Sp-brane accelerating cosmologies

We investigate time dependent solutions (S-brane solutions) for product manifolds consisting of factor spaces where only one of them is non-Ricci-flat. Our model contains minimally coupled free scalar field as a matter source. We discuss a possibility of generating late time acceleration of the Universe. The analysis is performed in conformally related Brans-Dicke and Einstein frames. Dynamical behavior of our Universe is described by its scale factor. Since the scale factors of our Universe are described by different variables in both frames, they can have different dynamics. Indeed, we show that with our S-brane ansatz in the Brans-Dicke frame the stages of accelerating expansion exist for all types of the external space (flat, spherical and hyperbolic). However, applying the same ansatz for the metric in the Einstein frame, we find that a model with flat external space and hyperbolic compactification of the internal space is the only one with the stage of the accelerating expansion. Scalar field can prevent this acceleration. It is shown that the case of hyperbolic external space in Brans-Dicke frame is the only model which can satisfy experimental bounds for the fine structure constant variations. We obtain a class of models where a pare of dynamical internal spaces have fixed total volume. It results in fixed fine structure constant. However, these models are unstable and external space is non-accelerating.Comment: 17 pages, 4 figures, accepted in PR

Acceleration from M theory and Fine-tuning

The compactification of M theory with time dependent hyperbolic internal space gives an effective scalar field with exponential potential which provides a transient acceleration in Einstein frame in four dimensions. Ordinary matter and radiation are present in addition to the scalar field coming from compactification. We find that we have to fine-tune the initial conditions of the scalar field so that our Universe experiences acceleration now. During the evolution of our Universe, the volume of the internal space increases about 12 times. The time variation of the internal space results in a large time variation of the fine structure constant which violates the observational constraint on the variation of the fine structure constant. The large variation of the fine structure constant is a generic feature of transient acceleration models.Comment: 9 pages, 3 figures, use iopart, v2; references updated, accepted for publication in Class. Quantum Gra

Dynamical dark energy from extra dimensions

We consider multidimensional cosmological model with a higher-dimensional product manifold M = R x R^{d_0} x H^{d_1}/\Gamma where R^{d_0} is d_0-dimensional Ricci-flat external (our) space and H^{d_1}/\Gamma is d_1-dimensional compact hyperbolic internal space. M2-brane solution for this model has the stage of accelerating expansion of the external space. We apply this model to explain the late time acceleration of our Universe. Recent observational data (the Hubble parameter at the present time and the redshift when the deceleration parameter changes its sign) fix fully all free parameters of the model. As a result, we find that considered model has too big size of the internal space at the present time and variation of the effective four-dimensional fine structure constant strongly exceeds the observational limits.Comment: 5 pages, 3 figures, LaTex, a few remarks and reference adde

Models of G time variations in diverse dimensions

A review of different cosmological models in diverse dimensions leading to a relatively small time variation of the effective gravitational constant G is presented. Among them: 4-dimensional general scalar-tensor model, multidimensional vacuum model with two curved Einstein spaces, multidimensional model with multicomponent anisotropic "perfect fluid", S-brane model with scalar fields and two form field etc. It is shown that there exist different possible ways of explanation of relatively small time variation of the effective gravitational constant G compatible with present cosmological data (e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional cosmological models with different matter sources. The experimental bounds on G-dot may be satisfied ether in some restricted interval or for all allowed values of the synchronous time variable.Comment: 27 pages, Late

Modelling biocide release based on coating properties

Growth of micro-organisms on coated substrates is a common problem, since it reduces the performance of materials, in terms of durability as well as aesthetics. In order to prevent microbial growth biocides are frequently added to coatings. Unfortunately, early release of these biocides reduces the biocidal protection of these coatings. Furthermore, since biocides leach to the environment, their release rate is a key parameter determining the environmental impact of biocides. As a result of legislation, the use of biocides and the corresponding concentrations is restricted. Understanding how coating and biocide properties determine biocide release rate is crucial to understand durability, and enable new product development and evaluation of environmental impact. The present study aims to establish this connection via modelling of the release process. The proposed model is presented with an initial validation. The release is viewed as a combination of water diffusion, dissolution of the embedded biocide and release from the material. The key parameters necessary for the model are bulk parameters determined from experiments. The resulting model shows that release can be proportional to water exposure time or to its square root, depending on the dominating processes in a certain regime. In application conditions high variabilities exist in terms of materials and conditions, therefore the effect of environmental conditions is investigated by a statistical approach. The results show that the variability in the time of protection (the time during which biocides are still present in the coating) is high, due to the variabilities in the weather conditions. A variation of 1 year (95% interval) was seen on an average time of protection of 1.5 years. The model can be used to assess new approaches towards material development and helps underpinning product performance claims

Water transport in multilayer coatings

Coatings form the interface between structures and the environment in many application domains. They play a crucial role in providing protection, e.g. against corrosion, they form a barrier against an aggressive environment and they create the aesthetic appearance. To fulfill such functionalities, coatings are often composed of multiple layers, wherein each layer is designed to add a specific feature to the coating. The top coat serves as a barrier, whereas a base coat provides color and primers secure a good adhesion and corrosion protection. Nowadays, it is common that most objects are protected and decorated by coatings.Water penetration often forms a key factor in deterioration of a coating and its substrate. For example, water penetration may contribute to blister formation, leeching of water soluble components, hygroscopic stresses or freeze-thaw damages. Furthermore, water may act as a solvent or an electrolyte carrier, and may promote biodegradation. Consequently, a profound understanding of water penetration into (multilayer) coatings lays the foundation for improved product durability. Such a fundamental approach includes water penetration kinetics, water induced structural changes in coatings and clarification of the state of water in coatings. Most of the published studies are devoted to single layer coatings and only little is known about the water penetration in multilayer systems. Investigation of water transport in multilayer systems requires experimental techniques that measure transient water distributions and evaluate water-polymer interactions. The only technique combining these features is high resolution NMR (nuclear magnetic resonance) imaging. This thesis presents a study into water transport in multilayer coatings. This investigation was performed in two ways: experimentally with high resolution NMR imaging, combined with relaxometry and theoretically with an introduction and verification of a model for water transport. The thesis starts with an overview of knowledge about water in coatings in Chapter 2. This chapter focuses on water in polymers, since in organic coatings water mainly permeates in polymeric binder. The relation between the state of water in the polymer matrix and equilibrium sorption is addressed and the mechanisms of water transport kinetics are discussed. Further, the thesis proceeds with a description of NMR principles in Chapter 3. It discusses the NMR basics, principles of imaging and how information about molecular motions of the measured species is present in the NMR signal. The key results of the study are presented in the Chapters 4-8. First, water uptake was studied in two-layer systems consisting of hydrophilic base coats and hydrophobic top coats. The base coat consisted of acrylic, polyurethane and pigment particles and the formulation included a polymeric pigment dispersant. Chapter 4 presents the results of visualization of water transport in the two-layered systems on the basis of high resolution NMR imaging. The water transport rate in the studied systems appeared to be barrier limited. Mainly the base coat absorbed water, where it quickly redistributed. The swelling of the base coat showed to be linearly related with the absorbed water quantity. Further, Chapter 5 presents the results of a relaxation analysis of the NMR signal, which was introduced to identify water and polymeric phases in the base coat. Polymeric dispersant plasticization was observed. This polymeric dispersant appeared to be a major contributor to the water uptake capacity of the base coat. It was found with NMR diffusometry that high water mobility occurred under saturated conditions, whereas in partially saturated systems water mobility decreased and bonding to the polymer played a key role. In Chapter 6 a theoretical model was formulated for the transport in multilayer coatings, composed of a hydrophobic top coat and a hydrophilic base coat. This model basically assumed instantaneous equilibration of water in the base coat and Fickian water transport in the top coat. According to the model, the sorption isotherm of the base coat is the driving force of the process and the top coat permeability to water determines the timescale of transport. The asymmetry of the observed uptake/drying rates appeared to be due to the non-linear sorption isotherm of the base coat. Furthermore, top coats applied on base coats showed a higher permeability than free films and water uptake capacities of base coats decreased, when thicker top coats were applied. The ability of the model to predict water transport using the base coat sorption isotherm and the top coat permeability was validated experimentally. Practically, it is sufficient to measure the base coat sorption isotherm and the top coat permeability to understand water transport. This enables design of a simple and efficient procedure to test water resistance of this type of coatings. Such procedure will include only basic and cost-efficient equipment, like balances for gravimetric measurements of sorption isotherms and permeability. In the next step, the model was used for a theoretical investigation of the response of multilayer coatings to relative humidity fluctuations, which is presented in Chapter 7. The theoretical investigation showed that in most of the considered systems the response time is in the order of days for high humidities and has values of a few hours at low humidities. The response time is determined by the product of the timescale of penetration through the top coat and the differential solubility of the base coat. This means that most two-layer polymeric base coat/top coat systems with hydrophilic base coats are relatively inert to most humidity fluctuations, as the response in terms of water content will be damped. As accelerated and artificial weathering tests include only humidity fluctuations with typical timescales less than a day, this raises concerns if these tests can be compared to natural moisture exposure. Finally, the thesis addresses migration of plasticizer into a top coat. It was found that stress relaxation results in non-Fickian kinetics of the uptake process. The plasticizer, which is tributyl phosphate, causes a steep increase in the permeability of the film and may cause degradation of mechanical properties of the coating. This implies that, practically, that tributyl phosphate (the main component of common aerospace hydraulic fluids) makes multilayer protective coatings more permeable and causes significant stresses in the system

Modelling biocide release based on coating properties

\u3cp\u3eGrowth of micro-organisms on coated substrates is a common problem, since it reduces the performance of materials, in terms of durability as well as aesthetics. In order to prevent microbial growth biocides are frequently added to coatings. Unfortunately, early release of these biocides reduces the biocidal protection of these coatings. Furthermore, since biocides leach to the environment, their release rate is a key parameter determining the environmental impact of biocides. As a result of legislation, the use of biocides and the corresponding concentrations is restricted. Understanding how coating and biocide properties determine biocide release rate is crucial to understand durability, and enable new product development and evaluation of environmental impact. The present study aims to establish this connection via modelling of the release process. The proposed model is presented with an initial validation. The release is viewed as a combination of water diffusion, dissolution of the embedded biocide and release from the material. The key parameters necessary for the model are bulk parameters determined from experiments. The resulting model shows that release can be proportional to water exposure time or to its square root, depending on the dominating processes in a certain regime. In application conditions high variabilities exist in terms of materials and conditions, therefore the effect of environmental conditions is investigated by a statistical approach. The results show that the variability in the time of protection (the time during which biocides are still present in the coating) is high, due to the variabilities in the weather conditions. A variation of 1 year (95% interval) was seen on an average time of protection of 1.5 years. The model can be used to assess new approaches towards material development and helps underpinning product performance claims.\u3c/p\u3

Natural versus accelerated weathering : understanding water kinetics in bilayer coatings

Exposure to water is a key issue in the performance of multilayer coatings. It may take place in different forms, i.e. as rainfall, dew and humidity variation. Consequently, coatings will experience time-dependent water activity fluctuations. In industrial practice, coatings are subjected to artificial water activity fluctuations in weathering tests. Little is known about the connection between these tests and the reality experience by a coating. This article presents a theoretical investigation of the response of multilayer coatings to water activity fluctuations. This investigation is performed on the basis of a validated model for water transport in hydrophilic base coat/hydrophobic top coat systems. The study aims to understand how permeability and sorption properties determine the overall coating response to fluctuations. It is concluded that present accelerated weathering tests do not mimic natural weathering due to the response time of the considered systems, which are insensitive to rapid fluctuations