainly interacts with the lipids but also by the overall composition of the PEM film.In chapter 3, SUVs prepared by a mixture of 30:70 DOPC:DOPS are assembled on top of a PEM cushion of PAH/PSS and the electrical properties of the bilayer are studied byelectrochemical impedance spectroscopy (EIS). The bilayer supported on the PEMs shows a high resistance, in the order of 107 ¿ cm2 which is indicative of a continuous, dense bilayer. Such resistance is comparable with the resistance of black lipid membranes, being the first time that these values are obtained for lipid bilayers supported on PEMs. The assembly of polyelectrolytes on top of a lipid bilayer decreases the resistance of the bilayer up to 2 orders of magnitude. Thus the assembly of the polyelectrolytes on the lipid bilayer induces defects or pores in the bilayer followed by a subsequent decrease in resistance.Finally, this thesis addresses, in Chapter 4, the fusion of immunostimulating reconstituted influenza virosomes (IRIVs) with the functional viral envelope glycoprotein, hemagglutinin (HA), to artificial supported lipid membranes assembled on PAH/PSS PEMs on both colloidal particles and planar substrates. R18 assay is used to prove the IRIVs fusion in dependence of pH, temperature and HA concentration. IRIVs display a pH-dependent fusion mechanism, fusing at low pH in analogy to the influenza virus. The pH dependent behaviour is confirmed by QCM-D. AFM imaging shows that at low pH virosomes are integrated in the supported membrane displaying flatered features and a reduced vertical thickness. IRIVs fusion offers a new strategy for transferring biological functions on artificial supported membranes with potential applications in targeted delivery and sensing