arXiv subjects

Andreas Heuer

Publications and source records attributed to Andreas Heuer.

At least 55 records · Page 3Linked to original sources

arXiv2017

Comparing kinetic Monte Carlo and Thin-Film Modeling of Transversal Instabilities of Ridges on Patterned Substrates

We employ kinetic Monte Carlo (KMC) simulations and a thin-film continuum model to comparatively study the transverse (i.e., Plateau-Rayleigh) instability of ridges formed by molecules on pre-patterned substrates. It is demonstrated that the evolution of the occurring instability qualitatively agrees between the two models for a single ridge as well as for two weakly interacting ridges. In particular, it is shown for both models that the instability occurs on well defined length and time scales which are, for the KMC model, significantly larger than the intrinsic scales of thermodynamic fluctuations. This is further evidenced by the similarity of dispersion relations characterising the linear instability modes.

cond-mat.soft↗

arXiv2016

How to model the interaction of charged Janus particles

We analyse the interaction of charged Janus particles including screening effects. The explicit interaction is mapped via a least square method on a variable number $n$ of systematically generated tensors that reflect the angular dependence of the potential. For $n = 2$ we show that the interaction is equivalent to a model previously described by Erdmann, Kröger and Hess (EKH). Interestingly, this mapping is not able to capture the subtleties of the interaction for small screening lengths. Rather, a larger number of tensors has to be used. We find that the characteristics of the Janus type interaction plays an important role for the aggregation behaviour. We obtained cluster structures up to the size of $13$ particles for $n = 2$ and $36$ and screening lengths $κ^{-1} = 0.1$ and $1.0$ via Monte Carlo simulations. The influence of the screening length is analysed and the structures are compared to results for an electrostatic-type potential and for multipole-expanded Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We find that a dipole-like potential (EKH or dipole DLVO approximation) is not able to sufficiently reproduce the anisotropy effects of the potential. Instead, a higher order expansion has to be used to obtain clusters structures that are identical to experimental results for up to $N = 8$ particles. The resulting minimum-energy clusters are compared to those of sticky hard sphere systems. Janus particles with a short-range screened interaction resemble sticky hard sphere clusters for all considered particle numbers, whereas for long-range screening even very small clusters are structurally different.

cond-mat.soft↗

arXiv2016

Area confined position control of molecular aggregates

We report an experimental approach to control the position of molecular aggregates on surfaces by vacuum deposition. The control is accomplished by regulating the molecular density on the surface in a confined area. The diffusing molecules are concentrated at the centre of the confined area, producing a stable cluster when reaching the critical density for nucleation. Mechanistic aspects of that control are obtained from kinetic Monte Carlo simulations. The dimensions of the position can further be controlled by varying the beam flux and the substrate temperature.

physics.chem-ph↗

arXiv2015

Understanding the Nonlinear Dynamics of Driven Particles in Supercooled Liquids in Terms of an Effective Temperature

In active microrheology the mechanical properties of a material are tested by adding probe particles which are pulled by an external force. In case of supercooled liquids, strong forcing leads to a thinning of the host material which becomes more pronounced as the system approaches the glass transition. In this work we provide a quantitative theoretical description of this thinning behavior based on the properties of the Potential Energy Landscape (PEL) of a model glass-former. A key role plays the trap-like nature of the PEL. We find that the mechanical properties in the strongly driven system behave the same as in a quiescent system at an enhanced temperature, giving rise to a well-characterized effective temperature. Furthermore, this effective temperature turns out to be independent of the chosen observable and individually shows up in the thermodynamic and dynamic properties of the system. Based on this underlying theoretical understanding, we can estimate its dependence on temperature and force by the PEL-properties of the quiescent system. We furthermore critically discuss the relevance of effective temperatures obtained by scaling relations for the description of out-of-equilibrium situations.

cond-mat.soft↗

arXiv2015

Instabilities of layers of deposited molecules on chemically stripe patterned substrates: Ridges vs. drops

A mesoscopic continuum model is employed to analyse the transport mechanisms and structure formation during the redistribution stage of deposition experiments where organic molecules are deposited on a solid substrate with periodic stripe-like wettability patterns. Transversally invariant ridges located on the more wettable stripes are identified as very important transient states and their linear stability is analysed. It is found that there exist two different instability modes that result (i) at large ridge volume in the formation of bulges that spill from the more wettable stripes onto the less wettable bare substrate and (ii) at small ridge volume in the formation of small droplets located on the more wettable stripes. These predictions are confirmed by direct numerical simulations of the fully nonlinear evolution equation for two-dimensional substrates. In addition, the influence of different transport mechanisms during redistribution is investigated focusing on the cases of convective transport with no-slip at the substrate, transport via diffusion in the film bulk and via diffusion at the film surface. In particular, it is shown that the transport process does neither influence the linear stability thresholds nor the sequence of morphologies observed in the time simulation, but only the ratio of the time scales of the different process phases.

cond-mat.mes-hall↗

arXiv2014

Chain End Mobilities in Polymer Melts - A Computational Study

The Rouse model can be regarded as the standard model to describe the dynamics of a short polymer chain under melt conditions. In this contribution, we explicitly check one of the fundamental assumptions of this model, namely that of a uniform friction coefficient for all monomers, on the basis of MD simulation data of a poly(ethylene oxide) (PEO) melt. This question immediately arises from the fact that in a real polymer melt the terminal monomers have on average more intermolecular neighbors than the central monomers, and one would expect that exactly these details affect the precise value of the friction coefficient. The mobilities are determined by our recently developed statistical method, which provides detailed insights about the local polymer dynamics. Moreover, it yields complementary information to that obtained from the mean square displacement (MSD) or the Rouse mode analysis. It turns out that the Rouse assumption of a uniform mobility is fulfilled to a good approximation for the PEO melt. However, a more detailed analysis reveals that the underlying microscopic dynamics is highly affected by different contributions from intra- and intermolecular excluded volume interactions, which cannot be taken into account by a modified friction coefficient. Minor deviations occur only for the terminal monomers on larger time scales, which can be attributed to the presence of two different escape mechanisms from their first coordination sphere. These effects remain elusive when studying the dynamics with the MSD only.

cond-mat.soft↗

arXiv2014

Physical mechanisms of nonlinear conductivity: a model analysis

Nonlinear effects are omnipresent in thin films of ion conducting materials showing up as a significant increase of the conductivity. For a disordered hopping model general physical mechanisms are identified giving rise to the occurrence of positive or negative nonlinear effects, respectively. Analytical results are obtained in the limit of high but finite dimensions. They are compared with the numerical results for 3D up to 6D systems. A very good agreement can be found, in particular for higher dimensions. The results can also be used to rationalize previous numerical simulations. The implications for the interpretation of nonlinear conductivity experiments on inorganic ion conductors are discussed.

cond-mat.dis-nn↗

arXiv2014

A combined theoretical and experimental study of the influence of different anion ratios on lithium ion dynamics in ionic liquids

In this paper we have investigated via experimental and simulations techniques the transport properties, in terms of total ionic conductivity and ion diffusion coefficients, of ionic liquids and lithium salt mixtures composed of two anions, bis(fluorosulfonyl)imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI), and two cations, N-ethyl-N-methylimidazolium (emim) and lithium. The comparison of the experimental results with the simulations shows an exceptional agreement over a wide temperature range. Of particular interest is the high agreement achieved at lower temperature, which is a result of the extended simulation length of, at least, a factor of 5 compared with previous work. The results show as the addition of TFSI favor the formation of lithium dimers (Li+ - TFSI- - Li+). A closer analysis of such dimers shows that involved lithium ions move nearly as fast as single lithium ions although they have a different coordination and much slower TFSI exchange rates.

cond-mat.mtrl-sci↗

arXiv2013

A Simulation Study of the Lithium Ion Transport Mechanism in Ternary Polymer Electrolytes - The Critical Role of the Segmental Mobility

We present an extensive molecular dynamics (MD) simulation study of the lithium ion transport in ternary polymer electrolytes consisting of poly(ethylene oxide) (PEO), lithium-bis(trifluoromethane)sulfonimide (LiTFSI) and the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonimide (PYR13TFSI). In particular, we focus on two different strategies by which the ternary electrolytes can be devised, namely by (a) adding the ionic liquid to PEO20LiTFSI, and (b) substituting the PEO chains in PEO20LiTFSI by the ionic liquid. In order to grasp the changes of the overall lithium transport mechanism, we employ an analytical, Rouse-based cation transport model (Maitra et al., Phys. Rev. Lett., 2007, 98, 227802), which has originally been devised for binary PEO-based electrolytes. This model distinguishes three different microscopic transport mechanisms, each quantified by an individual time scale. In the course of our analysis, we extend this mathematical description to account for an entirely new transport mechanism, namely the TFSI-supported diffusion of lithium ions decoupled from the PEO chains, which emerges for certain stoichiometries. We find that the segmental mobility plays a decisive role in PEO-based polymer electrolytes. That is, whereas the addition of the ionic liquid to PEO20LiTFSI plasticizes the polymer network and thus also increases the lithium diffusion, the amount of free, mobile ether oxygens reduces when substituting the PEO chains by the ionic liquid, which compensates the plasticizing effect. In total, our observations allow us to formulate some general principles about the lithium ion transport mechanism in ternary polymer electrolytes. Moreover, our insights also shed light on recent experimental observations (Joost et al., Electrochim. Acta, 2012, 86, 330).

cond-mat.soft↗

arXiv2013

Particle dynamics in two-dimensional random energy landscapes - experiments and simulations

The dynamics of individual colloidal particles in random potential energy landscapes were investigated experimentally and by Monte Carlo simulations. The value of the potential at each point in the two-dimensional energy landscape follows a Gaussian distribution. The width of the distribution, and hence the degree of roughness of the energy landscape, was varied and its effect on the particle dynamics studied. This situation represents an example of Brownian dynamics in the presence of disorder. In the experiments, the energy landscapes were generated optically using a holographic set-up with a spatial light modulator, and the particle trajectories were followed by video microscopy. The dynamics are characterized using, e.g., the time-dependent diffusion coefficient, the mean squared displacement, the van Hove function and the non-Gaussian parameter. In both, experiments and simulations, the dynamics are initially diffusive, show an extended sub-diffusive regime at intermediate times before diffusive motion is recovered at very long times. The dependence of the long-time diffusion coefficient on the width of the Gaussian distribution agrees with theoretical predictions. Compared to the dynamics in a one-dimensional potential energy landscape, the localization at intermediate times is weaker and the diffusive regime at long times reached earlier, which is due to the possibility to avoid local maxima in two-dimensional energy landscapes.

cond-mat.soft↗

arXiv2013

Low concentrated hydroxyectoine solutions in presence of DPPC lipid bilayers: a computer simulation study

The influence of hydroxyectoine on the properties of the aqueous solution in presence of DPPC lipid bilayers is studied via semi-isotropic constant pressure (NPT) Molecular Dynamics simulations. We investigate the solvent-co-solute behavior in terms of Kirkwood-Buff integrals as well as hydrogen bond life times for an increasing hydroxyectoine concentration up to 0.148 mol/L. The observed preferential exclusion mechanism identifies hydroxyectoine as a kosmotropic osmolyte. Our findings in regards to the DPPC lipid bilayer indicate an increase of the surface pressure as well as the solvent accessible surface area in presence of higher hydroxyectoine concentrations. The results are in agreement to the outcome of recent experiments. With this study, we are able to validate the visibility of co-solute-solute-solvent effects for low and physiologically relevant osmolyte concentrations.

physics.bio-ph↗

arXiv2013

Deposition of model chains on surfaces: anomalous relation between flux and stability

Model chains are studied via Monte Carlo simulations which are deposited with a fixed flux on a substrate. They may represent, e.g., stiff lipophilic chains with an head group and tail groups mimicking the alkyl chain. After some subsequent fixed simulation time we determine the final energy as a function of flux and temperature. Surprisingly, in some range of temperature and flux the final energy increases with decreasing flux. The physical origin of this counterintuitive observation is elucidated. In contrast, when performing equivalent cooling experiments no such anomaly is observed. Furthermore, it is elaborated whether flux experiments give rise to configurations with lower energies as compared to cooling experiments. These results are related to recent experiments by the Ediger group where very stable configurations of glass-forming systems have been generated via flux experiments.

cond-mat.mtrl-sci↗

arXiv2013

Lithium Ion Transport Mechanism in Ternary Polymer Electrolyte-Ionic Liquid Mixtures - A Molecular Dynamics Simulation Study

The lithium transport mechanism in ternary polymer electrolytes, consisting of PEO/LiTFSI and various fractions of the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonimide, are investigated by means of MD simulations. This is motivated by recent experimental findings [Passerini et al., Electrochim. Acta 2012, 86, 330-338], which demonstrated that these materials display an enhanced lithium mobility relative to their binary counterpart PEO/LiTFSI. In order to grasp the underlying microscopic scenario giving rise to these observations, we employ an analytical, Rouse-based cation transport model [Maitra at al., PRL 2007, 98, 227802], which has originally been devised for conventional polymer electrolytes. This model describes the cation transport via three different mechanisms, each characterized by an individual time scale. It turns out that also in the ternary electrolytes essentially all lithium ions are coordinated by PEO chains, thus ruling out a transport mechanism enhanced by the presence of ionic-liquid molecules. Rather, the plasticizing effect of the ionic liquid contributes to the increased lithium mobility by enhancing the dynamics of the PEO chains and consequently also the motion of the attached ions. Additional focus is laid on the prediction of lithium diffusion coefficients from the simulation data for various chain lengths and the comparison with experimental data, thus demonstrating the broad applicability of our approach.

cond-mat.soft↗

arXiv2013

The frequency-dependence of nonlinear conductivity in disordered systems: an analytically solvable model

For the hopping dynamics in a one-dimensional model, containing energy and barrier disorder, we determine the linear and nonlinear response to an external field for arbitrary external frequencies. The calculation is performed in analytical terms. We systematically analyze the parameter space and find three different regimes, corresponding to qualitatively different frequency dependencies of the nonlinear response. Two regimes agree with the results of recent conductivity experiments on inorganic ion conductors and ionic liquids, respectively. The ratio of the nonlinear and linear conductivity in the dc-regime can be explicitly expressed in terms of the disorder parameters. As a generic feature the nonlinear conductivity displays a minimum as a function of frequency which can be identified with forward-backward dynamics in a double-well potential. The magnitude and sign of the nonlinear conductivity around the minimum is a measure of the disorder, inherent in this model. Surprisingly, the frequency of the minimum is hardly influenced by the disorder.

cond-mat.dis-nn↗

arXiv2012

Anomalous diffusion of driven particles in supercooled liquids

We have performed non-equilibrium dynamics simulations of a binary Lennard-Jones mixture in which an external force is applied on a single tagged particle. For the diffusive properties of this particle parallel to the force superdiffusive behavior at intermediate times as well as giant long-time diffusivity is observed. A quantitative description of this non-trivial behavior is given by a continuous time random walk analysis of the system in configuration space. We further demonstrate, that the same physical properties which are responsible for the superdiffusivity in non-equilibrium systems also determine the non-Gaussian parameter in equilibrium systems.

cond-mat.stat-mech↗

arXiv2012

Microrheology of supercooled liquids in terms of a continuous time random walk

Molecular dynamics simulations of a glass-forming model system are performed under application of a microrheological perturbation on a tagged particle. The trajectory of that particle is studied in its underlying potential energy landscape. Discretization of the configuration space is achieved via a metabasin analysis. The linear and nonlinear responses of drift and diffusive behavior can be interpreted and analyzed in terms of a continuous time random walk. In this way the physical origin of linear and nonlinear response can be identified. Critical forces are determined and compared with predictions from literature.

cond-mat.stat-mech↗

arXiv2012

How coupled elementary units determine the dynamics of macroscopic glass-forming systems

We investigate the dynamics of a binary mixture Lennard-Jones system of different system sizes with respect to the importance of the properties of the underlying potential energy landscape (PEL). We show that the dynamics of small systems can be very well described within the continuous time random walk formalism, which is determined solely by PEL parameters. Finite size analysis shows that the diffusivity of large and small systems are very similar. This suggests that the PEL parameters of the small system also determine the local dynamics in large systems. The structural relaxation time, however, displays significant finite size effects. Furthermore, using a non-equilibrium configuration of a large system, we find that causal connections exist between close-by regions of the system. These findings can be described by the coupled landscape model for which a macroscopic system is described by a superposition of elementary systems each described by its PEL. A minimum coupling is introduced which accounts for the finite size behavior. The coupling strength, as the single adjustable parameter, becomes smaller closer to the glass transition.

cond-mat.stat-mech↗

arXiv2012

Towards the perfect prediction of soccer matches

We present a systematic approach to the prediction of soccer matches. First, we show that the information about chances for goals is by far more informative than about the actual results. Second, we present a multivariate regression approach and show how the prediction quality increases with increasing information content. This prediction quality can be explicitly expressed in terms of just two parameters. Third, by disentangling the systematic and random components of soccer matches we can identify the optimum level of predictability. These concepts are exemplified for the German Bundesliga.

physics.data-an↗