Origin of spin incommensurability in hole-doped S=1 Y2-xCaxBaNiO5 chains

Spin incommensurability (IC) has been recently experimentally discovered in the hole-doped Ni-oxide chain compound Y2-xCaxBaNiO5 [G. Xu et al., Science 289, 419 (2000)]. Here a two orbital model for this material is studied using computational techniques. Spin IC is observed in a wide range of densities and couplings. The phenomenon originates in antiferromagnetic correlations across holes dynamically generated to improve hole movement, as it occurs in the one-dimensional Hubbard model and in recent studies of the two-dimensional extended t-J model. The close proximity of ferromagnetic and phase-separated states in parameter space is also discussed.

Shape-independent expansion for the 3S1 - 3D1 mixing parameter

A low-energy shape-independent expansion is suggested for the function tan(2εBB)/(2k2), where εBB is the Blatt-Biedenharn mixing parameter for the 3S1 - 3D1 channel. This expansion allows an evaluation of the mixing parameter εBB from a knowledge of the deuteron asymptotic D to S ratio, pion mass and other low-energy observables, such as the scattering lengths, deuteron binding etc., of the nucleon-nucleon system. We demonstrate that the correct long range behavior of the tensor potential is essential for a realistic reproduction of εBB.

Unmanned platform for long-range remote analysis of volatile compounds in air samples

This paper describes a long-range remotely controlled CE system built on an all-terrain vehicle. A four-stroke engine and a set of 12-V batteries were used to provide power to a series of subsystems that include drivers, communication, computers, and a capillary electrophoresis module. This dedicated instrument allows air sampling using a polypropylene porous tube, coupled to a flow system that transports the sample to the inlet of a fused-silica capillary. A hybrid approach was used for the construction of the analytical subsystem combining a conventional fused-silica capillary (used for separation) and a laser machined microfluidic block, made of PMMA. A solid-state cooling approach was also integrated in the CE module to enable controlling the temperature and therefore increasing the useful range of the robot. Although ultimately intended for detection of chemical warfare agents, the proposed system was used to analyze a series of volatile organic acids. As such, the system allowed the separation and detection of formic, acetic, and propionic acids with signal-to-noise ratios of 414, 150, and 115, respectively, after sampling by only 30 s and performing an electrokinetic injection during 2.0 s at 1.0 kV.

Linear parameter varying control design for rotating systems supported by journal bearings

Linear parameter varying (LPV) control is a model-based control technique that takes into account time-varying parameters of the plant. In the case of rotating systems supported by lubricated bearings, the dynamic characteristics of the bearings change in time as a function of the rotating speed. Hence, LPV control can tackle the problem of run-up and run-down operational conditions when dynamic characteristics of the rotating system change significantly in time due to the bearings and high vibration levels occur. In this work, the LPV control design for a flexible shaft supported by plain journal bearings is presented. The model used in the LPV control design is updated from unbalance response experimental results and dynamic coefficients for the entire range of rotating speeds are obtained by numerical optimization. Experimental implementation of the designed LPV control resulted in strong reduction of vibration amplitudes when crossing the critical speed, without affecting system behavior in sub- or supercritical speeds. (C) 2012 Elsevier Ltd. All rights reserved.

Exploration of the parameter space in an agent-based model of tuberculosis spread: emergence of drug resistance in developing vs developed countries

In this work we present an agent-based model for the spread of tuberculosis where the individuals can be infected with either drug-susceptible or drug-resistant strains and can also receive a treatment. The dynamics of the model and the role of each one of the parameters are explained. The whole set of parameters is explored to check their importance in the numerical simulation results. The model captures the beneficial impact of the adequate treatment on the prevalence of tuberculosis. Nevertheless, depending on the treatment parameters range, it also captures the emergence of drug resistance. Drug resistance emergence is particularly likely to occur for parameter values corresponding to less efficacious treatment, as usually found in developing countries.

Experimental investigation of gravitational gas separation in an inclined annular channel

The oil industry uses gas separators in production wells as the free gas present in the suction of the pump reduces the pumping efficiency and pump lifetime. Therefore, free gas is one of the most important variables in the design of pumping systems. However, in the literature there is little information on these separators. It is the case of the inverted-shroud gravitational gas separator. It has an annular geometry due to the installation of a cylindrical container in between the well casing and pioduction pipe (tubing). The purpose of the present study is to understand the phenomenology and behavior of inverted-shroud separator. Experimental tests were performed in a 10.5-m-length inclinable glass tube with air and water as working fluids. The water flow rate was in the range of 8.265-26.117 l/min and the average inlet air mass flow rate was 1.1041 kg/h, with inclination angles of 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 80 degrees and 85 degrees. One of the findings is that the length between the inner annular level and production pipe inlet is one of the most important design parameters and based on that a new criterion for total gas separation is proposed. We also found that the phenomenology of the studied separator is not directly dependent on the gas flow rate, but on the average velocity of the free surface flow generated inside the separator. Maps of efficiency of gas separation were plotted and showed that liquid flow rate, inclination angle and pressure difference between casing and production pipe outlet are the main variables related to the gas separation phenomenon. The new data can be used for the development of design tools aiming to the optimized project of the pumping system for oil production in directional wells. (C) 2012 Elsevier Inc. All rights reserved.

Calculation of parameter ranges for robust gain tuning of power system controllers

This work proposes a computational tool to assist power system engineers in the field tuning of power system stabilizers (PSSs) and Automatic Voltage Regulators (AVRs). The outcome of this tool is a range of gain values for theses controllers within which there is a theoretical guarantee of stability for the closed-loop system. This range is given as a set of limit values for the static gains of the controllers of interest, in such a way that the engineer responsible for the field tuning of PSSs and/or AVRs can be confident with respect to system stability when adjusting the corresponding static gains within this range. This feature of the proposed tool is highly desirable from a practical viewpoint, since the PSS and AVR commissioning stage always involve some readjustment of the controller gains to account for the differences between the nominal model and the actual behavior of the system. By capturing these differences as uncertainties in the model, this computational tool is able to guarantee stability for the whole uncertain model using an approach based on linear matrix inequalities. It is also important to remark that the tool proposed in this paper can also be applied to other types of parameters of either PSSs or Power Oscillation Dampers, as well as other types of controllers (such as speed governors, for example). To show its effectiveness, applications of the proposed tool to two benchmarks for small signal stability studies are presented at the end of this paper.

Self assembled monolayers for engineering of structured inorganic materials in the micrometer and submicrometer range

Deutsch:In der vorliegenden Arbeit konnten neue Methoden zur Synthese anorganischer Materialien mit neuartiger Architektur im Mikrometer und Nanometer Maßstab beschrieben werden. Die zentrale Rolle der Formgebung basiert dabei auf der templatinduzierten Abscheidung der anorganischen Materialien auf selbstorganisierten Monoschichten. Als Substrate eignen sich goldbedampfte Glasträger und Goldkolloide, die eine Mittelstellung in der Welt der Atome bzw. Moleküle und der makroskopischen Welt der ausgedehnten Festkörper einnehmen. Auf diesen Substraten lassen sich Thiole zu einer monomolekularen Schicht adsorbieren und damit die Oberflächeneigenschaften des Substrates ändern. Ein besonderer Schwerpunkt bei dieser Arbeit stellt die Synthese speziell auf die Bedürfnisse der jeweiligen Anwendung ausgerichteten Thiole dar.Im ersten Teil der Arbeit wurden goldbedampfte Glasoberflächen als Template verwendet. Die Abscheidung von Calciumcarbonat wurde in Abhängigkeit der Schichtdicke der adsorbierten Monolage untersucht. Aragonit, eine der drei Hauptphasen des Calciumcarbonat Systems, wurde auf polyaromatischen Amid - Oberflächen mit Schichtdicken von 5 - 400 nm Dicke unter milden Bedingung abgeschieden. Die einstellbaren Parameter waren dabei die Kettenlänge des Polymers, der w-Substituent, die Bindung an die Goldoberfläche über Verwendung verschiedener Aminothiole und die Kristallisationstemperatur. Die Schichtdickeneinstellung der Polymerfilme erfolgte hierbei über einen automatisierten Synthesezyklus.Titanoxid Filme konnten auf Oberflächen strukturiert werden. Dabei kam ein speziell synthetisiertes Thiol zum Einsatz, das die Funktionalität einer Styroleinheit an der Oberflächen Grenze als auch eine Möglichkeit zur späteren Entfernung von der Oberfläche in sich vereinte. Die PDMS Stempeltechnik erzeugte dabei Mikrostrukturen auf der Goldoberfläche im Bereich von 5 bis 10 µm, die ihrerseits über die Polymerisation und Abscheidung des Polymers in den Titanoxid Film überführt werden konnten. Drei dimensionale Strukturen wurden über Goldkolloid Template erhalten. Tetraethylenglykol konnte mit einer Thiolgruppe im Austausch zu einer Hydroxylgruppe monofunktionalisiert werden. Das erhaltene Molekül wurde auf kolloidalem Gold selbstorganisiert; es entstand dabei ein wasserlösliches Goldkolloid. Die Darstellung erfolgte dabei in einer Einphasenreaktion. Die so erhaltenen Goldkolloide wurden als Krstallisationstemplate für die drei dimensionale Abscheidung von Calciumcarbonat verwendet. Es zeigte sich, dass Glykol die Kristallisation bzw. den Habitus des krsitalls bei niedrigem pH Wert modifiziert. Bei erhöhtem pH Wert (pH = 12) jedoch agieren die Glykol belegten Goldkolloide als Template und führen zu sphärisch Aggregaten. Werden Goldkolloide langkettigen Dithiolen ausgesetzt, so führt dies zu einer Aggregation und Ausfällung der Kolloide aufgrund der Vernetzung mehrer Goldkolloide mit den Thiolgruppen der Alkyldithiole. Zur Vermeidung konnte in dieser Arbeit ein halbseitig geschütztes Dithiol synthetisiert werden, mit dessen Hilfe die Aggregation unterbunden werden konnte. Das nachfolgende Entschützten der Thiolfunktion führte zu Goldkolloiden, deren Oberfläche Thiol funktionalisiert werden konnte. Die thiolaktiven Goldkolloide fungierten als template für die Abscheidung von Bleisulfid aus organisch/wässriger Lösung. Die Funktionsweise der Schutzgruppe und die Entschützung konnte mittels Plasmonenresonanz Spektroskopie verdeutlicht werden. Titanoxid / Gold / Polystyrol Komposite in Röhrenform konnten synthetisiert werden. Dazu wurde ein menschliches Haar als biologisches Templat für die Formgebung gewählt.. Durch Bedampfung des Haares mit Gold, Assemblierung eines Stryrolmonomers, welches zusätzlich eine Thiolfunktionalität trug, Polymerisation auf der Oberfläche, Abscheidung des Titanoxid Films und anschließendem Auflösen des biologischen Templates konnte eine Röhrenstruktur im Mikrometer Bereich dargestellt werden. Goldkolloide fungierten in dieser Arbeit nicht nur als Kristallisationstemplate und Formgeber, auch sie selbst wurden dahingehend modifiziert, dass sie drahtförmige Agglormerate im Nanometerbereich ausbilden. Dazu wurden Template aus Siliziumdioxid benutzt. Zum einen konnten Nanoröhren aus amorphen SiO2 in einer Sol Gel Methode dargestellt werden, zum anderen bediente sich diese Arbeit biologischer Siliziumoxid Hohlnadeln aus marinen Schwämmen isoliert. Goldkolloide wurden in die Hohlstrukturen eingebettet und die Struktur durch Ausbildung von Kolloid - Thiol Netzwerken mittels Dithiol Zugabe gefestigt. Die Gold-Nanodrähte im Bereich von 100 bis 500 nm wurden durch Auflösen des SiO2 - Templates freigelegt.

Analytical challenges in the fields of Nanobioscience and Nanobiotecnology: integrated methods for separation and characterization

Nano(bio)science and nano(bio)technology play a growing and tremendous interest both on academic and industrial aspects. They are undergoing rapid developments on many fronts such as genomics, proteomics, system biology, and medical applications. However, the lack of characterization tools for nano(bio)systems is currently considered as a major limiting factor to the final establishment of nano(bio)technologies. Flow Field-Flow Fractionation (FlFFF) is a separation technique that is definitely emerging in the bioanalytical field, and the number of applications on nano(bio)analytes such as high molar-mass proteins and protein complexes, sub-cellular units, viruses, and functionalized nanoparticles is constantly increasing. This can be ascribed to the intrinsic advantages of FlFFF for the separation of nano(bio)analytes. FlFFF is ideally suited to separate particles over a broad size range (1 nm-1 μm) according to their hydrodynamic radius (rh). The fractionation is carried out in an empty channel by a flow stream of a mobile phase of any composition. For these reasons, fractionation is developed without surface interaction of the analyte with packing or gel media, and there is no stationary phase able to induce mechanical or shear stress on nanosized analytes, which are for these reasons kept in their native state. Characterization of nano(bio)analytes is made possible after fractionation by interfacing the FlFFF system with detection techniques for morphological, optical or mass characterization. For instance, FlFFF coupling with multi-angle light scattering (MALS) detection allows for absolute molecular weight and size determination, and mass spectrometry has made FlFFF enter the field of proteomics. Potentialities of FlFFF couplings with multi-detection systems are discussed in the first section of this dissertation. The second and the third sections are dedicated to new methods that have been developed for the analysis and characterization of different samples of interest in the fields of diagnostics, pharmaceutics, and nanomedicine. The second section focuses on biological samples such as protein complexes and protein aggregates. In particular it focuses on FlFFF methods developed to give new insights into: a) chemical composition and morphological features of blood serum lipoprotein classes, b) time-dependent aggregation pattern of the amyloid protein Aβ1-42, and c) aggregation state of antibody therapeutics in their formulation buffers. The third section is dedicated to the analysis and characterization of structured nanoparticles designed for nanomedicine applications. The discussed results indicate that FlFFF with on-line MALS and fluorescence detection (FD) may become the unparallel methodology for the analysis and characterization of new, structured, fluorescent nanomaterials.

Phasenverhalten von Polyethylen in Mischsystemen

Die vorliegende Arbeit beschäftigt sich mit dem Phasenverhalten von Polyethylen (PE) in nicht-reaktiven und in reaktiven Systemen. Von drei eng verteilten Polyethylenen (Mw = 6,4, 82 bzw. 380 kg/mol) in n-Hexan sowie für das System 2,2-Dimethylbutan / PE 82 wurde die Entmischung in Abhängigkeit von der Zusammensetzung, dem Druck und der Temperatur experimentell bestimmt. Die Modellierung der Trübungskurven erfolgte nach der Theorie von Sanchez und Lacombe. Dieser Ansatz beschreibt die Ergebnisse qualitativ und kann in einem engen Temperatur- und Druckbereich für gegebenes Molekulargewicht die kritische Temperatur und den kritischen Druck quantitativ vorhersagen. Durch Extrapolation der kritischen Temperatur der verschiedenen Lösungen von PE in n-Hexan auf unendliches Molekulargewicht nach Shultz-Flory wurde im Druckbereich von 20 bis 100 bar und im Temperaturbereich von 130 bis 200 °C eine Grenzlinie bestimmt. Diese Linie trennt unabhängig vom Molekulargewicht des Polymers und der Zusammensetzung der Mischung das Zweiphasengebiet vom homogenen Bereich. Im Fall des Mischlösungsmittels n-Hexan / 2,2-Dimethylbutan wurde für eine annähernd kritische Polymerkonzentration die Abhängigkeit der Entmischungsbedingungen von der Zusammensetzung untersucht. Durch einfache Erweiterung der Sanchez-Lacombe-Theorie und Einführen eines Fitparameters konnte das ternäre System beschrieben werden. An einer breit verteilten PE-Probe wurden Experimente zur Fraktionierung von PE in n-Hexan durchgeführt. Die Analyse der in den koexistenten Phasen enthaltenen Polymere lieferte Informationen über die Konzentration und die Molekulargewichtsverteilung des PE in diesen Phasen sowie die kritische Zusammensetzung der Mischung. Von verschiedenen PE-Lösungen (Mw = 0,5 kg/mol) wurde die polymerisationsinduzierte Phasenseparation in Isobornylmethacrylat mit und ohne Vernetzer untersucht. Mit 15 Gew.-% PE und in Abwesenheit von Vernetzer findet die Entmischung erst bei hohen Umsätzen statt. Die Charakterisierung der resultierenden Proben zeigte, dass sich etwas mehr als 5 Gew.-% PE im Polyisobornylmethacrylat lösen. Die Glasübergangstemperaturen der Polymermischungen steigen mit steigender Vernetzer- und sinkender Polyethylenkonzentration. Bei Proben mit 15 Gew.-% PE zeigte sich folgendes: 5 Gew.-% Vernetzer führen zu großen PE-Bereichen (150 - 200 nm) in der Matrix und der Kristallinitätsgrad ist gering. Bei der Polymermischung mit 10 Gew.-% Vernetzer bilden sich sehr kleine Polyethylenkristalle (< 80 nm) und der Kristallinitätsgrad ist hoch. Ohne Vernetzer hängt der Kristallinitätsgrad - wie bei reinem PE - von der Abkühlrate ab, mit Vernetzer ist er von ihr unabhängig.

Production, radiochemical separation and chemical coupling of radioactive arsenic isotopes to synthesize radiopharmaceuticals for molecular imaging

Noninvasive molecular-imaging technologies are playing a keyrole in drug discovery, development and delivery. Positron Emission Tomography (PET) is such a molecular imaging technology and a powerful tool for the observation of various diseases. However, it is limited by the availability of agents with high selectivity to the target and a physical half-life of the used positron emitting nuclide which matches the biological half-life of the observed process. For the long lasting enrichment of antibodies in tumor tissue few suitable isotopes for PET imaging are currently available. The element arsenic provides a range of isotopes, which could be used for diagnosis and also for endoradiotherapy. This work describes the development of radiochemical separation procedures to separate arsenic isotopes in no-carrier-added (nca) purity from reactor or cyclotron irradiated targets, the development and evaluation of a labeling chemistry to attach these separated arsenic isotopes to monoclonal antibodies, the in vitro and in vivo evaluation of antibodies labeled with radioactive arsenic isotopes and the molecular imaging using small animal PET.

Control of the long-range self-organization of polycyclic aromatic hydrocarbons for device applications

Electronic devices based on organic semiconductors have gained increased attention in nanotechnology, especially applicable to the field of field-effect transistors and photovoltaic. A promising class of materials in this reseach field are polycyclic aromatic hydrocarbons (PAHs). Alkyl substitution of these graphenes results in the selforganization into one-dimensional columnar superstructures and provides solubility and processibility. The nano-phase separation between the π-stacking aromatic cores and the disordered peripheral alkyl chains leads to the formation of thermotropic mesophases. Hexa-peri-hexabenzocoronenes (HBC), as an example for a PAH, exhibits some of the highest values for the charge carrier mobility for mesogens, which makes them promising candidates for electronic devices. Prerequisites for efficient charge carrier transport between electrodes are a high purity of the material to reduce possible trapping sites for charge carriers and a pronounced and defect-free, long-range order. Appropriate processing techniques are required to induce a high degree of aligned structures in the discotic material over macroscopic dimensions. Highly-ordered supramolecular structures of different discotics, in particular, of HBC derivatives have been obtained by solution processing using the zone-casting technique, zone-melting or simple extrusion. Simplicity and fabrication of highly oriented columnar structures over long-range are the most essential advantages of these zone-processing methods. A close relation between the molecular design, self-aggregation and the processing conditions has been revealed. The long-range order achieved by the zone-casting proved to be suitable for field effect transistors (FET).

Statistische Betrachtungen und mößbauerspektroskopische Untersuchungen von Ordnungszuständen in Biotiten

Im Mittelpunkt der vorliegenden Arbeit standen Untersuchungen zu strukturellen Ordnungs- und Unordnungsphänomenen an natürlichen, substitutionellen Mischkristallen. Aufgrund der enormen Vielfalt an potentiellen Austauschpartnern wurden hierfür Vertreter der Biotit-Mischkristallreihe "Phlogopit-Annit" ausgewählt. Ihr modulartiger Aufbau ermöglichte die gezielte Beschreibung von Verteilungsmustern anionischer und kationischer Merkmalsträger innerhalb der Oktaederschicht der Biotit-Mischkristalle. Basierend auf der postulierten Bindungsaffinität zwischen Mg2+ und F- einerseits und Fe2+ und OH- andererseits, wurde die strukturelle Separation einer Fluor-Phlogopit-Komponente als primäre Ausprägungsform des Mg/F-Ordnungsprinzips abgeleitet. Im Rahmen dieser Modellvorstellung koexistieren im makroskopisch homogenen Biotit-Mischkristall Domänen zweier chemisch divergenter Phasen nebeneinander: Eine rein Mg2+/F- - führende Phlogopit-Phase und eine Wirtskristallphase, die mit fortschreitender Separation bzw. Entmischung der erstgenannten Phase sukzessive reicher an einer hydroxylführenden eisenreichen Annit-Komponente wird. Zwecks numerischer Beschreibung diverser Stadien der Entmischungsreaktion wurden die Begriffe der "Relativen" und "Absoluten Domänengröße" eingeführt. Sie stellen ein quantitatives Maß zur Beurteilung der diskutierten Ordnungsphänomene dar. Basierend auf einem sich stetig ändernden Chemismus der Wirtskristallphase kann jeder Übergangszustand zwischen statistischer Verteilung und vollständiger Ordnung durch das korrespondierende Verteilungsmuster der interessierenden Merkmalsträger ( = Nahordnungskonfiguration und Besetzungswahrscheinlichkeit) charakterisiert und beschrieben werden. Durch mößbauerspektroskopische Untersuchungen konnten die anhand der entwickelten Modelle vorhergesagten Ausprägungsformen von Ordnungs-/Unordnungsphänomenen qualitativ und quantitativ verifiziert werden. Es liessen sich hierbei zwei Gruppen von Biotit-Mischkristallen unterscheiden. Eine erste Gruppe, deren Mößbauer-Spektren durch den OH/F-Chemismus als dominierendes Differenzierungsmerkmal geprägt wird, und eine zweite Gruppe, deren Mößbauer-Spektren durch Gruppierungen von höherwertigen Kationen und Vakanzen ( = Defektchemie) geprägt wird. Auf der Basis von Korrelationsdiagrammen, die einen numerischen und graphischen Bezug zwischen absoluter und relativer Domänengröße einerseits und experimentell zugänglichem Mößbauer-Parameter A (= relativer Flächenanteil, korrespondierend mit der Besetzungswahrscheinlichkeit einer bestimmten Nahordnungs-konstellation) andererseits herstellen, konnten für die erste Gruppe die Volumina der beiden miteinander koexistierenden Komponenten "Hydroxyl-Annit reicher Wirtskristall" und "Fluor-Phlogopit" exakt quantifiziert werden. Das Spektrum der untersuchten Proben umfasste hierbei Kristallspezies, die einerseits durch geringe bis mittlere Mg2+/F- - bzw. Fe2+/OH- -Ordnungsgrade gekennzeichnet sind, andererseits eine nahezu vollständige Ordnung der interessierenden Merkmalsträger Mg2+, Fe2+, OH- und F- widerspiegeln. Desweiteren konnte der Nachweis geführt werden, dass für ausgewählte Proben eine quantitative Bestimmung der Defektvolumina möglich ist.

Optical resonances of sphere-on-plane geometries

The optical resonances of metallic nanoparticles placed at nanometer distances from a metal plane were investigated. At certain wavelengths, these “sphere-on-plane” systems become resonant with the incident electromagnetic field and huge enhancements of the field are predicted localized in the small gaps created between the nanoparticle and the plane. An experimental architecture to fabricate sphere-on-plane systems was successfully achieved in which in addition to the commonly used alkanethiols, polyphenylene dendrimers were used as molecular spacers to separate the metallic nanoparticles from the metal planes. They allow for a defined nanoparticle-plane separation and some often are functionalized with a chromophore core which is therefore positioned exactly in the gap. The metal planes used in the system architecture consisted of evaporated thin films of gold or silver. Evaporated gold or silver films have a smooth interface with their substrate and a rougher top surface. To investigate the influence of surface roughness on the optical response of such a film, two gold films were prepared with a smooth and a rough side which were as similar as possible. Surface plasmons were excited in Kretschmann configuration both on the rough and on the smooth side. Their reflectivity could be well modeled by a single gold film for each individual measurement. The film has to be modeled as two layers with significantly different optical constants. The smooth side, although polycrystalline, had an optical response that was very similar to a monocrystalline surface while for the rough side the standard response of evaporated gold is retrieved. For investigations on thin non-absorbing dielectric films though, this heterogeneity introduces only a negligible error. To determine the resonant wavelength of the sphere-on-plane systems a strategy was developed which is based on multi-wavelength surface plasmon spectroscopy experiments in Kretschmann-configuration. The resonant behavior of the system lead to characteristic changes in the surface plasmon dispersion. A quantitative analysis was performed by calculating the polarisability per unit area /A treating the sphere-on-plane systems as an effective layer. This approach completely avoids the ambiguity in the determination of thickness and optical response of thin films in surface plasmon spectroscopy. Equal area densities of polarisable units yielded identical response irrespective of the thickness of the layer they are distributed in. The parameter range where the evaluation of surface plasmon data in terms of /A is applicable was determined for a typical experimental situation. It was shown that this analysis yields reasonable quantitative agreement with a simple theoretical model of the sphere-on-plane resonators and reproduces the results from standard extinction experiments having a higher information content and significantly increased signal-to-noise ratio. With the objective to acquire a better quantitative understanding of the dependence of the resonance wavelength on the geometry of the sphere-on-plane systems, different systems were fabricated in which the gold nanoparticle size, type of spacer and ambient medium were varied and the resonance wavelength of the system was determined. The gold nanoparticle radius was varied in the range from 10 nm to 80 nm. It could be shown that the polyphenylene dendrimers can be used as molecular spacers to fabricate systems which support gap resonances. The resonance wavelength of the systems could be tuned in the optical region between 550 nm and 800 nm. Based on a simple analytical model, a quantitative analysis was developed to relate the systems’ geometry with the resonant wavelength and surprisingly good agreement of this simple model with the experiment without any adjustable parameters was found. The key feature ascribed to sphere-on-plane systems is a very large electromagnetic field localized in volumes in the nanometer range. Experiments towards a quantitative understanding of the field enhancements taking place in the gap of the sphere-on-plane systems were done by monitoring the increase in fluorescence of a metal-supported monolayer of a dye-loaded dendrimer upon decoration of the surface with nanoparticles. The metal used (gold and silver), the colloid mean size and the surface roughness were varied. Large silver crystallites on evaporated silver surfaces lead to the most pronounced fluorescence enhancements in the order of 104. They constitute a very promising sample architecture for the study of field enhancements.

Dynamics, rheology and critical properties of colloidal fluid mixtures

Liquids under the influence of external fields exhibit a wide range of intriguing phenomena that can be markedly different from the behaviour of a quiescent system. This work considers two different systems — a glassforming Yukawa system and a colloid-polymer mixture — by Molecular Dynamics (MD) computer simulations coupled to dissipative particle dynamics. The former consists of a 50-50 binary mixture of differently-sized, like-charged colloids interacting via a screened Coulomb (Yukawa) potential. Near the glass transition the influence of an external shear field is studied. In particular, the transition from elastic response to plastic flow is of interest. At first, this model is characterised in equilibrium. Upon decreasing temperature it exhibits the typical dynamics of glassforming liquids, i.e. the structural relaxation time τα grows strongly in a rather small temperature range. This is discussed with respect to the mode-coupling theory of the glass transition (MCT). For the simulation of bulk systems under shear, Lees-Edwards boundary conditions are applied. At constant shear rates γ˙ ≫ 1/τα the relevant time scale is given by 1/γ˙ and the system shows shear thinning behaviour. In order to understand the pronounced differences between a quiescent system and a system under shear, the response to a suddenly commencing or terminating shear flow is studied. After the switch-on of the shear field the shear stress shows an overshoot, marking the transition from elastic to plastic deformation, which is connected to a super-diffusive increase of the mean squared displacement. Since the average static structure only depends on the value of the shear stress, it does not discriminate between those two regimes. The distribution of local stresses, in contrast, becomes broader as soon as the system starts flowing. After a switch-off of the shear field, these additional fluctuations are responsible for the fast decay of stresses, which occurs on a time scale 1/γ˙ . The stress decay after a switch-off in the elastic regime, on the other hand, happens on the much larger time scale of structural relaxation τα. While stresses decrease to zero after a switch-off for temperatures above the glass transition, they decay to a finite value for lower temperatures. The obtained results are important for advancing new theoretical approaches in the framework of mode-coupling theory. Furthermore, they suggest new experimental investigations on colloidal systems. The colloid-polymer mixture is studied in the context of the behaviour near the critical point of phase separation. For the MD simulations a new effective model with soft interaction potentials is introduced and its phase diagram is presented. Here, mainly the equilibrium properties of this model are characterised. While the self-diffusion constants of colloids and polymers do not change strongly when the critical point is approached, critical slowing down of interdiffusion is observed. The order parameter fluctuations can be determined through the long-wavelength limit of static structure factors. For this strongly asymmetric mixture it is shown how the relevant structure factor can be extracted by a diagonalisation of a matrix that contains the partial static structure factors. By presenting first results of this model under shear it is demonstrated that it is suitable for non-equilibrium simulations as well.