arXiv subjects

Andreas Heuer

Publications and source records attributed to Andreas Heuer.

At least 37 records · Page 2Linked to original sources

arXiv2022

ChaTEAU: A Universal Toolkit for Applying the Chase

What do applications like semantic optimization, data exchange and integration, answering queries under dependencies, query reformulation with constraints, and data cleaning have in common? All these applications can be processed by the Chase, a family of algorithms for reasoning with constraints. While the theory of the Chase is well understood, existing implementations are confined to specific use cases and application scenarios, making it difficult to reuse them in other settings. ChaTEAU overcomes this limitation: It takes the logical core of the Chase, generalizes it, and provides a software library for different Chase applications in a single toolkit.

cs.DB↗

arXiv2022

Wetting dynamics under periodic switching on different scales: Characterization and mechanisms

The development of substrates with a switchable wettability is on a fast pace. The limit of switching frequencies and contact angle differences between substrate states are steadily pushed further. We investigate the behavior of a droplet on a homogeneous substrate, which is switched between two wettabilities for a large range of switching frequencies. Here, we are particularly interested in the dependence of the wetting behavior on the switching frequency. We show, that results obtained on the particle level via molecular dynamics simulations and on the continuum level via the thin-film model are consistent. Predictions of simple models as the molecular theory of wetting (MKT) and analytical calculations based on the MKT also show good agreement and offer deeper insights into the underlying mechanisms.

physics.flu-dyn↗

arXiv2021

Frame-guided assembly from a theoretical perspective

The molecular self-assembly of various structures such as micelles and vesicles has been the subject of comprehensive studies. Recently, a new approach to design these structures, the frame-guided assembly, has been developed to progress towards fabrics of predefined shape and size, following an initially provided frame of guiding elements. Here we study frame-guided assembly into a two-dimensional membrane via computer simulations, based on a single-bead coarse grained surfactant model in continuous space. In agreement with the experiment the assembly process already starts for surfactant concentrations below the critical micelle concentration. Furthermore, upon decreasing temperature the formation process gets more localized. Additionally, we consider a lattice gas model of the membrane plane including guiding elements where the particle concentration is varied via a chemical potential. It behaves similar to the continuous model and additionally allows the formulation of analytical mean-field predictions which provide a fundamental understanding of frame-guided assembly.

physics.chem-ph↗

arXiv2021

Impact of Charged Surfaces on the Structure and Dynamics of Polymer Electrolytes: Insights from Atomistic Simulations

Polymer electrolytes are intensely investigated for use as solid electrolytes in next generation lithium-ion and lithium-metal batteries. However, little is known about the structural and dynamical properties of polymer electrolytes close to electrode surfaces. Here, a PEO-LiTFSI polymer electrolyte, confined between two oppositely charged graphite-like electrodes, is studied via molecular dynamics simulations. Three different surface charges of $σ_S = 0$, $\pm 0.5$ and $\pm 1$ $e$/nm$^2$ are considered. Upon charging, a very strong and component-specific layering is observed. Only for the highest surface charge, lithium ions get desolvated and come into direct contact with the negative electrode. The layer structure goes along with the emergence of free energy barriers, which lead to a reduction of the lithium-ion dynamics, as quantified by spatially resolved mean square displacements, corrected for a drift component. Interchain transfers that are known to be very important for long-range lithium-ion transport in polymer electrolytes play no significant role for transitions of lithium ions between different layers.

cond-mat.soft↗

arXiv2021

Controlling $\text{Li}^+$ transport in ionic liquid electrolytes through salt content and anion asymmetry: A mechanistic understanding gained from molecular dynamics simulations

In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide ($\text{TFSI}^-$) anion or its asymmetric analog 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide ($\text{TFSAM}^-$). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the $\text{TFSAM}^-$-based ILEs that has been observed in recent experiments: For increased salt doping, the lithium ions must compete for the more attractive cyano over oxygen coordination and a fragmented landscape of solvation geometries emerges, in which lithium appears to be less strongly bound. We present a novel, yet statistically straightforward methodology to quantify the extent to which lithium and its solvation shell are dynamically coupled. By means of a Lithium Coupling Factor (LCF) we demonstrate that the shell anions do not constitute a stable lithium vehicle, which suggests for this electrolyte material the commonly termed "vehicular" lithium transport mechanism could be more aptly pictured as a correlated, flow-like motion of lithium and its neighbourhood. Our analysis elucidates two separate causes why lithium and shell dynamics progressively decouple with higher salt content: On the one hand, an increased sharing of anions between lithium limits the achievable LCF of individual lithium-anion pairs. On the other hand, weaker binding configurations naturally entail a lower dynamic stability of the lithium-anion complex, which is particularly relevant for the $\text{TFSAM}^-$-containing ILEs.

physics.chem-ph↗

arXiv2021

Effect of Cholesterol vs. Ergosterol on DPPC Bilayer Properties: Insights from Atomistic Simulations

Sterols have been ascribed a major role in the organization of biological membranes, in particular for the formation of liquid ordered domains in complex lipid mixtures. Here, we employed molecular dynamics simulations to compare the effects of cholesterol and ergosterol as the major sterol of mammalian and fungal cells, respectively, on binary mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as a proxy for saturated lipids. In agreement with previous work, we observe that the addition of sterol molecules modifies the order of DPPC both in the gel phase and in the liquid phase. When disentangling the overall tilt angle and the structure of the tail imposed by trans/gauche configurations of torsion angles in the tail, respectively, a more detailed picture of the impact of sterols can be formulated, revealing, e.g., an approximate temperature-concentration superposition ranging from the liquid to the gel phase. Furthermore, a new quantitative measure to identify the presence of collective sterol effects is discussed. Moreover, when comparing both types of sterols, addition of cholesterol has a noticeably stronger impact on phospholipid properties than of ergosterol. The observed differences can be attributed to higher planarity of the cholesterol ring system. This planarity combined with an inherent asymmetry in its molecular interactions leads to better alignment and hence stronger interaction with saturated acyl chains. Our results suggest that the high order demonstrated for ergosterol in fungal plasma membranes must therefore be generated via additional mechanisms.

physics.bio-ph↗

arXiv2021

Chain ordering of phospholipids in membranes containing cholesterol: What matters?

Cholesterol (CHOL) drives lipid segregation and is thus a key player for the formation of lipid rafts and followingly for the ability of a cell to, e.g., enable selective agglomeration of proteins. The lipid segregation is driven by cholesterol's affinity for saturated lipids, which stands directly in relation to the ability of cholesterol to order the individual phospholipid (PL) acyl chains. In this work, Molecular Dynamics simulations of DPPC (Dipalmitoylphosphatidylcholine, saturated lipid) and DLiPC (Dilineoylphosphatidylcholine, unsaturated lipid) mixtures with cholesterol are used to elucidate the underlying mechanisms of the cholesterol ordering effect. To this end, all enthalpic contributions, experienced by the PL molecules, are recorded as a function of the PL's acyl chain order. This involves, the PL-PL, the PL-cholesterol interaction, the interaction of the PLs with water, and the interleaflet interaction. This systematic analysis allows one to unravel differences of saturated and unsaturated lipids in terms of the different interaction factors. It turns out that cholesterol's impact on chain ordering stems not only from direct interactions with the PLs but is also indirectly present in the other energy contributions. Furthermore, the analysis sheds light on the relevance of the entropic contributions, related to the degrees of freedom of the acyl chain.

physics.bio-ph↗

arXiv2021

Privacy Aspects of Provenance Queries

Given a query result of a big database, why-provenance can be used to calculate the necessary part of this database, consisting of so-called witnesses. If this database consists of personal data, privacy protection has to prevent the publication of these witnesses. This implies a natural conflict of interest between publishing original data (provenance) and protecting these data (privacy). In this paper, privacy goes beyond the concept of personal data protection. The paper gives an extended definition of privacy as intellectual property protection. If the provenance information is not sufficient to reconstruct a query result, additional data such as witnesses or provenance polynomials have to be published to guarantee traceability. Nevertheless, publishing this provenance information might be a problem if (significantly) more tuples than necessary can be derived from the original database. At this point, it is already possible to violate privacy policies, provided that quasi identifiers are included in this provenance information. With this poster, we point out fundamental problems and discuss first proposals for solutions.

cs.DB↗

arXiv2020

Polymer electrolytes in strong external electric fields: Modification of structure and dynamics

We present the results from an extensive atomistic molecular dynamics simulation study of poly(ethylene oxide) (PEO) doped with various amounts of lithium-bis(trifluoromethane)sulfonimide (LiTFSI) salt under the influence of external electric field strengths up to $1\,$V/nm. The motivation stems from recent experimental reports on the nonlinear response of mobilities to the application of an electric field in such electrolyte systems and arising speculations on field-induced alignment of the polymer chains, creating channel-like structures that facilitate ion passage. Hence, we systematically examine the electric field impact on the lithium coordination environment, polymer structure as well as ionic transport properties and further present a procedure to quantify the susceptibility of both structural and dynamical observables to the external field. Our investigation reveals indeed a coiled-to-stretched transformation of the PEO strands along with a concurrent nonlinear behavior of the dynamic properties. However, from studying the temporal response of the unperturbed electrolyte system to field application we are able to exclude a structurally conditioned enhancement of ion transport and surprisingly observe a slowing down. A microscopic understanding is supplied.

cond-mat.soft↗

arXiv2020

From identification of random contributions to determination of the optimum forecast of a soccer match

The forecasting of sports events is of broad interest from the applied but also from the theoretical perspective. In this work the question is addressed for the example of the German soccer Bundesliga how a theoretically optimum forecast of the goal difference of a match can be characterized. This involves a careful analysis of the random contributions in a match and its disentanglement from the informative contributions, resulting from the individual team strengths. An important aspect is the consideration of the time dependence of the team strength which turns out to mainly fluctuate around a team-specific value during the course of a season. Two types of time-dependent properties have to be distinguished, one being uncorrelated between different match-days, the other being correlated and thus accessible by an appropriate correlation analysis. For some performance indicators, which may be used to estimate the team strength, the quality of the respective forecast is compared to the theoretical optimum. Knowledge of the informative contribution allows one to conclude that the offensive team strength is more important than the defensive team strength for the final success.

physics.soc-ph↗

arXiv2020

Identification of relevant performance indicators in round-robin tournaments

A myriad of different data are generated to characterize a soccer match. Here we discuss which performance indicators are particularly helpful to forecast the future results of a team via an estimation of the underlying team strengths with minimum statistical uncertainty. We introduce an appropriate statistical framework and exemplify it for different performance indicators for the German premier soccer league. Two aspects are involved: (i) It is quantified how well the estimation process would work if no statistical noise due to finite information is present. The related score directly expresses to which degree the chosen performance indicator reflects the underlying team strength. (ii) Additionally, the reduction of the forecasting quality due to statistical noise is determined. From both pieces of information a normalized value can be constructed which is a direct measure of the overall forecasting quality. It turns out that the so-called packing rate works best. New perspectives of performance indicators enter when trying to understand the outcome of single matches based on match-specific observations from the same match. Implications for the purpose of forecasting as well as consequences for the interpretation of team strengths are discussed.

physics.soc-ph↗

arXiv2019

Non-equilibrium effects of micelle formation as studied by a minimum particle-based model

The formation of self assembled structures such as micelles has been intensively studied and is well understood. The ability of a solution of amphiphilic molecules to develop micelles is depending on the concentration and characterized by the critical micelle concentration (cmc), above which micelle formation does occur. Recent studies use a lattice approach in order to determine cmc and show that the correct modelling and analysis of cluster formations is highly non-trivial. We developed a minimalistic coarse grained model for amphiphilic molecules in the continuum and simulated the time evolution via dynamic Monte Carlo simulations in the canonical (NVT) ensemble. Starting from a homogeneous system we observed and characterized how the initial fluctuations, yielding small aggregates of amphiphilic molecules, end up in the growth of complete micelles. Our model is sufficiently versatile to account for different structures of surfactant systems such as membranes, micelles of variable radius and tubes at high particle densities by adjusting particle density and potential properties. Particle densities and micellization rates are investigated and an order parameter is introduced, so that the dependence of the micellization process on temperature and surfactant density can be studied. The constant density of free particles for concentrations above cmc, e. g. as expected from theoretical considerations, can be reproduced when choosing a careful definition of free volumes. In the cmc regime at low temperatures different non-equilibrium effects are reported, occurring even for very long time-scales.

cond-mat.soft↗

arXiv2019

Relating local structures, energies, and occurrence probabilities in a two-dimensional silica network

Recently, it became possible to experimentally generate and characterize a very thin silica system on a substrate which can be basically described as a 2D random network. The key structural properties, in particular related to the ring statistics, could be numerically reproduced by performing molecular dynamics simulations with an appropriately chosen 2D force field. Using a maximum entropy formulation it is shown that the probability distribution of the individual rings and triplets can be related to the ring and triplet energies, respectively, obtained from the simulations. Using additional Lagrange parameters, the correct average properties of random networks are guaranteed. In agreement with previous work, based on distributions of complementary rings and triplets, respectively, one finds a Boltzmann-type relation albeit with an effective temperature which largely deviates from the bath temperature. Furthermore, it is shown that the ring and triplet energies can be estimated based on the properties of their average inner angles. This calculation supports, on a quantitative level, the previously suggested angle mismatch theory. It suggests that correlations among adjacent rings originate from the net mismatch in the inner ring angles in a triplet of rings. By taking into account an average effect from the surrounding rings of a triplet, an even better estimate of the correlations can be provided. That approach is also applied to estimate of the Aboav-Wearie parameter.

cond-mat.dis-nn↗

arXiv2018

Ring statistics in 2D-silica: effective temperatures in equilibrium

The thermodynamic properties of subsystems in strong interaction with the neighborhood can largely differ from the standard behavior. Here we study the thermodynamic properties of rings and triplets in equilibrated disordered 2D-silica. Their statistics follows a Boltzmann behavior, albeit with a strongly reduced temperature. This effective temperature strongly depends on the length scale of the chosen subsystem. From a systematic analysis of the 1D Ising model and an analytically solvable model we suggest that these observations reflect the presence of strong local positive energy correlations.

cond-mat.dis-nn↗

arXiv2018

Shearing small glass-forming systems: a potential energy landscape perspective

Understanding the yielding of glass-forming systems upon shearing is notoriously difficult since it is a strong non-equilibrium effect. Here we show that the concept of the potential energy landscape (PEL), developed for the quiescent state, can be extended to shearing. When introducing an appropriate coarse graining of the extended PEL for sheared systems, one can distinguish two fundamentally different types of plastic events, namely inherent structures (IS) vs. minimized structure (MS) transitions. We apply these concepts to non-cyclic and cyclic shearing of small systems, which allow us to characterize the properties of elementary plastic events. Whereas the general properties of the stress-strain curves are similar to larger systems, a closer analysis reveals significantly different properties. This allows one to identify the impact of the elastic coupling in larger systems. The concept of MS enables us to relate the stress overshoot of a single trajectory to the emergence of an MS transition. Furthermore, the occurrence of limit cycles can be characterized in great detail for the small systems and connections to the properties of the PEL can be formulated. Possible implications of our small-system results for the macroscopic limit are discussed.

cond-mat.soft↗

arXiv2018

Boundary-induced nucleation control: A theoretical perspective

The pre-patterning of a substrate to create energetically more attractive or repulsive regions allows one to generate a variety of structures in physical vapor deposition experiments. A particular interesting structure is generated if the energetically attractive region is forming a rectangular grid. For specific combinations of the particle flux, the substrate temperature and the lattice size it is possible to generate exactly one cluster per cell, giving rise to nucleation control. Here, we show that the experimental observations of nucleation control can be very well understood from a theoretical perspective. For this purpose we perform, on the one hand, kinetic Monte Carlo simulations and, on the other hand, use analytical scaling arguments to rationalize the observed behavior. For several observables, characterizing nucleation control, a very good agreement is found between experiment and theory. This underlines the generality of the presented mechanism to control the deposition of material by manipulation of the direct environment.

physics.chem-ph↗

arXiv2017

Nudged Elastic Band calculation of the binding potential for liquids at interfaces

The wetting behavior of a liquid on solid substrates is governed by the nature of the effective interaction between the liquid-gas and the solid-liquid interfaces, which is described by the binding or wetting potential $g(h)$ which is an excess free energy per unit area that depends on the liquid film height $h$. Given a microscopic theory for the liquid, to determine $g(h)$ one must calculate the free energy for liquid films of any given value of $h$; i.e. one needs to create and analyze out-of-equilibrium states, since at equilibrium there is a unique value of $h$, specified by the temperature and chemical potential of the surrounding gas. Here we introduce a Nudged Elastic Band (NEB) approach to calculate $g(h)$ and illustrate the method by applying it in conjunction with a microscopic lattice density functional theory for the liquid. We show too that the NEB results are identical to those obtained with an established method based on using a fictitious additional potential to stabilize the non-equilibrium states. The advantages of the NEB approach are discussed.

cond-mat.soft↗

arXiv2017

Exploring the free energy gain of phase separation via Markov State Modeling

The gain of free energy upon unmixing is determined via application of Markov state modeling (MSM), using an Ising model with a fixed number of up- and down-spins. MSM yields reasonable estimates of the free energies. However, a closer look reveals significant differences which point to residual non-Markovian effects. These non-Markovian effects are rather unexpected since the typical criteria to study the quality of Markovianity indicate complete Markovian behavior. We identify the sparse connectivity between different Markov states as a likely reason for the observed bias. By studying a simple five state model system we can analytically elucidate different sources of the bias and thus explain the different deviations that were observed for the Ising model. Based on this insight we can modify the determination of the count matrix in the MSM approach. In this way, the estimation of the free energy is significantly improved.

physics.comp-ph↗