M Markus Hütter

Parameterization of a reactive force field using a Monte Carlo algorithm

Parameterization of a molecular dynamics force field is essential in realistically modeling the physicochemical processes involved in a molecular system. This step is often challenging when the equations involved in describing the force field are complicated as well as when the parameters are mostly empirical. ReaxFF is one such reactive force field which uses hundreds of parameters to describe the interactions between atoms. The optimization of the parameters in ReaxFF is done such that the properties predicted by ReaxFF matches with a set of quantum chemical or experimental data. Usually, the optimization of the parameters is done by an inefficient single‐parameter parabolic‐search algorithm. In this study, we use a robust metropolis Monte‐Carlo algorithm with simulated annealing to search for the optimum parameters for the ReaxFF force field in a high‐dimensional parameter space. The optimization is done against a set of quantum chemical data for MgSO4 hydrates. The optimized force field reproduced the chemical structures, the equations of state, and the water binding curves of MgSO4 hydrates. The transferability test of the ReaxFF force field shows the extend of transferability for a particular molecular system. This study points out that the ReaxFF force field is not indefinitely transferable.

Collective behaviour of dislocations in a finite medium

We derive the grand-canonical partition function of straight and parallel dislocation lines without making a priori assumptions on the temperature regime. Such a systematic derivation for dislocations has, to the best of our knowledge, not been carried out before, and several conflicting assumptions on the free energy of dislocations have been made in the literature. Dislocations have gained interest as they are the carriers of plastic deformation in crystalline materials and solid polymers, and they constitute a prototype system for two-dimensional Coulomb particles. Our microscopic starting level is the description of dislocations as used in the discrete dislocation dynamics (DDD) framework. The macroscopic level of interest is characterized by the temperature, the boundary deformation and the dislocation density profile. By integrating over state space, we obtain a field theoretic partition function, which is a functional integral of the Boltzmann weight over an auxiliary field. The Hamiltonian consists of a term quadratic in the field and an exponential of this field. The partition function is strongly non-local, and reduces in special cases to the sine–Gordon model. Moreover, we determine implicit expressions for the response functions and the dominant scaling regime for metals, namely the low-temperature regime.

Tracking a glassy polymer on its energy landscape in the course of elastic deformation

The response to deformation of a detailed computer model of glassy atactic polystyrene, represented as a collection of basins on its potential energy landscape, has been investigated. The volumetric behaviour of the polymer is calculated via ‘brute force’ molecular dynamics quenching simulations. Results are compared with corresponding estimates obtained by invoking the quasi-harmonic approximation (QHA) for a variety of temperatures below the glass temperature and with experimental data. The stress-controlled uniaxial deformations fall in the linear elastic regime and the resulting strains are calculated as ensemble averages of QHA estimates over 200 uncorrelated inherent structures of the potential energy landscape. The elastic constants (Youngs modulus and Poisson ratio) and their temperature dependence are in very good agreement with experiments for glassy atactic polystyrene. Additionally, a classification of the deformed inherent structures in respect to the geometry and general shape of their energy minima is undertaken. A distortion of the potential energy basins upon mechanical deformation in the elastic regime is observed in all cases.

GENERIC treatment of compressible two-phase flow: Convection mechanism of scalar morphological variables

Abstract A two-phase model is considered which resolves both the thermodynamics of the two phases and of the interface, as well as the morphology in terms of Minkowski functionals, as scalar morphological variables, assuming equal velocities of the phases. In order to discuss whether, in compressible flow, the morphological variables should transform as scalars, as scalar densities or as a mixture of both, the two-phase model is cast into the GENERIC framework of non-equilibrium thermodynamics. The Jacobi identity, representing the time structure invariance of the reversible dynamics, is found not to impose any restrictions on the convection of the morphological variables, i.e., on the divergence terms of the velocity field appearing in their evolution equations. As an application of the general compressible two-phase model, the implications of assuming both equal velocities and equal temperatures of the phases are elaborated. In particular, it is found that in such a description the volume fraction and the amount of interface per unit volume are neither scalars nor scalar densities, their intermediate convection mechanism being completely determined in terms of the material properties of the phases.

Modeling aging and mechanical rejuvenation of amorphous solids

Abstract The elasto-viscoplasticity of amorphous solids is modeled, with a focus on the effects of physical aging and mechanical rejuvenation. Using nonequilibrium thermodynamics, the concept of kinetic and configurational subsystems has been employed. The Hamiltonian structure of reversible dynamics is exploited to derive a constitutive relation for the stress tensor. Furthermore, it is demonstrated that accounting for mechanical rejuvenation results in a modification of the driving force for viscoplastic flow.

The influence of the degree of heterogeneity on the elastic properties of random sphere packings

The macroscopic mechanical properties of colloidal particle gels strongly depend on the local arrangement of the powder particles. Experiments have shown that more heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties than their more homogeneous counterparts at equal volume fraction. In this paper, packings of spherical particles are used as model structures to computationally investigate the elastic properties of coagulated particle gels as a function of their degree of heterogeneity. The discrete element model comprises a linear elastic contact law, particle bonding and damping. The simulation parameters were calibrated using a homogeneous and a heterogeneous microstructure originating from earlier Brownian dynamics simulations. A systematic study of the elastic properties as a function of the degree of heterogeneity was performed using two sets of microstructures obtained from Brownian dynamics simulation and from the void expansion method. Both sets cover a broad and to a large extent overlapping range of degrees of heterogeneity. The simulations have shown that the elastic properties as a function of the degree of heterogeneity are independent of the structure generation algorithm and that the relation between the shear modulus and the degree of heterogeneity can be well described by a power law. This suggests the presence of a critical degree of heterogeneity and, therefore, a phase transition between a phase with finite and one with zero elastic properties.

Effective mobility of dislocations from systematic coarse-graining

The dynamics of large amounts of dislocations governs the plastic response of crystalline materials. In this contribution we discuss the relation between the mobility of discrete dislocations and the resulting flow rule for coarse-grained dislocation densities. The mobilities used in literature on these levels are quite different, for example in terms of their intrinsic the stress dependence. To establish the relation across the scales, we have derived the macroscopic evolution equations of dislocation densities from the equations of motion of individual dislocations by means of systematic coarse-graining. From this, we can identify a memory kernel relating the driving force and the flux of dislocations. This kernel can be considered as an effective macroscopic mobility with two contributions; a direct contribution related to the overdamped motion of individual dislocations, and an emergent contribution that arises from time correlations of fluctuations in the Peach–Koehler force. Scaling analysis shows that the latter contribution is dominant for dislocations in metals at room temperature. We also discuss several concerns related to the separation of timescales.

Kinetic model for the mechanical response of suspensions of sponge-like particles

A dynamic two-scale model is developed that describes the stationary and transient mechanical behavior of concentrated suspensions made of highly porous particles. Particularly, we are interested in particles that not only deform elastically, but also can swell or shrink by taking up or expelling the viscous solvent from their interior, leading to rate-dependent deformability of the particles. The fine level of the model describes the evolution of particle centers and their current sizes, while the shapes are at present not taken into account. The versatility of the model permits inclusion of density- and temperature-dependent particle interactions, and hydrodynamic interactions, as well as to implement insight into the mechanism of swelling and shrinking. The coarse level of the model is given in terms of macroscopic hydrodynamics. The two levels are mutually coupled, since the flow changes the particle configuration, while in turn the configuration gives rise to stress contributions, that eventually determine the macroscopic mechanical properties of the suspension. Using a thermodynamic procedure for the model development, it is demonstrated that the driving forces for position change and for size change are derived from the same potential energy. The model is translated into a form that is suitable for particle-based Brownian dynamics simulations for performing rheological tests. Various possibilities for connection with experiments, e.g. rheological and structural, are discussed.

Effect of particle-size dynamics on flow properties of dense spongy-particle systems

Suspensions of poroelastic particles are indispensable for applications where tailoring the overall properties is a necessity. The single-particle elastic network gives rise to their elastic behavior, while the flow of the viscous solvent through the particle structure gives rise to their rate-dependent behavior. In this work, we study the effect of the single-particle elastic modulus and permeability on the properties of the entire poroelastic-particle suspension subject to simple-shear deformation. For this purpose, the dynamic two-scale model developed by Hutter et al. [Faraday Discuss. 158, 407–424 (2012)] is used. Upon deformation, both permeable- and impermeable-particle suspensions undergo a transition from a glassy state to a shear-induced ordered state. On the one hand, the particle permeability is found to affect the rate at which the ordered state is reached. At a fixed elastic modulus, increasing the particle permeability prolongs the time scale at which shear-induced ordering occurs. On the o...

Stress relaxation of dense spongy-particle systems

Spongy particles have a permeable structure that allows them to undergo rate-dependent volume changes as their elastic network takes up or expels the viscous suspending solvent. Their ability to be jammed well above random close-packing makes them particularly attractive in applications where tailoring the overall properties is a requirement such as pharmaceuticals and foods. In this work, we independently vary the particle modulus and the particle permeability to study their effect on the stress-relaxation behavior of jammed permeable-particle suspensions. The dynamic two-scale model developed by Hutter et al. [Faraday Discuss. 158, 407–424 (2012)], which explicitly accounts for the particle size dynamics, is used for this purpose. We perform flow-cessation simulations of dense permeable-particle systems subjected to different preshear deformations. The stress relaxation occurs on shorter time scales in the case of permeable particles compared to impermeable particles. In terms of particle dynamics, stress relaxation is found to be promoted primarily by the motion of the particles within the cages formed by the surrounding particles, rather than by cage escape. The stress-relaxation process is accelerated by the permeability of spongy particles, namely, due to the sustained volume change that was induced during preshear, which renders their cages less effective.Spongy particles have a permeable structure that allows them to undergo rate-dependent volume changes as their elastic network takes up or expels the viscous suspending solvent. Their ability to be jammed well above random close-packing makes them particularly attractive in applications where tailoring the overall properties is a requirement such as pharmaceuticals and foods. In this work, we independently vary the particle modulus and the particle permeability to study their effect on the stress-relaxation behavior of jammed permeable-particle suspensions. The dynamic two-scale model developed by Hutter et al. [Faraday Discuss. 158, 407–424 (2012)], which explicitly accounts for the particle size dynamics, is used for this purpose. We perform flow-cessation simulations of dense permeable-particle systems subjected to different preshear deformations. The stress relaxation occurs on shorter time scales in the case of permeable particles compared to impermeable particles. In terms of particle dynamics, stre...