Derivation and solution of effective-medium equations for bulk heterojunction organic solar cells

A drift-diffusion model for charge transport in an organic bulk-heterojunction solar cell, formed by conjoined acceptor and donor materials sandwiched between two electrodes, is formulated. The model accounts for (i) bulk photogeneration of excitons, (ii) exciton drift and recombination, (iii) exciton dissociation (into polarons) on the acceptor-donor interface, (iv) polaron recombination, (v) polaron dissociation into a free electron (in the acceptor) and a hole (in the donor), (vi) electron/hole transport and (vii) electron-hole recombination on the acceptor-donor interface. A finite element method is employed to solve the model in a cell with a highly convoluted acceptor/donor interface. The solutions show that, with physically realistic parameters, and in the power generating regime, the solution varies little on the scale of the microstructure. This motivates us to homogenise over the microstructure; a process that yields a far simpler one-dimensional effective medium model on the cell scale. The comparison between the solution of the full model and the effective medium (homogenised) model is very favourable for the applied voltages that are less than the built-in voltage (the power generating regime) but breaks down as the applied voltages increases above it. Furthermore, it is noted that the homogenisation technique provides a systematic way to relate effective medium modelling of bulk heterojunctions [19, 25, 36, 37, 42, 59] to a more fundamental approach that explicitly models the full microstructure [8, 38, 39, 58] and that it allows the parameters in the effective medium model to be derived in terms of the geometry of the microstructure. Finally, the effective medium model is used to investigate the effects of modifying the microstructure geometry, of a device with an interdigitated acceptor/donor interface, on its current-voltage curve

Geomorphological Dating of Scarps in Temperate Climate Using a Modified Diffusion Model

Geomorphological dating of a certain landform or geomorphological structure is based on the evolution of the landscape itself. In this context it is difficult to use common absolute dating techniques because they require datable material which is often not available. Additionally, these methods do not always date the time since the formation of these structures. For these reasons the application of geomorphological dating seems one possibility to date certain geomorphological features. The aim of this study was to relate present-day shapes of terrace risers to their ages. The time span since scarp formation ceased is reflected by the stage of degradation along with the rounding of the profile edges due to erosive processes. It is assumed that the average rate of downslope soil movement depends on the local slope and can be described by a diffusion equation. Furthermore, present-day scarps are often asymmetric and suggest a higher diffusivity at the base than at the toe of a slope. The diffusion equation has been modified and a linear approach with increasing diffusivity in downslope direction is suggested and assimilated in a model to obtain a better fit between observed and simulated profiles. In order to date the scarps, the model had to be calibrated. For this purpose, published diffusivities taken from literature were used as well as estimated diffusivities by fitting modelled profiles to observed ones of known age. Field data were collected in the area around Bonn, Germany, and in Valais, Switzerland. In addition, profiles from literature which have already been dated were digitized. For the general use of geomorphological dating, a Java tool was developed where, e.g., different initial profiles and diffusivities can be chosen. Results show a better match between simulated and observed profiles in comparison to models using a constant diffusivity. But a reliable calibration of the model finally failed which brings up the question whether the diffusion equation and the presented modification are really an adequate description of the geomorphological processes degrading a slope.Geomorphologische Datierung von Geländekanten im gemäßigten Klima unter Anwendung eines modifizierten Diffusionsmodels Die geomorphologische Datierung einer bestimmten Geländeform oder geomorphologischen Struktur basiert auf der Entwicklung der Landschaft selber. In diesem Zusammenhang ist es schwierig herkömmliche Datierungsmethoden anzuwenden, da diese in der Regel datierbares Material benötigen was oft nicht zur Verfügung steht. Außerdem datieren diese Methoden nicht immer die vergangene Zeit seit Entstehung der Strukturen. Aus diesen Gründen scheint die Anwendung der geomorphologischen Datierung eine Möglichkeit zu sein, bestimmte geomorphologische Merkmale zu datieren. Das Ziel dieser Arbeit war es, die heutige Form von Geländekanten in Form von Terrassenstufen und deren Alter in Bezug zu setzen. Der Degradationsgrad sowie die Abrundung der Profilober- und -unterkanten durch erosive Prozesse reflektieren dabei die Zeitspanne seit Beendigung der Terrassenformation. Es wird weiterhin angenommen, dass die durchschnittliche Erosionsrate hangabwärts von der lokalen Hangneigung abhängt und somit durch eine Diffusionsgleichung beschrieben werden kann. Außerdem sind rezente Kanten oft asymmetrisch und deuten eine höhere Diffusivität an der Unterkante als an der Oberkante an. Die Diffusionsgleichung wurde modifiziert und ein linearer Ansatz mit hangabwärts zunehmender Diffusivität angenommen. Dieser wurde in ein Modell integriert um eine bessere Anpassung zwischen natürlichen und simulierten Profilen zu erreichen. Zur Datierung von Geländekanten musste das Model kalibriert werden. Dazu wurden neben Daten aus der Literatur auch Diffusivitäten abgeschätzt indem simulierte Profile an natürliche Profile bekannten Alters angepasst wurden. Felddaten wurden in Gebieten rund um Bonn in Deutschland sowie im Wallis in der Schweiz erhoben. Zusätzlich wurden bereits datierte Profile aus der Literatur digitalisiert. Zur einfacheren Handhabung dieser Datierungsmethode wurde ein Java Tool entwickelt in dem z.B. das Initialprofil sowie die Diffusivität verändert werden kann. Die Ergebnisse zeigen eine bessere Übereinstimmung von simulierten und natürlichen Profilen als bei der Anwendung von Modellen mit konstanter Diffusivität. Aber eine verlässliche Kalibrierung des Modells konnte letztendlich nicht erreicht werden, so dass in Frage gestellt wird, ob die geomorphologischen Erosionsprozesse wirklich adäquat durch die hier vorgestellte modifizierte Diffusionsgleichung beschrieben werden

Electrokinetic concentration enrichment within a microfluidic device integrated with a hydrogel microplug

A simple and efficient technique for the concentration enrichment of charged species within a microfluidic device was developed. The functional component of the system is a hydrogel microplug photopolymerized inside the microfluidic channel. The fundamental properties of the nanoporous hydrogel microplug in modulating the electrokinetic transport during the concentration enrichment were investigated. The physicochemical properties of the hydrogel plug play a key role in determining the mode of concentration enrichment. A neutral hydrogel plug acts as a physical barrier to the electrophoretic transport of charged analytes resulting in size-based concentration enrichment. In contrast, an anionic hydrogel plug introduces concentration polarization effects, facilitating a size and charge-based concentration enrichment. The concentration polarization effects result in redistribution of the local electric field and subsequent lowering of the extent of concentration enrichment. In addition, an electroosmotic flow originating inside the pores of the anionic hydrogel manipulates the location of concentration enrichment. A theoretical model qualitatively consistent with the experimental observations is provided

Numerical modeling of multiphase flow and phase separation phenomena in the system H2O−NaCl with applications to magmatic hydrothermal systems at fast-spreading mid-ocean ridges

At mid-ocean ridges, the inner workings of hydrothermal systems are difficult to access directly and have to be investigated indirectly through geophysical measurements, petrological studies of disclosed oceanic crust brought to the surface, or geochemical analysis of venting hydrothermal fluids. Numerical models have become a beneficial tool to study hydrothermal fluid dynamics and allow researchers to better synthesize and understand observations and interpretations obtained by other scientific disciplines. At fast-spreading ridges, hydrothermal fluids are heated by shallow magmatism separating the fluids into a high-salinity liquid and a low-salinity vapor phase. In the first application of a self-developed hydrothermal simulator, I investigate the brine formation and mobilization in hydrothermal systems driven by a transient basal temperature boundary condition, which represents the axial magma lens. It was found that basal heating results in rapid phase segregation and the formation of a stable brine layer that thermally insulates the driving heat source of flow circulation. While this brine layer is stable under steady-state conditions, a reduction of the heat input mobilizes the brines. The second application studies dike intrusions for conditions found at the East Pacific Rise (EPR). At EPR 9°50.3’N, vent fluid salinity and temperature at individual vents in the axial summit trough have been repeatedly measured over a 25+ year-long period. After the 1991/92 diking event it was observed that low salinities are followed by higher salinities after a few years. The simulation analysis includes brine accumulation close to the dike as well as two-phase flow and the delayed brine upflow when the dike has cooled. In a comprehensive suite of model runs, I have identified key parameters, which control the vent salinity evolution, These are rock permeability and porosity plus the background fluid temperature and salinity

Nanoscale heat conduction with applications in nanoelectronics and thermoelectrics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references.When the device or structure characteristic length scales are comparable to the mean free path and wavelength of energy carriers (electrons, photons, phonons, and molecules) or the time of interest is on the same order as the carrier relaxation time, conventional heat transfer theory is no longer valid. Tremendous progress has been made in the past two decades to understand and characterize heat transfer in nanostructures. However most work in the last decade has focused on heat transfer in simple nanostructures, such as thin films, superlattices and nanowires. In reality, there is a demand to study transport process in complex nanostructures for engineering applications, such as heat transfer in nanoelectronic devices and the thermal conductivity in nanocomposites which consists of nanowires or nanoparticles embedded in a matrix material. Another class of problems which are rich in physics and might be explored for better design of both nanoelectronic devices and energy conversion materials and devices are coupled electron and phonon transport. Experimentally, most past work has been focused on thermal conductivity characterization of various nanostructures and very little has been done on the fundamental transport properties of energy carriers.(cont.) This thesis work contributes to the following aspects of heat transfer, nanoelectronics, and thermoelectrics. 1) Simulation tools are developed for transient phonon transport in multidimensional nanostructures and used to predict the size effect on the temperature rise surrounding a nanoscale heat source, which mimics the heating issue in nano-MOSFETs. 2) Semiconductor nanocomposites are proposed for highly efficient thermoelectric materials development where low thermal conductivity is a blessing for efficiency enhancement. Both the deterministic solution and Monte Carlo simulation of the phonon Boltzmann equation are established to study the size effect on the thermal conductivity of nanocomposites where nanoparticles and nanowires are embedded in a host material. 3) Explored the possibility of creating nonequilibrium conditions between electrons and phonons in thermoelectric materials using high energy flux coupling to electrons through surface plasmons, and thus to develop highly efficient thermoelectric devices.(cont.) 4) Established a sub-pico second optical pump-probe measurement system where a femtosecond laser is employed and explored the possibility of extracting phonon reflectivity at interfaces and the phonon relaxation time in a material, which are the two most fundamental phonon properties for nanoscale energy transport from the pump-probe measurements.by Ronggui Yang.Ph.D

Development of nanostencil lithography and its applications for plasmonics and vibrational biospectroscopy

Thesis (Ph.D.)--Boston UniversityDevelopment of low cost nanolithography tools for precisely creating a variety of nanostructure shapes and arrangements in a high-throughput fashion is crucial for next generation biophotonic technologies. Although existing lithography techniques offer tremendous design flexibility, they have major drawbacks such as low-throughput and fabrication complexity. In addition the demand for the systematic fabrication of sub-100 nm structures on flexible, stretchable, non-planar nanoelectronic/photonic systems and multi-functional materials has fueled the research for innovative fabrication methods in recent years. This thesis research investigates a novel lithography approach for fabrication of engineered plasmonic nanostructures and metamaterials operating at visible and infrared wavelengths: The technique is called Nanostencil Lithography (NSL) and relies on direct deposition of materials through nanoapertures on a stencil. NSL enables high throughput fabrication of engineered antenna arrays with optical qualities similar to the ones fabricated by standard electron beam lithography. Moreover, nanostencils can be reused multiple times to fabricate series of plasmonic nanoantenna arrays with identical optical responses enabling high throughput manufacturing. Using nanostencils, very precise nanostructures could be fabricated with 10 nm accuracy. Furthermore, this technique has flexibility and resolution to create complex plasmonic nanostructure arrays on the substrates that are difficult to work with e-beam and ion beam lithography tools. Combining plasmonics with polymeric materials, biocompatible surfaces or curvilinear and non-planar objects enable unique optical applications since they can preserve normal device operation under large strain. In this work, mechanically tunable flexible optical materials and spectroscopy probes integrated on fiber surfaces that could be used for a wide range of applications are demonstrated. Finally, the first application of NSL fabricated low cost infrared nanoantenna arrays for plasmonically enhanced vibrational biospectroscopy is presented. Detection of immunologically important protein monolayers with thickness as small as 3 nm, and antibody assays are demonstrated using nanoantenna arrays fabricated with reusable nanostencils. The results presented indicate that nanostencillithography is a promising method for reducing the nano manufacturing cost while enhancing the performance of biospectroscopy tools for biology and medicine. As a single step and low cost nanofabrication technique, NSL could facilitate the manufacturing of biophotonic technologies for real-world applications

Microphysiological Systems for the Evaluation of Biomaterials in Regenerative Therapies

[eng] The design of bioresponsive materials capable of stimulating the body’s innate regenerative potential is opening unprecedented possibilities to treat tissue and organ failure, which is one of the most important burdens of healthcare systems worldwide. Unfortunately, their development is hampered by the lack of adequate preclinical models, which are essential in the successful transition of a biomaterial to the clinical trials phase. Most of the experiments rely on animal models, which usually fail to predict the material interactions with the human body, as they are unable to recapitulate the complexities of our physiology. During the last decades, the advancements in the field of microtechnology have allowed to create advanced cell culture systems capable of replicating tissue and organ-level physiology by mimicking relevant conditions such as cell organization or microenvironmental cues. These platforms, known as microphysiological systems (MPS), have shown in different studies their great potential in predicting mechanisms of action, safety, and efficacy of different drugs, attracting a lot of attention from the pharmaceutical industry and regulatory agencies. However, few studies have explored the possibility of using microphysiological systems for the preclinical testing of biomaterials. The goal of this thesis is to fill this knowledge gap by developing microfluidic cell culture systems that allow to reliably predict the actual in vivo response of different materials. One of the proposed platforms is aimed at assessing the potential of a biomaterial to stimulate endothelial progenitor cell recruitment in a bone tissue microenvironment. This is a critical step in the neovascularization and bone regeneration process that has not been properly studied due to the lack of adequate models. The proposed device allowed to identify the role of calcium ions in stimulating the recruitment of rat endothelial progenitor cells (rEPC) to the site of injury, which is mediated by an increase in the release of osteopontin, a chemotactic and mitogenic protein produced by rat bone-marrow mesenchymal stromal cells (BM-rMSC). The platform was also used to evaluate a calcium-releasing biomaterial based on electrospun polylactic acid (PLA) fibers with calcium-phosphate (CaP) nanoparticles. The results show a significant increase in terms of rEPC recruitment and the release of osteopontin and other pro-angiogenic and inflammatory proteins by BM-rMSC with respect to a regular PLA control, which is in close agreement with previous experiments performed in a murine in vivo model. The other platform proposed in this thesis is aimed at providing a physiologically relevant model of cardiac tissue to study a myocardial ischemia-reperfusion injury. There are currently no reliable in vitro models to mimic this disease, making these contributions extremely relevant for cardiac regeneration studies. A first prototype of the platform based on the combination of aligned electrospun PLA fibers with a user-friendly electrical stimulation setup in a microfluidic cell culture platform produced a biomimetic cardiac tissue in 2D. This was confirmed by the high anisotropy of the tissue constructs, based on the co- culture of neonatal mouse cardiomyocytes with cardiac fibroblasts, as well as the upregulation of several key cardiac markers such as contractile and structural proteins. In order to make the model more physiologically relevant, a second device was developed to obtain human-derived 3D tissues. This platform is based on the self-assembling of primary cardiac fibroblasts (hCF) co-cultured with human pluripotent stem cell-derived cardiomyocytes (hPSC-CM) in a fibrin-based hydrogel around two microposts structures, which exert a passive mechanical tension that stimulates tissue maturation and cell alignment. We first performed a screening using 2D assays based on hPSC-CM monolayers to select the best environmental conditions to mimic an ischemia-reperfusion injury. We then characterized the response of the human- derived cardiac organoids to an ischemia-reperfusion injury, consisting of an 8 h culture period at 0 % oxygen in an ischemic solution that replicates the acidic and hyperkalemic conditions observed in vivo, followed by a refreshment with fully supplemented cell media and recovery of 21 % environmental oxygen concentrations. We observed a drastic increase in cell death by necrosis and apoptosis as well as a strong fibrotic response, characterized by an increase in hCF proliferation, differentiation towards myofibroblasts and collagen I deposition. Taken together, we believe that the platforms developed in this thesis constitute an extremely valuable and versatile tool to perform preclinical studies, offering a promising alternative to animal studies for the development of new biomaterials and drug discovery.[spa] El diseño de biomateriales capaces de estimular la capacidad innata del cuerpo de regenerarse está abriendo una oportunidad sin precedentes para el tratamiento y reemplazamiento de órganos y tejidos, una de las principales cargas en los sistemas de salud a nivel mundial. Desafortunadamente, el desarrollo de estas terapias se ve lastrado por la falta de modelos preclínicos adecuados, que son esenciales en la transición exitosa de un biomaterial a la aplicación clínica. La mayoría de estos experimentos se basan en el uso de modelos animales, que habitualmente fallan en la predicción de las interacciones que ocurren en el cuerpo humano, debido a las diferencias inherentes que existen en términos de fisiología. Durante las últimas décadas, los avances en el campo de la microtecnología han permitido crear plataformas de cultivo celular capaces de replicar elementos fisiológicos a nivel de tejidos y órganos denominados sistemas microfisiológicos. Estos sistemas han demostrado su gran utilidad en la predicción de mecanismos de acción, seguridad y eficacia de diferentes fármacos, atrayendo una gran atención por parte de las agencias regulatorias. Sin embargo, pocos estudios han explorado la posibilidad de usar este tipo de sistemas para la evaluación preclínica de biomateriales. El objetivo de esta tesis es realizar contribuciones en este campo mediante el desarrollo de sistemas microfluídicos de cultivo celular capaces de predecir la respuesta in vivo de diferentes materiales. En esta tesis se presentan principalmente dos modelos diferentes de sistemas microfisiológicos. El primer está relacionado con el reclutamiento de células progenitoras endoteliales en un entorno de regeneración ósea para el estudio de la vascularización de biomateriales, mientras que el segundo busca generar un modelo de tejido cardíaco fisiológicamente relevante para estudiar una lesión por isquemia-reperfusión y posibles terapias regenerativas

Experiments on the fragmentation of a buoyant liquid volume in another liquid

We present experiments on the instability and fragmentation of volumes of heavier liquid released into lighter immiscible liquids. We focus on the regime defined by small Ohnesorge numbers, density ratios of order one, and variable Weber numbers. The observed stages in the fragmentation process include deformation of the released fluid by either Rayleigh-Taylor instability or vortex ring roll-up and destabilization, formation of filamentary structures, capillary instability, and drop formation. At low and intermediate Weber numbers, a wide variety of fragmentation regimes is identified. Those regimes depend on early deformations, which mainly result from a competition between the growth of Rayleigh-Taylor instabilities and the roll-up of a vortex ring. At high Weber numbers, turbulent vortex ring formation is observed. We have adapted the standard theory of turbulent entrainment to buoyant vortex rings with initial momentum. We find consistency between this theory and our experiments, indicating that the concept of turbulent entrainment is valid for non-dispersed immiscible fluids at large Weber and Reynolds numbers