Morphogens are signaling molecules that play a key role in animal development. They spread from a restricted source into an adjacent target tissue forming a concentration gradient. The fate of cells in the target tissue is determined by the local concentration of such morphogens. Morphogen transport through the tissue has been studied in experiments which lead to the suggestion of several transport mechanisms. While diffusion in the extracellular space contributes to transport, recent experiments on the morphogen Decapentaplegic (Dpp) in the fruit fly Drosophila provide evidence for the importance of a cellular transport mechanism that was termed "planar transcytosis". In this mechanism, morphogens are transported through cells by repeated rounds of internalization and externalization. Starting from a microscopic theoretical description of these processes, we derive systems of nonlinear transport equations which describe the interplay of transcytosis and passive diffusion. We compare the results of numerical calculations based on this theoretical description of morphogen transport to recent experimental data on the morphogen Dpp in the Drosophila wing disk. Agreement with the experimental data is only achieved if the parameters entering the theoretical description are chosen such that transcytosis contributes strongly to transport. Analyzing the derived transport equations, we find that transcytosis leads to an increased robustness of the created gradients with respect to morphogen over-expression. Indications for this kind of robustness have been found in experiments. Furthermore, we theoretically investigate morphogen gradient formation in disordered systems. Here, an important question is how the position of concentration thresholds can be defined with high precision in the noisy environment present in typical developing tissues. Among other things, we find that the dimensionality of the system in which the gradient is formed plays an important role for the precision. Comparing gradients formed by transcytosis to those formed by extracellular diffusion, we find substantial differences that may result in a higher precision of gradients formed by transcytosis. Finally, we suggest several experiments to test the theoretical predictions of this work.

Thermal conductivity and thermopower measurements in strongly correlated electron systems at low temperatures

Bei der Vertical Gradient Freeze-Züchtung von Halbleiterkristallen bis zu einem Durchmesser von 2″ wurde in einer speziell entwickelten Anlage die Beeinflussung des Wärme- und Stofftransports in der Schmelze durch ein rotierendes Magnetfeld (RMF) untersucht. An GaAs- und Ge-Kristallen konnte bei Einwirkung des RMF eine verminderte Durchbiegung der Phasengrenze aufgrund geringerer radialer Temperaturgradienten festgestellt werden. Die Analyse der Segregationsprozesse ergab unter RMF-Einfluss signifikante Änderungen der Ausdehnung der Diffusionsgrenzschicht vor der Phasengrenze. Dies spiegelte sich in einer verbesserten axialen und radialen Homogenität der Ladungsträgerverteilung in den Kristallen wider. Die kritische magnetische Taylor-Zahl, die den Übergang von stationärer zu instationärer RMF-Konvektion charakterisiert, konnte für verschiedene Geometrieverhältnisse der Schmelze experimentell bestimmt werden.

Morphogens are signaling molecules that play a key role in animal development. They spread from a restricted source into an adjacent target tissue forming a concentration gradient. The fate of cells in the target tissue is determined by the local concentration of such morphogens. Morphogen transport through the tissue has been studied in experiments which lead to the suggestion of several transport mechanisms. While diffusion in the extracellular space contributes to transport, recent experiments on the morphogen Decapentaplegic (Dpp) in the fruit fly Drosophila provide evidence for the importance of a cellular transport mechanism that was termed "planar transcytosis". In this mechanism, morphogens are transported through cells by repeated rounds of internalization and externalization. Starting from a microscopic theoretical description of these processes, we derive systems of nonlinear transport equations which describe the interplay of transcytosis and passive diffusion. We compare the results of numerical calculations based on this theoretical description of morphogen transport to recent experimental data on the morphogen Dpp in the Drosophila wing disk. Agreement with the experimental data is only achieved if the parameters entering the theoretical description are chosen such that transcytosis contributes strongly to transport. Analyzing the derived transport equations, we find that transcytosis leads to an increased robustness of the created gradients with respect to morphogen over-expression. Indications for this kind of robustness have been found in experiments. Furthermore, we theoretically investigate morphogen gradient formation in disordered systems. Here, an important question is how the position of concentration thresholds can be defined with high precision in the noisy environment present in typical developing tissues. Among other things, we find that the dimensionality of the system in which the gradient is formed plays an important role for the precision. Comparing gradients formed by transcytosis to those formed by extracellular diffusion, we find substantial differences that may result in a higher precision of gradients formed by transcytosis. Finally, we suggest several experiments to test the theoretical predictions of this work.

Im Grundwasser mobile Feststoff-Partikel stehen im Verdacht, den Transport schwerlöslicher Schadstoffe zu begünstigen. Die Partikel-Konzentration im Wasser lässt sich aber nur mittels einer aufwändigen Probenahme bestimmen. Vor diesem Hintergrund wurden im Rahmen dieser Arbeit Feld- und Laborversuche durchgeführt. In Feldversuchen zur Probenahme wurde festgestellt, dass für eine Stabilisierung der Partikel-Anzahl-Konzentration (CN in Partikel>Nachweisgrenze/L) die Dauer der Probenahme ausschlaggebend ist, nicht das insgesamt geförderte Volumen. Eine hohe Förderrate hat dabei nicht automatisch eine stärkere Mobilisierung von Partikeln aus dem Partikel-Depot im Umfeld der Messstelle zur Folge, wie es häufig postuliert wird. Die Gewinnung einer repräsentativen Partikelprobe macht eine Probenahme-Dauer von mindestens 5, häufig sogar über 10 oder 12 Stunden erforderlich. Empfehlungen zur Probenahme wurden erarbeitet. Es wurde aber festgestellt, dass sich stabilisierte Werte für CN als Funktion der Förderdauer (t in min) nach CN = a ? t^(-½) ungefähr abschätzen lassen, wenn der Parameter a über CN nach ca. 60 Minuten ermittelt wird. Bei Wässern mit Sauerstoff-Konzentrationen über ca. 1 mg/L kann überdies auch die Verteilung der Partikel auf einzelne Größenklassen bereits nach dieser Zeit ermittelt werden; bei sauerstoffärmeren Wässern verändert sie sich später noch. Angesichts einer starken Variation von CN einer Messstelle ist es generell aber nur möglich, eine Größenordnung für die Hintergrundkonzentration anzugeben. Von Messstelle zu Messstelle sind große Unterschiede bei CN festzustellen. Die Konzentration ist dabei nicht eindeutig abhängig von der Lithologie des Grundwasserleiters oder der Zusammensetzung des Grundwassers. Allerdings konnte eine schwache Korrelation mit dem Redox-Potential (Eh in mV) des Wassers gefunden werden (CN = 1,8?10^6?e^( 0,0087?Eh) [Partikel>2,58µm/L]; r² ≈ 0,46). Hierüber besteht die Möglichkeit, CN eines Grundwassers auch ohne Partikelmessung abzuschätzen. Insgesamt bewegen sich die Partikel-Massen-Konzentrationen (C in mg/L) der beprobten Wässer meist im Bereich von <1 mg/L, öfters sogar <0,1 mg/L, wenn die im Verlaufe von Probenahme und Messung ausgefällten Fe- und Mn-Oxide nicht berücksichtigt werden. Solche Konzentrationen sind vermutlich kaum in der Lage, relevante Mengen selbst sehr hydrophober Schadstoffen aufzunehmen und zu transportieren. Versuche mit Phenanthren ergaben zudem, dass es anscheinend weniger an bereits in Suspension befindliche Partikel sorbiert, sondern vielmehr in sorbiertem oder kristallinen Zustand aus dem Depot erodiert wird. In Laborversuchen wurde weiter gezeigt, dass in natürlichen Sedimenten ein großes Depot mobilisierbarer Partikel vorhanden ist, das Partikel über einen sehr langen Zeitraum kontinuierlich abgeben kann. Durchbruchsversuche ergaben außerdem, dass ein Großteil zugegebener Partikel bei der Passage durch eine Sedimentprobe zurückgehalten und nur sehr allmählich wieder abgegeben wird. Allerdings war auch ein schneller (präferentieller) Durchbruch zu verzeichnen. In Modellrechnungen konnte gezeigt werden, dass sich ein solcher Partikeltransport weder über eine Filterfunktion noch über die Transportgleichung zufrieden stellend berechnen lässt. Daher ist es nötig, einerseits einen bevorzugten Transport und andererseits eine starke Retardation zu berücksichtigen. Letzteres kann am besten über verschiedene Retardationsfaktoren oder ein dynamisches Partikel-Depot mit Anlagerungs- und Ablösungskonstanten geschehen. / Solid particles that are mobile in groundwater are suspected to enhance the transport of hardly soluble contaminants. But particle concentrations in water can only be measured using time-consuming sampling-procedures. On this background field- and laboratory-experiments were conducted in this work. In field experiments on sampling it turned out, that sampling time is crucial for stabilizing particle number-concentration (CN in particles>detection limit/L), not the volume sampled. A high sampling rate does not -as often argued- automatically result into higher mobilization of particles from the particle-depot in the vicinity of the sampling-well. Obtaining a representative particle sample requires a sampling-time of at least 5, often even more than 10 or 12 hours. In this work recommendations on sampling are given. It was noticed that stabilized values of CN can be estimated as function of sampling time (t in min) by CN = A ? t^(-½), if parameter A is calculated using CN after about 60 minutes. In waters having oxygen-concentrations above approximately 1 mg/L even distribution of the particles into size classes can be estimated after this time; in oxygen-poor waters size-distributions stabilized much later. Because of strong variations of CN in a single measuring well it is generally only possible to give the magnitude of the background-particle-concentration. But among several measuring wells CN may differ by several magnitudes. The concentrations do neither definitely depend upon the lithology of the aquifer nor on the groundwater-composition. But a weak correlation to the redox-potential (Eh in mV) can be found (CN = 1.8 ? 10^6 ? e^( 0.0087 ? Eh) [particles>2.58µm/L]; r² ≈ 0,46). Using this connection it is possible to estimate a magnitude of CN of a groundwater without even measuring particles. Over all particle mass-concentrations (C in mg/L) of all sampled groundwaters were almost always <1 mg/L, often even <0.1 mg/L, at least if iron- and manganese-oxides that precipitated during measurements were ignored. Such particle concentrations are probably hardly capable of adsorbing and carrying relevant amounts of contaminants, even very hydrophobic ones. Experiments using phenanthrene in contaminated sand additionally showed that it is probably hardly adsorbed onto already suspended particles, but mostly eroded from the particle depot in adsorbed or crystalline state. In laboratory experiments it was further shown that there is a huge depot of mobilizable particles in natural sediments that can continually release particles over a very long period of time. Break-through-experiments showed in addition that a large part of particles fed into the system are retained during the passage through a sediment sample and that they are re-released only very slowly. However, there also was a fast (preferential) break-through. In model calculations it could be shown that such a particle transport can neither be sufficiently described by the filter-function nor by the transport equation. Because of that it is necessary to take into consideration a preferential transport on the one hand and a strong retardation on the other. The last one can at the best be described by several retardation-factors or a dynamic particle-depot having constant attachment- and detachment rates.

A fundamental challenge is to understand nonequilibrium statistical mechanics starting from microscopic chaos in the equations of motion of a many-particle system. In this thesis we summarize recent theoretical advances along these lines. We focus on two different approaches to nonequilibrium transport: One considers Hamiltonian dynamical systems under nonequilibrium boundary conditions, another one suggests a non-Hamiltonian approach to nonequilibrium situations created by external electric fields and by temperature or velocity gradients. A surprising result related to the former approach is that in simple low-dimensional periodic models the deterministic transport coefficients are typically fractal functions of control parameters. These fractal transport coefficients yield the first central theme of this thesis. We exemplify this phenomenon by deterministic diffusion in a simple chaotic map. We then construct an arsenal of analytical and numerical methods for computing further transport coefficients such as electrical conductivities andchemical reaction rates. These methods are applied to hierarchies of chaotic dynamical systems that are successively getting more complex, starting from abstract one-dimensional maps generalizing a simple random walk on the line up to particle billiards that should be directly accessible in experiments. In all cases, the resulting transport coefficients turn out to be either strictly fractal, or at least to be profoundly irregular. The impact of random perturbations on these quantities is also investigated. We furthermore provide some access roads towards a physical understanding of these fractalities. The second central theme is formed by a critical assessment of the non-Hamiltonian approach to nonequilibrium transport. Here we consider situations where the nonequilibrium constraints pump energy into a system, hence there must be some thermal reservoir that prevents the system from heating up. For this purpose a deterministic and time-reversible modeling of thermal reservoirs was proposed in form of Gaussian and Nose-Hoover thermostats. This approach yielded simple relations between fundamental quantities of nonequilibrium statistical mechanics and of dynamical systems theory. Our goal is to critically assesses the universality of these results. As a vehicle of demonstration we employ the driven periodic Lorentz gas, a toy model for the classical dynamics of an electron in a metal under application of an electric field. Applying different types of thermal reservoirs to this system we compare the resulting nonequilibrium steady states with each other. Along the same lines we discuss an interacting many-particle system under shear and heat. Finally, we outline an unexpected relationship between deterministic thermostats and active Brownian particles modeling biophysical cell motility.

The production of raw materials through mining projects is nowadays very challenging, mainly due to the rapid progress in the industrial and technological fields. On the one hand, they have to fulfill industries' requirements in their demand for materials while making a profit based on the current technologies. On the other hand, they should consider all other limitations, primarily environmental and social challenges that are confronting. The transportation system in any mining project is one of the most significant parts, especially in the technical and economic issues. It must transfer the planned volume of ore/waste that the whole stream of the mining process would not be interrupted and, it can cover the technical challenges and the costs imposed on the project. Additionally, it should be designed and selected to have the lowest environmental impact and the highest safety during the operation. Accordingly, a transportation system selection process that considers all these factors is one of the challenging issues in any mining project. Although the Truck-Shovel system is known as the conventional transportation in open-pit mines, which is preferable because of the low capital cost and high flexibility, it still imposes a high rate of operating costs, safety issues as well as environmental footprints. In-Pit Crushing and Conveying (IPCC) systems are the alternative transportation systems for the Truck-Shovel systems, in which the material is crushed inside the mine’s pit limit and transferred into the outside through conveyor belts. Although these systems are not new, they are mostly neglected as a transportation option basically due to the high capital cost and low flexibility. On the contrary, they can offer more environmentally friendly and safer working areas and a lower operating cost. According to these facts, each transportation system is preferable in a couple of technical, economic, environmental, safety, and social issues. Accordingly, in each circumstance, one or more of these systems can be used in the mining project. However, there is not yet a way or tool that investigates the transportation system selection along with the mine life that takes into account all of these factors. To fill this gap, this project aims to define a model to introduce all these elements while it is interactively connected throughout the mine life. For this and as the first step, the system dynamics modeling is defined and used to build the model for all the technical, economic, environmental, safety, and social factors. As an output of this step, software entitled “TEcESaS Indexes” is designed and produced through Venapp that makes working with the model comfortable. As the second step, a selection method based on the Analytical Hierarchy Process (AHP) is performed that the transportation system selection regarding all the mentioned factors can be made. As the output in this step, the “Sustainability Index” software programmed in the Java language is developed. Considering a hypothetical copper open-pit mine as the case study and implementing the designed software, the results show although the Truck-Shovel system should be used in the first two years of the project (2016 and 2017) in the single expert and deterministic mode, the Fully Mobile In-Pit Crushing and Conveying (FMIPCC) system shows the highest sustainability index among other transportation systems from 2018 until the end of the mine life. While in the group decision making and deterministic simulation, the Truck-Shovel system should be utilized from 2016 to 2020. Additionally, in the group decision making and stochastic mode, the FMIPCC is the selected transportation system with the highest sustainability index probability.

A fundamental challenge is to understand nonequilibrium statistical mechanics starting from microscopic chaos in the equations of motion of a many-particle system. In this thesis we summarize recent theoretical advances along these lines. We focus on two different approaches to nonequilibrium transport: One considers Hamiltonian dynamical systems under nonequilibrium boundary conditions, another one suggests a non-Hamiltonian approach to nonequilibrium situations created by external electric fields and by temperature or velocity gradients. A surprising result related to the former approach is that in simple low-dimensional periodic models the deterministic transport coefficients are typically fractal functions of control parameters. These fractal transport coefficients yield the first central theme of this thesis. We exemplify this phenomenon by deterministic diffusion in a simple chaotic map. We then construct an arsenal of analytical and numerical methods for computing further transport coefficients such as electrical conductivities andchemical reaction rates. These methods are applied to hierarchies of chaotic dynamical systems that are successively getting more complex, starting from abstract one-dimensional maps generalizing a simple random walk on the line up to particle billiards that should be directly accessible in experiments. In all cases, the resulting transport coefficients turn out to be either strictly fractal, or at least to be profoundly irregular. The impact of random perturbations on these quantities is also investigated. We furthermore provide some access roads towards a physical understanding of these fractalities. The second central theme is formed by a critical assessment of the non-Hamiltonian approach to nonequilibrium transport. Here we consider situations where the nonequilibrium constraints pump energy into a system, hence there must be some thermal reservoir that prevents the system from heating up. For this purpose a deterministic and time-reversible modeling of thermal reservoirs was proposed in form of Gaussian and Nose-Hoover thermostats. This approach yielded simple relations between fundamental quantities of nonequilibrium statistical mechanics and of dynamical systems theory. Our goal is to critically assesses the universality of these results. As a vehicle of demonstration we employ the driven periodic Lorentz gas, a toy model for the classical dynamics of an electron in a metal under application of an electric field. Applying different types of thermal reservoirs to this system we compare the resulting nonequilibrium steady states with each other. Along the same lines we discuss an interacting many-particle system under shear and heat. Finally, we outline an unexpected relationship between deterministic thermostats and active Brownian particles modeling biophysical cell motility.

The work discusses the development of molecular electronics based on the possibility of the usage of anorganic quantum dots and organic molecules as basis material. Of special interest are the properties of semiconductor quantum dots and their modification due to the coupling of quantum dots from different materials. Eventually these are proper candidates to avoid the fast recombination of excitons which is a problem in many organic photovoltaic materials, by local separation of charge carriers. Another materials class investigated are the so called charge transfer dimers. On the way to usable molecular building blocks switching and rectification behavior are important properties, therefore they were of special interest in the investigation. Especially the usage of charge transfer materials in rectification was already suggested in the 70’s, but could be realized till now only with a quiet limited success. Already around the millennium it was shown that a too strong coupling between the components leads to a resymmetrization of the I-V-characteristics. For all systems the electronic structure was investigated by means of density functional theory. Additional the charge transport in between gold leads was computed based on non equilibrium Greens functions. For the system of coupled quantum dots it is shown how the combination of several gates can be used to adjust the transport properties. This work shows that the rectification effect within weakly coupled charge transfer systems stays also small because also in this case a resymmetrization of the I-V-characteristics takes place.:1 Introduction 2 Molecular Electronics 3 Theoretical background 4 Computational details and software packages 5 Modeling 6 Results and Discussion 6.1 Quantum dots 6.2 Transport through coupled quantum dots 6.3 Charge transfer dimers 6.4 Transport through charge transfer dimers 7 Conclusion 8 Outlook Acknowledgement List of Figures List of Tables Bibliography List of own Publications / Die Arbeit befasst sich mit der Entwicklung der molekularen Elektronik und insbesondere mit der Prüfung der Verwendbarkeit von anorganischen Quantenpunkten und organischen Molekülen für diesen Bereich. Quantenpunkte aus Halbleitermaterialien besitzen eine grosse Bandbreite von Eigenschaften. Es wird untersucht, wie die Eigenschaften durch die Kopplung von Quantenpunkten unterschiedlicher Materialien modifiziert werden können. Eine Idee besteht in der lokalen Trennung von Ladungsträgern um die schnelle Rekombination von Exzitonen zu vermeiden, welche in organischen Solarzellen häufig ein Problem darstellt. Als weitere Materialklasse werden molekulare Ladungstransferdimere untersucht. Auf dem Weg zu nutzbaren Bauelementen sind das Schalt- und Gleichrichtverhalten wichtige Eigenschaften, daher sind sie von besonderem Interesse. Insbesondere die Frage des Ladungstransfers in Bezug auf das Gleichrichten wurde schon in den 1970ern vorgeschlagen, konnte aber bisher immer nur mit begrenztem Erfolg realisiert werden. Schon um die Jahrtausendwende wurde gezeigt, dass Systeme mit einer zu starken Kopplung zu einer Symmetrisierung der Strom-Spannungs-Kennlinie führen. Bei beiden Systemen wird jeweils die elektronische Struktur im Sinne der Dichtefunktionaltheorie berechnet. Zusätzlich wird jeweils der Ladungstransport zwischen Goldkontakten mittels Nichtgleichgewichts-Greenschen Funktionen berechnet. Für die Systeme gekoppelter Quantenpunkte wird gezeigt, wie die Transporteigenschaften mittels Gatespannungen eingestellt werden können. In der vorliegenden Arbeit wird gezeigt, dass es auch im Fall schwach gekoppelter Ladungstransferdimere zu weitgehend symmetrischen Strom-Spannungs-Kennlinien kommt und es auch für diese Systeme nur zu einem schwachen Gleichrichtverhalten kommt.:1 Introduction 2 Molecular Electronics 3 Theoretical background 4 Computational details and software packages 5 Modeling 6 Results and Discussion 6.1 Quantum dots 6.2 Transport through coupled quantum dots 6.3 Charge transfer dimers 6.4 Transport through charge transfer dimers 7 Conclusion 8 Outlook Acknowledgement List of Figures List of Tables Bibliography List of own Publications

Im Grundwasser mobile Feststoff-Partikel stehen im Verdacht, den Transport schwerlöslicher Schadstoffe zu begünstigen. Die Partikel-Konzentration im Wasser lässt sich aber nur mittels einer aufwändigen Probenahme bestimmen. Vor diesem Hintergrund wurden im Rahmen dieser Arbeit Feld- und Laborversuche durchgeführt. In Feldversuchen zur Probenahme wurde festgestellt, dass für eine Stabilisierung der Partikel-Anzahl-Konzentration (CN in Partikel>Nachweisgrenze/L) die Dauer der Probenahme ausschlaggebend ist, nicht das insgesamt geförderte Volumen. Eine hohe Förderrate hat dabei nicht automatisch eine stärkere Mobilisierung von Partikeln aus dem Partikel-Depot im Umfeld der Messstelle zur Folge, wie es häufig postuliert wird. Die Gewinnung einer repräsentativen Partikelprobe macht eine Probenahme-Dauer von mindestens 5, häufig sogar über 10 oder 12 Stunden erforderlich. Empfehlungen zur Probenahme wurden erarbeitet. Es wurde aber festgestellt, dass sich stabilisierte Werte für CN als Funktion der Förderdauer (t in min) nach CN = a ? t^(-½) ungefähr abschätzen lassen, wenn der Parameter a über CN nach ca. 60 Minuten ermittelt wird. Bei Wässern mit Sauerstoff-Konzentrationen über ca. 1 mg/L kann überdies auch die Verteilung der Partikel auf einzelne Größenklassen bereits nach dieser Zeit ermittelt werden; bei sauerstoffärmeren Wässern verändert sie sich später noch. Angesichts einer starken Variation von CN einer Messstelle ist es generell aber nur möglich, eine Größenordnung für die Hintergrundkonzentration anzugeben. Von Messstelle zu Messstelle sind große Unterschiede bei CN festzustellen. Die Konzentration ist dabei nicht eindeutig abhängig von der Lithologie des Grundwasserleiters oder der Zusammensetzung des Grundwassers. Allerdings konnte eine schwache Korrelation mit dem Redox-Potential (Eh in mV) des Wassers gefunden werden (CN = 1,8?10^6?e^( 0,0087?Eh) [Partikel>2,58µm/L]; r² ≈ 0,46). Hierüber besteht die Möglichkeit, CN eines Grundwassers auch ohne Partikelmessung abzuschätzen. Insgesamt bewegen sich die Partikel-Massen-Konzentrationen (C in mg/L) der beprobten Wässer meist im Bereich von <1 mg/L, öfters sogar <0,1 mg/L, wenn die im Verlaufe von Probenahme und Messung ausgefällten Fe- und Mn-Oxide nicht berücksichtigt werden. Solche Konzentrationen sind vermutlich kaum in der Lage, relevante Mengen selbst sehr hydrophober Schadstoffen aufzunehmen und zu transportieren. Versuche mit Phenanthren ergaben zudem, dass es anscheinend weniger an bereits in Suspension befindliche Partikel sorbiert, sondern vielmehr in sorbiertem oder kristallinen Zustand aus dem Depot erodiert wird. In Laborversuchen wurde weiter gezeigt, dass in natürlichen Sedimenten ein großes Depot mobilisierbarer Partikel vorhanden ist, das Partikel über einen sehr langen Zeitraum kontinuierlich abgeben kann. Durchbruchsversuche ergaben außerdem, dass ein Großteil zugegebener Partikel bei der Passage durch eine Sedimentprobe zurückgehalten und nur sehr allmählich wieder abgegeben wird. Allerdings war auch ein schneller (präferentieller) Durchbruch zu verzeichnen. In Modellrechnungen konnte gezeigt werden, dass sich ein solcher Partikeltransport weder über eine Filterfunktion noch über die Transportgleichung zufrieden stellend berechnen lässt. Daher ist es nötig, einerseits einen bevorzugten Transport und andererseits eine starke Retardation zu berücksichtigen. Letzteres kann am besten über verschiedene Retardationsfaktoren oder ein dynamisches Partikel-Depot mit Anlagerungs- und Ablösungskonstanten geschehen. / Solid particles that are mobile in groundwater are suspected to enhance the transport of hardly soluble contaminants. But particle concentrations in water can only be measured using time-consuming sampling-procedures. On this background field- and laboratory-experiments were conducted in this work. In field experiments on sampling it turned out, that sampling time is crucial for stabilizing particle number-concentration (CN in particles>detection limit/L), not the volume sampled. A high sampling rate does not -as often argued- automatically result into higher mobilization of particles from the particle-depot in the vicinity of the sampling-well. Obtaining a representative particle sample requires a sampling-time of at least 5, often even more than 10 or 12 hours. In this work recommendations on sampling are given. It was noticed that stabilized values of CN can be estimated as function of sampling time (t in min) by CN = A ? t^(-½), if parameter A is calculated using CN after about 60 minutes. In waters having oxygen-concentrations above approximately 1 mg/L even distribution of the particles into size classes can be estimated after this time; in oxygen-poor waters size-distributions stabilized much later. Because of strong variations of CN in a single measuring well it is generally only possible to give the magnitude of the background-particle-concentration. But among several measuring wells CN may differ by several magnitudes. The concentrations do neither definitely depend upon the lithology of the aquifer nor on the groundwater-composition. But a weak correlation to the redox-potential (Eh in mV) can be found (CN = 1.8 ? 10^6 ? e^( 0.0087 ? Eh) [particles>2.58µm/L]; r² ≈ 0,46). Using this connection it is possible to estimate a magnitude of CN of a groundwater without even measuring particles. Over all particle mass-concentrations (C in mg/L) of all sampled groundwaters were almost always <1 mg/L, often even <0.1 mg/L, at least if iron- and manganese-oxides that precipitated during measurements were ignored. Such particle concentrations are probably hardly capable of adsorbing and carrying relevant amounts of contaminants, even very hydrophobic ones. Experiments using phenanthrene in contaminated sand additionally showed that it is probably hardly adsorbed onto already suspended particles, but mostly eroded from the particle depot in adsorbed or crystalline state. In laboratory experiments it was further shown that there is a huge depot of mobilizable particles in natural sediments that can continually release particles over a very long period of time. Break-through-experiments showed in addition that a large part of particles fed into the system are retained during the passage through a sediment sample and that they are re-released only very slowly. However, there also was a fast (preferential) break-through. In model calculations it could be shown that such a particle transport can neither be sufficiently described by the filter-function nor by the transport equation. Because of that it is necessary to take into consideration a preferential transport on the one hand and a strong retardation on the other. The last one can at the best be described by several retardation-factors or a dynamic particle-depot having constant attachment- and detachment rates.