Site-specific modelling of turbulent fluxes on the Tibetan Plateau
- The Tibetan Plateau attracts attention in recent decades due to its influence on the East-Asian Monsoon and regional hydrology. Therefore estimates of the regional energy and water balance have come into the focus, utilising remote sensing and regional model approaches, but such attempts require surface-specific flux data of high quality for validation. Eddy-covariance measurements are qualified for this task, but these are scarce on the Tibetan Plateau, incomplete due to quality filtering and potentially biased due to the well-known closure gap of the observed energy balance as well as small-scale heterogeneity. This thesis is related to the infrastructural EU project CEOP-AEGIS, aiming at a standardised processing of eddy-covariance data – including correction of the energy balance closure and gap-filling – on the Tibetan Plateau.
In a pre-analysis step, particular issues about data quality of turbulent fluxes (sensible heat flux and latent heat flux/evapotranspiration) at Tibetan Plateau sites have been addressed. One of them is the degradation of data quality due to the frequent occurrence of near-ground free convective conditions. Another issue arises from coordinate rotation for non-omnidirectional sonic anemometer, which requires a careful handling. In consequence, a sector-wise planar-fit is recommended, disregarding the sector influenced by the anemometer's mounting structure. This can reduce occurrences of invalid momentum flux data, whilst no effect on scalar fluxes can be seen.
As a main topic, this thesis investigates the application of process-based modelling to estimate turbulent flux exchange between the surface and the atmosphere for typical surface types on the Tibetan Plateau. Therefore a case study has been carried out at Nam Co, Tibetan Plateau. Turbulent flux measurements over dry and wet grassland as well as over a shallow lake have been conducted during the summer monsoon season of 2009, and modelled with the land surface scheme SEWAB and a hydrodynamic multilayer model for the lake. Adaptations were implemented to the land surface scheme with regard to the special conditions on the Tibetan Plateau, such as extreme diurnal variation of surface temperature and variation in soil moisture, further called TP version. The analysis includes a consequent model comparison with eddy-covariance data, using model parameters derived independently rather than applying optimisation strategies. Specific attention has been devoted to the impact of observed energy balance closure and its correction, establishing a new correction method according to the Buoyancy flux.
The land surface model reasonably represented the dry and the wet grassland site by only setting the site-specific model parameters, and the TP version performed overall better than the original version, while laboratory measurements of soil parameters failed to improve model performance in comparison to standard parameter values. Soil temperature and moisture measurements as well as field based knowledge of the soil type have been identified as minimum requirements for model parameter acquisition. Lake surface fluxes have been modelled reliably, the lake depth has been taken into account. These results can be transferred to any lake on the Tibetan Plateau given the required forcing data including a representative lake surface temperature.
The choice of the surface model and the selection of the energy balance closure correction method are inter-related problems. The correction partitions the balance residual to the sensible and latent heat flux. This can be typically done according to the Bowen ratio, or according to the presented new method which attributes a larger fraction to the sensible heat flux. Testing both methods leads to partly ambiguous model performance, especially with respect to the used parameter sets. It clearly leads to shifts in model bias, while the R² metric suggests higher model compatibility to the Bowen ratio correction. The latter agrees with previous findings with respect to SEWAB modelling, but is in contradiction with recent experimental findings, attributing the closure gap to secondary circulations, driven by buoyancy. Future research on model structure should account for such processes.
As expected, the flux measurements showed distinct differences between the investigated land use types in magnitude and dynamics. The used models were able to resolve these differences in general with contrasts between surface types exceeding model errors. This must be considered when validating regional flux estimates with eddy-covariance data from the dry Nam Co station. The findings from this thesis provide the basis to process eddy-covariance data on the required level as described above.
Optimal sensor placement for linear systems
- The aim of sensor placement is to observe the state of a dynamical system while using only a small part of the available output information. Thus, the observer does not need sensors at every possible node of the system. We use sensor placement because it is not practical for large-scale networks, such as power grids, to place sensors at each node. With an optimal sensor placement we obtain a subset of sensors which minimizes the observer error in comparison to any other subset of the same size. This means we generate an optimal observation with the given number of sensors.
We compute the observer error, for the linear dynamical systems we consider, with the H2-norm of the observer error system. In this approach, we optimize both the subset of selected sensors and the observer gain matrix in parallel. The optimization problem is non-convex both in a constraint, which bounds the H2-norm, as well as in the objective function which uses a l0-norm to count the used sensors. To obtain a semidefinite program, we first relax the l0-norm by an
iterative reweighted l1-norm. Second, we use a reformulation of the H2-norm with linear matrix inequalities to replace an occuring bilinear and therefore non-convex term.
We use this computationally efficient formulation of the sensor placement problem to derive three algorithms. Furthermore, existing algorithms, which do not use the convex reformulation of the optimization problem, were implemented. The algorithms are compared extensively relating to execution time, performance of the chosen sensors, and the applicability on a practical problem. The practical problem is a model of a high-voltage power grid with the aim to measure the phase angles and the frequencies at every node. The result of the comparison is that a algorithm with a greedy approach solves the optimization problem fast and usually with a good solution. However, this algorithm is problematic because the shortsighted greedy approach cannot exclude that a worst case solution is generated. The best results in general were produced by a novel approach made in this thesis. This novel algorithm iteratively solves the relaxed optimization problem and finds near-optimal sensor subsets.
Organic Electronics by Self-Assembly
- This thesis deals with the synthesis of new materials and their applications in n type field-effect transistors in order to improve device performance by using simple fabrication procedures at low temperatures.
The first approach deals with field effect transistors based on zinc oxide nanoparticles. Stable dispersions of zinc oxide nanoparticles allow the fabrication of zinc oxide transistors by solution processes. Despite the high intrinsic electron mobilities of zinc oxide, a large surface to volume ratio of nanoparticles often prevents their use as active material in field-effect transistors. Dangling zinc bonds are present at the surface, which act as electron donor, leading to increased conductivity. Switching the transistor off becomes challenging. The use of a tailored perylene bisimide, chemically linked to pyrrolidone groups, allowed the passivation of the dangling zinc bonds at the particle surface. Mixing of this perylene bisimide into nanoparticulate zinc oxide dispersions allowed transistor fabrication by spin-coating at low temperatures. On/off-ratios of the resulting transistors could be enhanced by 3 orders of magnitude to 10^3, so that zinc oxide nanoparticles turn into a semiconductor.
The second approach represents the main part of this thesis focusing on n type self assembled monolayer field-effect transistors (SAMFETs). It is known that approximately 90% of charge transport in field-effect transistors is managed just by an ultra-thin layer close to the dielectric. Due to the absence of bulk current, on/off ratios in SAMFETs are enhanced without disadvantages to charge mobility or threshold voltage. The molecules for SAMFET applications typically consist of a semiconducting core, an endcapper on one side, and a reactive group on the other side, which is fixed to the core via a spacer. In this thesis, the well known perylene bisimides were chosen as semiconducting core. A branched alkyl tail acted as endcapper, whereas a linear C11 alkyl tail takes over the spacer part. As reactive group a phosphonic acid was chosen, which enables covalent fixation to aluminium oxide, a common dielectric for transistor applications. SAMFETs were fabricated by submerging transistor substrates into a dilute solution of the active molecule. During the immersion, the perylene bisimides reacts spontaneously to the aluminium oxide dielectric, forming a monomolecular layer. X-ray photoelectron spectroscopy (XPS) revealed a dense, homogeneous, and smooth monolayer on top of aluminium oxide. X-ray reflectivity (XRR) measurements confirmed the expected three regions, endcapper, semiconducting core, and spacer, perpendicularly ordered to the surface. In plane order was investigated by grazing incidence measurements (GIXD), which resulted in a nano-crystalline layer. SAMFETs showed bulk like electron mobilities of 10^-3 cm2/Vs. High on/off-ratios up to 10^5 and low threshold voltages were achieved. SAMFETs with channel length up to 100 micrometer were measured for the first time. The fact that all measured transistors, short channel as well as long channel, were working, indicated a high degree of reproducibility. Furthermore, by combining n-type and p type SAMFETs, the first CMOS bias inverter, solely based on SAMFETs, with large gain values up to 15, was realized.
To further improve SAMFET performance, the branched alkyl tail was replaced by a short linear fluorinated alkyl tail, with the intention to increase the surface coverage of the chromophores by a more slender design of the molecule. SAMFETs were fabricated with the same simple method as described above. XPS measurements showed a complete coverage of an organic layer with a thickness matching perfectly the simulated length of the molecule. In contrast to the previous SAMFETs, the phosphorous was located mainly at the aluminium oxide surface and not, as before, throughout the organic layer. The fluorine atoms were detected at the top of the layer. Both observations are indicative for higher order of the monolayer. XRR measurements gave a consistent structure of the layer. A tree layer structure was found in which the electron density of the outer layer was enhanced, due to the presence of electron rich fluorine atoms. The thicknesses of all three layers were in good agreement to the calculated distances. Furthermore, GIXD studies revealed an amorphous SAM on top of aluminium oxide, optimal for charge transport without disturbing grain boundaries.
The new SAMFETs were also highly reproducible and showed electron mobilities on the order of 10^-3 cm2/Vs, low onset voltages, and high on/off-ratios in the order of 10^6. Besides SAMFETs on common non-flexible silicon substrates, comparable n-type SAMFETs were also fabricated on polymer based substrates for the first time. Furthermore, a unipolar bias inverter was built, paving the way towards flexible organic electronics by self-assembly.
In summary, all three publications of this thesis deal with the synthesis of semiconducting perylene bisimides and their implementation in n-type field-effect transistors. Reliable transistors and the first integrated CMOS-like circuits based solely on SAMFETs with high performances were achieved, made by simplest solution processes at low temperatures.
Topics in health economics and social policy - abstracts of the 7th DIBOGS-Workshop, Düsseldorf 2012
Toufic M. El Masri
- Der DIBOGS e.V. ist aus dem Duisburg-Ilmenau-Bayreuther Oberseminar zur Gesund-heitsökonomik und Sozialpolitik (DIBOGS) hervorgegangen. Der Verein hat sich zum Ziel gesetzt, den wissenschaftlichen Nachwuchs auf dem Gebiet der Wirtschaftswissenschaften in den Forschungsfeldern Gesundheitsökonomik, Gesundheitspolitik und Sozialpolitik zu fördern. Zu den Aktivitäten des Vereins zählt u.a. ein gesundheitsökonomischer Workshop, der erstmalig im Jahr 2005 ausgerichtet wurde und der sich zentral an gesundheitsökonomische Nachwuchswissenschaftler richtet. Ziel ist es, einen möglichst regen Erfahrungsaustausch herbeizuführen und insofern Unterstützung bei der Vorbereitung und Durchführung von Dissertations- oder Habilitationsvorhaben, sowie sonstigen wissenschaftlichen Projekten oder Publikationen zu bieten.
Der Workshop stellt nicht die Präsentation an sich, sondern den Austausch über das jeweilige Thema in den Mittelpunkt. Alle Papiere gehen den Teilnehmern im Vorfeld zu. Während des Workshops stehen für jedes Papier 45 Minuten zur Verfügung. Ein Koreferent setzt sich in-tensiv mit dem Papier auseinander, darauf schließt sich eine Diskussion unter den Teilnehmern an. Mittlerweile (seit 2006) gibt es zu jedem Workshop einen Sammelband, der die (fach)politische Öffentlichkeit über diskutierte Themen informieren soll.
Der vorliegende Band „DIBOGS-Beiträge zur Gesundheitsökonomik und Sozialpolitik“ enthält - im Gegensatz zu den Überblicksartikeln aus den Vorjahren - die Zusammenfassungen/Abstracts ausgewählter Beiträge des siebten Duisburg-Ilmenau-Bayreuther Oberseminars zur Gesundheitsökonomik und Sozialpolitik (DIBOGS), das am 23. November 2012 in Düsseldorf stattgefunden hat. Die Zusammenfassung soll einen ersten Überblick über die diskutierten Papiere geben, die unter den ggf. jeweils angegeben Links auch im Detail gelesen werden können. Weitere Informationen zum Sammelband sowie zur Teilnahme am Workshop entnehmen Sie bitte der Webseite www.dibogs.biz.
Free convection and turbulent fluxes over complex terrain
- The impact of complex terrain on the land-atmosphere exchange is investigated in this thesis. Here, free convection, a very effective vertical transport mechanism as turbulence is predominantly driven by buoyant forces, is explicitly addressed. Recently, it was shown for certain situations over complex terrain that free convective injections of surface layer air masses into the atmospheric boundary layer (ABL) can alter the ABL properties significantly. This study aims at the general identification and description of such situations of near-ground free convection conditions (FCCs) over complex terrain. For this purpose, data obtained during the COPS (Convective and Orographically induced Precipitation Study) field campaign in summer 2007 were used. Within this project, several surface flux measurement stations were installed, mainly in valleys and on mountaintops of the Black Forest, southwestern Germany. Turbulent fluxes were calculated with the eddy-covariance (EC) method and were used to detect FCCs with the help of a stability parameter. The flux measurements were further combined with ABL profiling measurements (Sodar/RASS) and a large-eddy simulation (LES) model in order to investigate the impact of FCCs on ABL properties. The effect of complex terrain on the energy balance closure and on spatial and temporal flux differences was also studied with these flux data.
FCCs were detected on about 25% of the days during the three month COPS experiment. In situations of weak synoptic forcing, thermally driven orographic (e.g. valley winds) or local wind systems developed over the complex terrain due to heating differences. During the adaption of these wind systems to changing heating differences (e.g. during the reversal of the valley wind from down- to up-valley winds in the morning), the horizontal wind vanished. If, at the same time, the buoyancy flux was positive and enhanced, buoyant forces exceeded the usually prevailing shear forces in the surface layer and FCCs were detected. Moreover, it was demonstrated that FCCs are not restricted to the COPS region. Also, a data set of Nam Co station on the Tibetan Plateau showed FCCs during the reversal of a thermally driven land-lake breeze. However, at this high-altitude site, FCCs were more often detected in the afternoon compared to the COPS region due to the frequent change of heating differences during cloud cover periods.
The Sodar/RASS as well as the LES model showed the presence of coherent updraft
structures in the developing early-morning convective boundary layer (CBL) in the Kinzig valley (Black Forest) during FCCs. Spectral analysis of the EC data in these situations indicated the existence of large-eddy turbulent scales – typical for thermal updrafts in the CBL – already close to the ground. An ensemble and time mean analysis of the simulated flow field in the valley further confirmed that the Sodar/RASS was located preferably in an updraft region during FCCs. In a CBL over flat homogeneous terrain, the locations of convective structures would occur randomly. However, over the complex orography of the Kinzig valley, the updraft structures were found to develop in
quasi-stationary patterns at specific locations relative to the surrounding mountain ridges. The model further showed that the flux through the valley boundary layer is mainly determined by the flux within these coherent updrafts. In combination with the Sodar/RASS observations, the model also showed that these updrafts deeply penetrated into the stably stratified valley boundary layer up to approximately the height of the surrounding mountains leading to an effective upward counter-gradient transport of surface layer air mass properties during FCCs.
The analysis of the turbulent fluxes at the different COPS sites showed that the flux values were strongly determined by varying land surface characteristics. Also an increase of the Bowen ratio with increasing altitude could be detected. These findings are in accordance with former studies in this area. As expected, the energy balance was found to be unclosed on average during the entire COPS period, with values of the residual typical for heterogeneous landscapes. However, regarding only the periods with FCCs, no residual occurred on average. This is due to the fact that the landscape
heterogeneity is of minor importance in case of the more vertical oriented exchange regime during FCCs, so that missing advective flux components became strongly reduced in these situations. Moreover, it was found that in comparable periods with no FCCs, flux components were missing with exactly the proportions of the buoyancy flux ratio, thus suggesting a correction of the energy balance according to the buoyancy flux ratio approach. These results support recent publications on the energy balance closure
Interactions between hydrology and biogeochemistry within riparian wetlands
- Interactions between hydrology and biogeochemistry at various spatio-temporal scales are important control mechanisms within terrestrial and aquatic ecosystems and exist among different compartments and transition interfaces. Understanding the fundamental mechanistic couplings between hydrological and biogeochemical processes and how these couplings feed back into ecosystem services and functions is an interdisciplinary challenge that must be addressed especially in the context of humanly mediated climate change. Riparian wetlands, as a transition zone between terrestrial and aquatic ecosystems, occupy large fractions of terrestrial ecosystems and provide important ecohydrological services. Due to their anoxic environments, riparian wetlands are able to store significant amounts of carbon as peat and act as an effective nutrient sink e.g. for sulfur, phosphorous and nitrogen. Riparian wetlands are characterized by highly dynamical interactions between hydrologically controlled transport mechanisms and biogeochemically controlled substrate availability, which governs nutrient cycling as well as the sink and source functions of wetlands. Generally, these interactions and their potential implications on ecosystem functions are only poorly understood. The representation of the tight couplings between hydrology and biogeochemistry in mechanistic models is a very challenging task because they have revealed a complexity which is often beyond the capabilities of current models. The objective of this thesis is to investigate interactions between hydrology and biogeochemistry in riparian wetlands and to understand their potential implications for internal biogeochemical process distributions and solute mobilization. Additionally, one major focus of the thesis is the attempt to represent such fundamental couplings in a process-based, hydrological/biogeochemical modeling approach. To this end, this thesis uses a combination of field and virtual experiments, as well as catchment-scale numerical modeling, performed for the Lehstenbach catchment, which was exemplarily chosen as main study site.
Results from the virtual experiments show very complex small-scale hydrological dynamics within the riparian areas. Here, runoff generation processes are strongly influenced by the spatial structure of the wetland-typical micro-topography (hummocks and hollows). Surface flow is episodically generated by a highly dynamical, threshold-controlled process where extended surface flow networks drain large fractions of the wetland's area. During intensive rainstorm events these surface flow networks, which contribute to stream discharge due to a fill and spill mechanism, dominate runoff generation. These fast flow components are characterized by very low residence times (minutes to hours) and once they are activated, the surface flow networks are able to rapidly mobilize large amounts of solutes, like nitrate or dissolved organic carbon (DOC), out of the wetlands by bypassing deeper anoxic layers. The importance of fast flow components for the catchment-scale mobilization of DOC was further confirmed by field investigations and catchment-scale numerical modeling. High frequency measurements of DOC in runoff of the Lehstenbach catchment revealed that DOC export is subject to substantial short term variations at an hourly to daily timescale. During intense rainstorms, DOC concentrations are up to ten times higher (up to 40 mg/L) compared to low flow conditions (~3-5 mg/L). Short term variations together with the dramatic rise of DOC concentrations in runoff during rainstorms can be explained by the episodically activation of fast flow components in the wetland areas. At the catchment-scale, application of a hydraulic mixing-cell (HMC) methodology in combination with numerical modeling has revealed that fast flow components like saturated overland flow are exclusively generated in the wetland areas during intensive rainstorm events. On an annual basis, exemplarily for the hydrological year 2001, the HMC analysis quantified the relative contribution of saturated overland flow related to the total discharge with 19.5%, which highlights the importance of riparian wetlands for catchment-scale runoff generation.
Virtual experiments, additionally show that distinct shifts between surface and subsurface flow dominance, as a result of small-scale micro-topographic driven runoff generation in the wetlands, are responsible for very complex three-dimensional subsurface flow patterns showing a wide range of subsurface residence times. To investigate how these micro-topography induced subsurface flow patterns, together with the non-uniform hydrological and biogeogeochemical boundary conditions, affect the internal re-distribution and transformation of redox-sensitive species (like nitrate, sulfate or iron) a coupled hydrological/biogeogeochemical model was developed. In the model, wetland-typical biogeochemical processes are represented in a sequential stream tube approach where redox-sensitive processes are implemented as kinetic reactions. Simulations show the formation of local hot spots for redox-sensitive processes within the subsurface as a result of the complex subsurface flow paths and the transport-limited availability of electron acceptors and donors. Formation of hot spots was simulated for all key reduction processes including iron(III)-/sulfate reduction and denitrification as well as for the corresponding re-oxidation processes. These results offer a new perspective on hydrologically controlled biogeochemical transformation processes in riparian wetlands, which provides a dynamic framework to explain process heterogeneity in wetland soils and variability in process rates over space and time.
Findings from this thesis clearly prove how useful interdisciplinary approaches are in understanding processes and mechanisms in ecosystems and how important functions of ecosystems are affected by couplings among those. However, a lot of knowledge gaps still exist in understanding the nature of dependency between water and nutrient cycles across scales and how these interacting cycles feed back into humanly-mediated climate change in ecosystems. Development of new interdisciplinary methodologies and frameworks as well as an integrated way of thinking across the boundaries of the different environmental disciplines is necessary to address the grand challenges associated with climate change.
Tibet Plateau Atmosphere-Ecology-Glaciology Cluster Joint Kobresia Ecosystem Experiment: Documentation of the second Intensive Observation Period, Summer 2012 in KEMA, Tibet
- Experiment documentation of the second joined Kobresia ecosystem experiment conducted by the Atmosphere-Ecology-Glaciology Cluster within DFG SPP 1372 (Tibetan Plateau)in Kema, Tibet, China. The report provides background information about the field side, conducted measurements and participants.
The Re-implementation of Sharia in Northern Nigeria and the Education of Muslim Women 1999-2007
- This study investigates the impact of the re-implementation of Sharia on the education of Muslim women between 1999 and 2007, in four states in northern Nigeria namely, Zamfara, Kano, Bauchi and Kaduna. The study fills the research gap in the description of the status of women with respect to education under the Sharia dispensation re-introduced in 1999. Before this era, scholarly works have documented the state of female education in northern Nigeria; however, none deals directly with post re-implementation and its impact on Muslim women’s education. This period is particularly interesting because it coincides with the return of the country to democratic rule after a string of military rules. Using interviews with Sharia proponents, school administrators, women and reports from Sharia Commissions in these states, the work establishes that while Zamfara and Kano States underwent transformations in the educational sector as a result of the programs introduced by the Sharia governments, the changes in the educational domain in Bauchi and Kaduna were exceedingly triggered by the return of democracy in Nigeria after decades of military rule. The results arrived at in this study reveal interesting facets of the relationship between Sharia and the education of Muslim women. The research shows that proponents of Sharia used the premise (also supported by verses from the Qur’an and Prophet Mohammed’s teachings) that education is an important part of Islam to which both males and females have equal rights, to sell the Islamic-based educational programs introduced by the Sharia regime. Using religion to promote these programs, the predominantly male Sharia bodies, mitigate the often tough cultural practices and religious dogmas that have prevented women from having full access to formal, western education. However, the type of education offered to women under these Sharia programs is a hybrid of formal education and Islamic education which pays strict attention to such cultural and religious practices like the wearing of the hijab, the separation of males and females in schools, the teaching of practical, home craft skills such as knitting and baking to the women. So, instead of the education taking women out of the home domain in which they have been restricted in the past, it rather further maintains them while providing them access to formal education which they will hardly use in any public career.
Crystalline-core micelles based on triblock terpolymers with polyethylene middle blocks
- This thesis is focused on the crystallization-induced structure formation of polyethylene containing triblock terpolymers in organic solvents to surface-compartmentalized worm-like crystalline-core micelles (wCCMs). Obtaining profound knowledge of the parameters controlling the self-assembly process allowed the production of a variety of complex one-dimensional micellar architectures with many potential applications, such as adaptive surfactants.
At first, the basic parameters that control the crystallization-induced self-assembly were explored using symmetric polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymers and a PS-b-PE-b-PS triblock copolymer. In good solvents for the PE block, e.g. THF and toluene, the selective formation of wCCMs was observed over a wide range of concentration, applied crystallization temperature and polymer composition. Whereas wCCMs produced by PS-b-PE-b-PS showed a homogeneous PS corona, a patch-like compartmentalization of the corona was observed if the micelles were formed by PS-b-PE-b-PMMA. As THF shows equal solvent quality for both corona blocks, wCCMs with almost alternating PS and PMMA compartments of about 15 nm were observed in this solvent. However, if structure formation was conducted in bad solvents for PE, such as dioxane or dimethylacetamide, spherical micelles with amorphous PE cores were formed already before crystallization. Hence, the subsequent crystallization of PE resulted in spherical CCMs with a patchy or a homogeneous corona depending on the used triblock. These findings allow the highly selective production of stable spherical or worm-like CCMs from the same polymer.
As the corona structure of the patchy micelles self-assembled from triblock terpolymers was mainly deduced from transmission electron microscopy (TEM) performed on dried samples, a small-angle neutron scattering (SANS) study was performed in order to elucidate the morphology in solution. Therefore a partly deuterated triblock terpolymer was synthesized and measured at different contrasts to allow the selective detection of the different corona compartments. The resulting SANS curves could be interpreted using a form factor model for core-shell cylinders with alternating PS and PMMA hemishells including interparticle interactions, thus validating the TEM observations. Notably, Janus-type and patchy cylinders can be clearly distinguished using the applied form factor model.
Moreover, the controlled formation of wCCMs with tunable corona composition and structure was achieved using the cocrystallization of different triblock copolymers. Via random cocrystallization of PS-b-PE-b-PMMA and PS-b-PE-b-PS the corona morphology could be tuned continuously from a mixed corona at low PMMA content over spherical PMMA patches of increasing number and size to alternating PS and PMMA patches. This approach allows to manufacture wCCMs with predefined corona structure omitting the need to synthesize a new tailor-made triblock terpolymer for every desired morphology.
By establishing the controlled crystallization-driven self-assembly of triblock terpolymers with PE middle blocks, it was further possible to prepare wCCMs with predefined average lengths up to 500 nm and length polydispersities as low as Lw/Ln = 1.1. Here, self-assembled spherical CCMs of PS-b-PE-b-PS were used as seeds for the controlled growth of PS-b-PE-b-PS unimers. Upon further addition of PS-b-PE-b-PMMA unimers these grew epitaxially onto the preexisting wCCMs, resulting in triblock co-micelles that consisted of middle blocks with a homogeneous PS corona and outer blocks with alternating PS/PMMA compartments. These structures represent not only the first block co-micelles including blocks with a patchy corona, but also the first ones produced from purely organic block copolymers.
In view of application, the ability of patchy wCCMs formed by PS-b-PE-b-PMMA to stabilize interfaces was investigated using pendant-drop tensiometry. The observed reduction of the interfacial tension at the toluene/water interface was significantly higher than that of comparable triblock terpolymer single chains and that of wCCMs with a homogeneous PS corona. Interestingly, the obtained equilibrium interfacial tension equaled that of Janus cylinders with similar dimensions. To explain this unexpected finding the corona chains were proposed to adapt to the interface via selective collapse and shielding of the incompatible part of the corona chains. Studying wCCMs formed by several triblock terpolymers with different compositions, the interfacial activity was found to increase with increasing overall length of the corona chains, and to a certain extent with the molar fraction of PS units in the corona.
Silent Sound Art: Performing the Unheard
- This article is a reflection on silent sound art, exemplified here by the works of Peter Ablinger and Akio Suzuki, in the context of a partially historical con-sideration of the participatory subject in Installation Art, with a primary focus on artistic movements and selected works from the late 1950s to the early 1970s by John Cage, George Brecht, and La Monte Young.