Rudolf Weeber

ESPResSo 3.1: Molecular Dynamics Software for Coarse-Grained Models

ESPResSo is a package for Molecular Dynamics (MD) simulations of coarse-grained models. We present the most recent version 3.1 of our software, highlighting some recent algorithmic extensions to version 1.0 presented in a previous paper (Limbach et al. Comput Phys Commun 174:704–727, 2006). A major strength of our package is the multitude of implemented methods for calculating Coulomb and dipolar interactions in periodic and partially periodic geometries. Here we present some more recent additions which include methods for systems with dielectric contrasts that frequently occur in coarse-grained models of charged systems with implicit water models, and an alternative, completely local electrostatic solver that is based on the electrodynamic equations. We also describe our approach to rigid body dynamics that uses MD particles with fixed relative positions. ESPResSo now gained the ability to add bonds during the integration, which allows to study e.g. agglomeration. For hydrodynamic interactions, a thermalized lattice Boltzmann solver has been built into ESPResSo, which can be coupled to the MD particles. This computationally expensive algorithm can be greatly accelerated by using Graphics Processing Units. For the analysis of time series spanning many orders of magnitude in time scales, we implemented a hierarchical generic correlation algorithm for user-configurable observables.

Ferrofluids with shifted dipoles: ground state structures

In the past decades, ferrofluids have become relevant in many applications ranging from engineering to medicine, and have attracted the interest of scientists from many fields. To understand the physical mechanisms serving as a basis for these applications, over the last decades, many of the properties of ferrofluids have been studied and can now be controlled. However, in order to fine-tune various aspects of the interactions in the system and hopefully – in the long run – to build tailored structures, in recent years, magnetic nanoparticles and colloids that deviate from the model of a spherical particle with a dipole moment at its center were examined. Among them are dumbbells, magnetic core-shell particles, elongated ferro-particles, and colloidal particles with a magnetic cap. In this paper, we introduce and examine – using analytical calculations and Monte Carlo simulations – one such model system, namely, magnetic particles in which the dipole moment is shifted from the center of mass towards the particles surface. In this way, an additional anisotropy is introduced to the particles, which results in quite different and surprising microscopic properties of suspensions. Here, we mainly concentrate on ground states of small clusters of shifted-dipole particles, but also take a first glance on suspensions at finite temperature.

Deformation mechanisms in 2D magnetic gels studied by computer simulations

Magnetic gels, so-called ferrogels, consist of a polymer network, into which magnetic nanoparticles are embedded. The interesting properties of ferrogels originate from a complex interplay of the mechanical properties of the polymers with the magnetic interactions of the embedded nanoparticles. The ability to control the system by an external magnetic field may give rise to applications in medicine and engineering. In this paper, we propose and examine two microscopical simulation models for a 2D ferrogel which are suited to explain two distinct mechanisms of deformation in such a system. The first model focusses on deformation of the gel due to the dipole–dipole interaction between the magnetic nanoparticles. In an external magnetic field, a gel of this kind elongates in the direction parallel to the field and shrinks in the perpendicular direction. The second model deals with a distortion of the polymer matrix due to the transmission of torques from the magnetic nanoparticles to the polymer network. In this model, we observe an isotropic shrinking of the gel in an external magnetic field. As the observed deformations are very different in the two models, we conclude that the magnetoelastic behaviour of a magnetic gel strongly depends on the microscopical details of, both, the structure of the network and the coupling between the polymers and the magnetic nanoparticles. This may help to explain seemingly contradicting evidence from different experiments.

Buckling of paramagnetic chains in soft gels

We study the magneto-elastic coupling behavior of paramagnetic chains in soft polymer gels exposed to external magnetic fields. To this end, a laser scanning confocal microscope is used to observe the morphology of the paramagnetic chains together with the deformation field of the surrounding gel network. The paramagnetic chains in soft polymer gels show rich morphological shape changes under oblique magnetic fields, in particular a pronounced buckling deformation. The details of the resulting morphological shapes depend on the length of the chain, the strength of the external magnetic field, and the modulus of the gel. Based on the observation that the magnetic chains are strongly coupled to the surrounding polymer network, a simplified model is developed to describe their buckling behavior. A coarse-grained molecular dynamics simulation model featuring an increased matrix stiffness on the surfaces of the particles leads to morphologies in agreement with the experimentally observed buckling effects.

Colloids dragged through a polymer solution: Experiment, theory, and simulation

We present microrheological measurements of the drag force on colloids pulled through a solution of lambda-DNA (used here as a monodisperse model polymer) with an optical tweezer. The experiments show a drag force that is larger than expected from the Stokes formula and the independently measured viscosity of the DNA solution. We attribute this to the accumulation of DNA in front of the colloid and the reduced DNA density behind the colloid. This hypothesis is corroborated by a simple drift-diffusion model for the DNA molecules, which reproduces the experimental data surprisingly well, as well as by corresponding Brownian dynamics simulations.

Micro-rheology on (polymer-grafted) colloids using optical tweezers

Optical tweezers are experimental tools with extraordinary resolution in positioning (± 1 nm) a micron-sized colloid and in the measurement of forces (± 50 fN) acting on it-without any mechanical contact. This enables one to carry out a multitude of novel experiments in nano- and microfluidics, of which the following will be presented in this review: (i) forces within single pairs of colloids in media of varying concentration and valency of the surrounding ionic solution, (ii) measurements of the electrophoretic mobility of single colloids in different solvents (concentration, valency of the ionic solution and pH), (iii) similar experiments as in (i) with DNA-grafted colloids, (iv) the nonlinear response of single DNA-grafted colloids in shear flow and (v) the drag force on single colloids pulled through a polymer solution. The experiments will be described in detail and their analysis discussed.

Microstructure and magnetic properties of magnetic fluids consisting of shifted dipole particles under the influence of an external magnetic field

We investigate the structure of a recently proposed magnetic fluid consisting of shifted dipolar (SD) particles in an externally applied magnetic field via computer simulations. For standard dipolar fluids the applied magnetic field usually enhances the dipole-dipole correlations and facilitates chain formation whereas in the present system the effect of an external field can result in a break-up of clusters. We thoroughly investigate the origin of this phenomenon through analyzing first the ground states of the SD-particle systems as a function of an applied field. In a second step we quantify the microstructure of these systems as functions of the shift parameter, the effective interaction parameter, and the applied magnetic field strength. We conclude the paper by showing that with the proper choice of parameters, it is possible to create a system of SD-particles with highly interacting magnetic particles, whose initial susceptibility is below the Langevin susceptibility, and which remains spatially isotropic even in a very strong external magnetic field.

Magnetic Flux Topology of 2D Point Dipoles

Magnetic fields exhibit higher‐order, nonlinear singularities in the form of point‐dipole singularities. In addition, due to absence of divergence, they feature only a subset of invariant structures from traditional vector field topology. For magnetic fields of sets of point dipoles—widely present in physics and often used as an approximation—we present a technique revealing the topology of magnetic flux. The flux topology is identified with areas covered by field lines that directly connect pairs of dipoles. We introduce the dipole connectrix as a reduced one‐manifold representation of those areas. The set of connectrices serves as our concise visualization of the global structure of magnetic flux. In addition, the quantitative values of flux are displayed by the thickness of the connectrices. We evaluate our technique for simulations of ferroparticle monolayers and magnetic gels.

Modeling Nanoparticle Agglomeration using Local Interactions

Nanoparticle agglomeration plays an important role in processes such as spray drying and particle flame synthesis. These processes have in common that nanoparticles collide at low concentrations and get irreversibly linked at the point of contact due to plastic deformation. In this article, we investigate several models of irreversible connections, which require only local interactions between the colliding nanoparticles and thus allow for scalable simulations. The models investigated here connect the particles upon collision by non-bonded strongly attractive interactions, bonded interactions or by binding agents placed at the point of contact. Models using spherically symmetric interactions form compact agglomerates and are therefore unsuitable to study agglomeration. In contrast, models that are either based on both central and angular potentials (type one) or on binding agents (type two) efficiently prevent restructuring of the agglomerates, and are therefore useful for modeling contacts formed by plastic deformation. Moreover, both types of models allow to control the rigidity and by that the degree of restructuring. The first type of model is computationally more efficient at low fractional dimensions of the aggregates, while the second gives easy access to local shear forces, which is important when breaking of agglomerates is to be considered. As example applications, we reproduce the well-known diffusion-limited agglomeration (DLA) and report results on soot aggregation. Copyright 2014 American Association for Aerosol Research

Hydrodynamic interactions in active colloidal crystal microrheology.

In dense colloids it is commonly assumed that hydrodynamic interactions do not play a role. However, a found theoretical quantification is often missing. We present computer simulations that are motivated by experiments where a large colloidal particle is dragged through a colloidal crystal. To qualify the influence of long-ranged hydrodynamics, we model the setup by conventional Langevin dynamics simulations and by an improved scheme with limited hydrodynamic interactions. This scheme significantly improves our results and allows to show that hydrodynamics strongly impacts the development of defects, the crystal regeneration, as well as the jamming behavior.