Measurement of action spectra of light-activated processes

We report on a new experimental technique suitable for measurement of light-activated processes, such as fluorophore transport. The usefulness of this technique is derived from its capacity to decouple the imaging and activation processes, allowing fluorescent imaging of fluorophore transport at a convenient activation wavelength. We demonstrate the efficiency of this new technique in determination of the action spectrum of the light mediated transport of rhodamine 123 into the parasitic protozoan Giardia duodenalis. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Relaxation processes in room temperature ionic liquids: The case of 1-butyl-3-methyl imidazolium hexafluorophosphate

A detailed investigation on the nature of the relaxation processes occurring in a typical room temperature ionic liquid (RTIL), namely, 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF6]), is reported. The study was conducted using both elastic and inelastic neutron scattering over a wide temperature range from 10 to 400 K, accessing the dynamic features of both the liquid and glassy amorphous states. In this study, the inelastic fixed energy scan technique has been applied for the first time to this class of materials. Using this technique, the existence of two relaxation processes below the glass transition and a further diffusive process occurring above the glass-liquid transition are observed. The low temperature processes are associated with methyl group rotation and butyl chain relaxation in the glassy state and have been modeled in terms of two Debye-like, Arrhenius activated processes. The high temperature process has been modeled in terms of a Kohlraush-Williams-Watts relaxation, with a distinct Vogel-Fulcher-Tamman temperature dependence. These results provide novel information that will be useful in rationalizing the observed structural and dynamical behavior of RTILs in the amorphous state.

Thermally activated dynamics in La2CuO4+delta: tilts of the CuO6 octahedra and interstitial O

An intense peak of the elastic energy loss versus temperature is found in La2CuO4 at 150 K (in the LTO phase), at a vibration frequency of similar to 280 Hz. From the dependence of the dissipation curve on frequency it is deduced that the relaxation rate has an activation enthalpy of 0.23 eV. The peak is ascribed to a thermally activated dynamics of the tilts of the CuO6 octahedra which form the La2CuO4 lattice, a fraction of which are supposed to be able to switch between energetically equivalent configurations. The peak is suppressed by interstitial O; this is explained by supposing that each interstitial atom can block several octahedra into a configuration that can accommodate the distortion due to its presence. Increasing the content of excess O, two new thermally activated processes develop, attributed to the hopping of interstitial O atoms which are isolated and which are paired or otherwise aggregated. The activation enthalpy for the diffusion of O at low values of 6 is 0.48 eV.

Theoretical investigation of thermal tweezers for parallel manipulation of atoms and nanoparticles on surfaces

A major focus of research in nanotechnology is the development of novel, high throughput techniques for fabrication of arbitrarily shaped surface nanostructures of sub 100 nm to atomic scale. A related pursuit is the development of simple and efficient means for parallel manipulation and redistribution of adsorbed atoms, molecules and nanoparticles on surfaces – adparticle manipulation. These techniques will be used for the manufacture of nanoscale surface supported functional devices in nanotechnologies such as quantum computing, molecular electronics and lab-on-achip, as well as for modifying surfaces to obtain novel optical, electronic, chemical, or mechanical properties. A favourable approach to formation of surface nanostructures is self-assembly. In self-assembly, nanostructures are grown by aggregation of individual adparticles that diffuse by thermally activated processes on the surface. The passive nature of this process means it is generally not suited to formation of arbitrarily shaped structures. The self-assembly of nanostructures at arbitrary positions has been demonstrated, though these have typically required a pre-patterning treatment of the surface using sophisticated techniques such as electron beam lithography. On the other hand, a parallel adparticle manipulation technique would be suited for directing the selfassembly process to occur at arbitrary positions, without the need for pre-patterning the surface. There is at present a lack of techniques for parallel manipulation and redistribution of adparticles to arbitrary positions on the surface. This is an issue that needs to be addressed since these techniques can play an important role in nanotechnology. In this thesis, we propose such a technique – thermal tweezers. In thermal tweezers, adparticles are redistributed by localised heating of the surface. This locally enhances surface diffusion of adparticles so that they rapidly diffuse away from the heated regions. Using this technique, the redistribution of adparticles to form a desired pattern is achieved by heating the surface at specific regions. In this project, we have focussed on the holographic implementation of this approach, where the surface is heated by holographic patterns of interfering pulsed laser beams. This implementation is suitable for the formation of arbitrarily shaped structures; the only condition is that the shape can be produced by holographic means. In the simplest case, the laser pulses are linearly polarised and intersect to form an interference pattern that is a modulation of intensity along a single direction. Strong optical absorption at the intensity maxima of the interference pattern results in approximately a sinusoidal variation of the surface temperature along one direction. The main aim of this research project is to investigate the feasibility of the holographic implementation of thermal tweezers as an adparticle manipulation technique. Firstly, we investigate theoretically the surface diffusion of adparticles in the presence of sinusoidal modulation of the surface temperature. Very strong redistribution of adparticles is predicted when there is strong interaction between the adparticle and the surface, and the amplitude of the temperature modulation is ~100 K. We have proposed a thin metallic film deposited on a glass substrate heated by interfering laser beams (optical wavelengths) as a means of generating very large amplitude of surface temperature modulation. Indeed, we predict theoretically by numerical solution of the thermal conduction equation that amplitude of the temperature modulation on the metallic film can be much greater than 100 K when heated by nanosecond pulses with an energy ~1 mJ. The formation of surface nanostructures of less than 100 nm in width is predicted at optical wavelengths in this implementation of thermal tweezers. Furthermore, we propose a simple extension to this technique where spatial phase shift of the temperature modulation effectively doubles or triples the resolution. At the same time, increased resolution is predicted by reducing the wavelength of the laser pulses. In addition, we present two distinctly different, computationally efficient numerical approaches for theoretical investigation of surface diffusion of interacting adparticles – the Monte Carlo Interaction Method (MCIM) and the random potential well method (RPWM). Using each of these approaches we have investigated thermal tweezers for redistribution of both strongly and weakly interacting adparticles. We have predicted that strong interactions between adparticles can increase the effectiveness of thermal tweezers, by demonstrating practically complete adparticle redistribution into the low temperature regions of the surface. This is promising from the point of view of thermal tweezers applied to directed self-assembly of nanostructures. Finally, we present a new and more efficient numerical approach to theoretical investigation of thermal tweezers of non-interacting adparticles. In this approach, the local diffusion coefficient is determined from solution of the Fokker-Planck equation. The diffusion equation is then solved numerically using the finite volume method (FVM) to directly obtain the probability density of adparticle position. We compare predictions of this approach to those of the Ermak algorithm solution of the Langevin equation, and relatively good agreement is shown at intermediate and high friction. In the low friction regime, we predict and investigate the phenomenon of ‘optimal’ friction and describe its occurrence due to very long jumps of adparticles as they diffuse from the hot regions of the surface. Future research directions, both theoretical and experimental are also discussed.

pH sensing by single and multi-layer hydrogel coated Fiber Bragg Grating

In this paper we show a novel chemo-mechanical-optical sensing mechanism in single and multi-layer hydrogel coated Fiber Bragg Grating (FBG) and demonstrate specific application in pH activated processes. The sensing device is based on the ionizable monomers inside the hydrogel which reversibly dissociates as a function of the pH and consequently resulting in osmotic pressure difference between the gel and the solution. This pressure gradient causes the hydrogel to deform which in turn induces secondary strain on the FBG sensor resulting in shift in the Bragg wavelength. We also report on the sensitivity factor of single and multilayer hydrogel coated FBG at various different pH.

A influência da salinidade nos processos de tratamento de efluentes por lodos ativados.

O processo de tratamento de efluentes por lodos ativados é um dos processos de tratamento mais difundido em todo o mundo, devido principalmente a qualidade do efluente obtido. Entretanto, trata-se de um processo biológico dependente da atividade bacteriana para a estabilização da matéria orgânica proveniente dos esgotos e com isso, se faz necessária a manutenção das condições ideais para a sobrevivência e proliferação das bactérias e dos outros microrganismos envolvidos neste processo. Efluentes salinos causam um grande distúrbio na atividade celular dos microrganismos presentes neste processo. A análise biológica do lodo e a taxa de consumo de oxigênio são testes rápidos, práticos, baratos e sem geração de resíduos químicos e que foram adotados neste trabalho para acompanhar a eficiência do processo. Esses dois parâmetros são amplamente utilizados em pesquisas, porém ainda é sub-utilizado para efetivo monitoramento de Estações de Tratamento de Esgotos. Este presente trabalho investigou a influência da salinidade nos processos de lodos ativados através de parâmetros que levam em consideração a atividade metabólica dos organismos aeróbios e através do monitoramento da comunidade de protozoários indicadores biológicos do lodo ativado, os testes de respirometria e os testes da análise biológica do lodo, respectivamente. Os resultados da atividade metabólica são apresentados em forma de gráficos, em termos de taxa de consumo de oxigênio específico e porcentagem de inibição. Os resultados da qualidade biológica são apresentados em números de 0 a 10 de acordo com o Índice de Madoni (1994). Os resultados demonstraram a imediata intoxicação dos lodos ativados em concentrações de sal a partir da concentração mínima utilizada neste trabalho, que foi de 5 gL de NaCl. Para entender melhor o processo de intoxicação foi realizado experimentos de 96 horas de monitoramento após o choque de cloreto de sódio nos reatores com lodos ativados, os resultados não demonstraram melhoria no processo. Os resultados sugerem que os testes de taxa de consumo de oxigênio em consonância com os testes da qualidade biológica são eficientes e complementares para a avaliação da influência da salinidade no processo de lodos ativados.

Plane-strain discrete dislocation plasticity with climb-assisted glide motion of dislocations

A small-strain two-dimensional discrete dislocation plasticity (DDP) framework is developed wherein dislocation motion is caused by climb-assisted glide. The climb motion of the dislocations is assumed to be governed by a drag-type relation similar to the glide-only motion of dislocations: such a relation is valid when vacancy kinetics is either diffusion limited or sink limited. The DDP framework is employed to predict the effect of dislocation climb on the uniaxial tensile and pure bending response of single crystals. Under uniaxial tensile loading conditions, the ability of dislocations to bypass obstacles by climb results in a reduced dislocation density over a wide range of specimen sizes in the climb-assisted glide case compared to when dislocation motion is only by glide. A consequence is that, at least in a single slip situation, size effects due to dislocation starvation are reduced. By contrast, under pure bending loading conditions, the dislocation density is unaffected by dislocation climb as geometrically necessary dislocations (GNDs) dominate. However, climb enables the dislocations to arrange themselves into lower energy configurations which significantly reduces the predicted bending size effect as well as the amount of reverse plasticity observed during unloading. The results indicate that the intrinsic plasticity material length scale associated with GNDs is strongly affected by thermally activated processes and will be a function of temperature. © 2013 IOP Publishing Ltd.

Influence of the driving mechanism on the response of systems with athermal dynamics: The example of the random-field Ising model

We investigate the influence of the driving mechanism on the hysteretic response of systems with athermal dynamics. In the framework of local mean-field theory at finite temperature (but neglecting thermally activated processes), we compare the rate-independent hysteresis loops obtained in the random field Ising model when controlling either the external magnetic field H or the extensive magnetization M. Two distinct behaviors are observed, depending on disorder strength. At large disorder, the H-driven and M-driven protocols yield identical hysteresis loops in the thermodynamic limit. At low disorder, when the H-driven magnetization curve is discontinuous (due to the presence of a macroscopic avalanche), the M-driven loop is reentrant while the induced field exhibits strong intermittent fluctuations and is only weakly self-averaging. The relevance of these results to the experimental observations in ferromagnetic materials, shape memory alloys, and other disordered systems is discussed.

Estudo comparativo da queima rápida com a queima tradicional nas propriedades de materiais cerâmicos de base argilos

Dutra, R. P. S.; Varela,M. L.; Nascimento, R. M. ; Gomes, U. U. ; Martinelli1, A. E. ; Paskocimas, C. A. Estudo comparativo da queima rápida com a queima tradicional nas propriedades de materiais cerâmicos de base argilosa. Cerâmica [online]. 2009, vol.55, n.333, pp. 100-105. ISSN 0366-6913. doi:Disponivem em: . Acesso em: 04 out. 2010.

Temperature dependence of photoinduced birefringence in polystyrene doped with Disperse Red-1

A theoretical model on the basis of the free-volume concept is presented explaining the temperature dependence of photoinduced birefringence in polystyrene films that contain Disperse Red-1. Birefringence increases with temperature up to 180 K as the free volume for isomerization increases, and then decreases as thermally activated processes dominate. The fast process of birefringence decay has a time constant that increases with temperature at low temperatures, due to the change kin photoisomerization.

The Fokker-Planck equation for a bistable potential

The Fokker-Planck equation is studied through its relation to a Schrodinger-type equation. The advantage of this combination is that we can construct the probability distribution of the Fokker-Planck equation by using well-known solutions of the Schrodinger equation. By making use of such a combination, we present the solution of the Fokker-Planck equation for a bistable potential related to a double oscillator. Thus, we can observe the temporal evolution of the system describing its dynamic properties such as the time tau to overcome the barrier. By calculating the rates k = 1/tau as a function of the inverse scaled temperature 1/D, where D is the diffusion coefficient, we compare the aspect of the curve k x 1/D, with the ones obtained from other studies related to four different kinds of activated process. We notice that there are similarities in some ranges of the scaled temperatures, where the different processes follow the Arrhenius behavior. We propose that the type of bistable potential used in this study may be used, qualitatively, as a simple model, whose rates share common features with the rates of some single rate-limited thermally activated processes. (C) 2014 Elsevier B.V. All rights reserved.

Experimentelle Untersuchung von Phasenübergängen im kolloidalen Modellsystem harter Kugeln

Die vorliegende Arbeit behandelt den fluid-kristallinen Phasenübergang sowie den Glasübergang anhand von kolloidalen Hart-Kugel(HK)-Modellsystemen. Die Untersuchungen erfolgen dabei im Wesentlichen mit unterschiedlichen Lichtstreumethoden und daher im reziproken Raum. rnDie Analyse der Kristallisationskinetik zeigt, dass es bei der Kristallisation zu signifikanten Abweichungen vom Bild der klassischen Nukleationstheorie (CNT) kommt. Diese geht von einem einstufigen Nukleationsprozess aus, wohingegen bei den hier durchgeführten Experimenten ein mehrstufiger Prozess beobachtet wird. Vor der eigentlichen Kristallisation kommt es zunächst zur Nukleation einer metastabilen Zwischenphase, sogenannter Precursor. In einer zweiten Stufe erfolgt innerhalb der Precursor die eigentliche Nukleation der Kristallite. rnDurch weitere Analyse und den Vergleich des Kristallisations- und Verglasungsszenarios konnte das Konzept der Precursornukleation auf den Vorgang der Verglasung erweitert werden. Während die Kristallnukleation oberhalb des Glasübergangspunktes zum Erliegen kommt, bleibt der Prozess der Precursornukleation auch bei verglasenden Proben erhalten. Ein Glas erstarrt somit in einem amorphen Zustand mit lokalen Precursorstrukturen. Die Korrelation der gemessenen zeitlichen Entwicklung der strukturellen sowie der dynamischen Eigenschaften zeigt darüber hinaus, dass das bisher unverstandene Ageing-Phänomen von HK-Gläsern mit der Nukleation von Precursorn zusammenhängt.rnEin solches mehrstufiges Szenario wurde bereits in früheren Veröffentlichungen beobachtet. Die im Rahmen dieser Arbeit durchgeführten Messungen ermöglichten erstmals die Bestimmung von Kristallnukleationsratendichten (Kristall-NRD) und Ratendichten für die Precursornukleation bis über den Glasübergangspunkt hinaus. Die Kristall-NRD bestätigen die Resultate aus anderen experimentellen Arbeiten. Die weiteren Analysen der Kristall-NRD belegen, dass die fluid-kristalline Grenzflächenspannung bei der Nukleation entgegen den Annahmen der CNT nicht konstant ist, sondern mit ansteigendem Volumenbruch linear zunimmt. Die Erweiterung der CNT um eine linear zunehmende Grenzflächenspannung ermöglichte eine quantitative Beschreibung der gemessenen Kristall- sowie der Precursor-NRD, was den Schluss zulässt, dass es sich in beiden Fällen um einen Boltzmann-aktivierten Prozess handelt. rnUm die beobachteten Abweichungen des Nukleationsprozesses vom Bild der CNT näher zu untersuchen, wurden die kollektiven Partikeldynamiken in stabilen Fluiden und metastabilen Schmelzen analysiert. Im klassischen Bild wird angenommen, dass die kollektive Partikeldynamik beim Vorgang der Nukleation keine Rolle spielt. Anhand der Resultate zeigen sich Abweichungen in der Dynamik stabiler Fluide und metastabiler Schmelzen. Während die kollektive Partikeldynamik in der stabilen Schmelze von der Struktur entkoppelt ist, tritt oberhalb des Phasenübergangspunktes eine Kopplung von Struktur und Dynamik auf. Dabei treten die Abweichungen zunächst in der Umgebung des ersten Strukturfaktormaximums und somit bei den am stärksten besetzten Moden auf. Mit steigender Unterkühlung nehmen die Anzahl der abweichenden Moden sowie die Stärke der Abweichungen zu. Dieses Phänomen könnte einen signifikanten Einfluss auf den Nukleationsprozess und somit auf die Kristallisationskinetik haben. Die Analyse der Dynamik im stabilen Fluid zeigt darüber hinaus Hinweise auf eine Singularität bei Annäherung an den fluid-kristallinen Phasenübergangspunkt.rnDes Weiteren wurden im Rahmen der vorliegenden Arbeit erstmals Ratendichten für die heterogene Nukleation eines HK-Systems an einer flachen Wand mittels statischer Lichtstreuung (SLS) bestimmt. Die Ergebnisse der Messung zeigen, dass die Nukleationsbarriere der heterogenen Nukleation annähernd Null ist und folglich eine vollständige Benetzung der Wand mit einer kristallinen Monolage vorliegt. Die Erweiterung der Untersuchungen auf gekrümmte Oberflächen in Form von sphärischen Partikeln (Seeds) stellt die erste experimentelle Arbeit dar, die den Einfluss eines Ensembles von Seeds auf die Kristallisationskinetik in HK-Systemen untersucht. Die Kristallisationskinetik und die Mikrostruktur werden abhängig von Größe und Anzahldichte der Seed-Partikel signifikant beeinflusst. In Übereinstimmung mit konfokalmikroskopischen Experimenten und Simulationen spielt dabei das Radienverhältnis der Majoritäts- zur Minoritätskomponente eine entscheidende Rolle.

Temperature-Dependent Defect Dynamics in the Network Glass SiO2

We investigate the long time dynamics of a strong glass former, SiO2, below the glass transition temperature by averaging single-particle trajectories over time windows which comprise roughly 100 particle oscillations. The structure on this coarse-grained time scale is very well defined in terms of coordination numbers, allowing us to identify ill-coordinated atoms, which are called defects in the following. The most numerous defects are O-O neighbors, whose lifetimes are comparable to the equilibration time at low temperature. On the other hand, SiO and OSi defects are very rare and short lived. The lifetime of defects is found to be strongly temperature dependent, consistent with activated processes. Single-particle jumps give rise to local structural rearrangements. We show that in SiO2 these structural rearrangements are coupled to the creation or annihilation of defects, giving rise to very strong correlations of jumping atoms and defects.

Estudo comparativo da queima rápida com a queima tradicional nas propriedades de materiais cerâmicos de base argilos

Dutra, R. P. S.; Varela,M. L.; Nascimento, R. M. ; Gomes, U. U. ; Martinelli1, A. E. ; Paskocimas, C. A. Estudo comparativo da queima rápida com a queima tradicional nas propriedades de materiais cerâmicos de base argilosa. Cerâmica [online]. 2009, vol.55, n.333, pp. 100-105. ISSN 0366-6913. doi:Disponivem em: . Acesso em: 04 out. 2010.