arXiv subjects

Lars Pastewka

Publications and source records attributed to Lars Pastewka.

At least 19 recordsLinked to original sources

arXiv2026

Nonparametric multiscale modeling of boundary lubrication: hexadecane in highly pressurized gold asperity contacts

Boundary lubrication is governed by molecular processes at the sliding interface that are inaccessible to classical continuum descriptions. While molecular dynamics (MD) simulations can resolve these processes in atomistic detail, incorporating their output into engineering-scale models through semi-empirical constitutive laws becomes increasingly difficult as confinement approaches the molecular scale. Here, we apply a nonparametric multiscale framework based on Gaussian process (GP) regression to model boundary lubrication of hexadecane confined between gold surfaces under pressures up to 1 GPa and gap heights down to 1.4 nm. The GP surrogates are trained on nonequilibrium MD simulations of the confined fluid and directly provide stress predictions to a continuum thin-film solver, circumventing the need for fixed-form constitutive laws. The framework naturally captures molecular phenomena such as density layering, viscosity changes, and the strongly nonlinear wall slip that dominates the frictional response at high pressures and small gap heights. Our results reproduce the atomistic benchmark data of Codrignani et al., demonstrating that nonparametric surrogate models offer a flexible and physically transparent route toward predictive continuum modeling of boundary lubrication.

cond-mat.soft↗

arXiv2026

Bridging perturbation and variational approaches in brittle fracture

We present a variational reduced-order model for three-dimensional coplanar propagation of sharp cracks in heterogeneous perfectly brittle solids under mixed-mode I+II+III loading. The approach connects the variational fracture formulation of Francfort and Marigo (1998) and the perturbation theory of Rice (1985) by computing equilibrium crack-front configurations through minimization of the total energy defined as the sum of (i) the elastic potential energy, evaluated asymptotically from front deformations, and (ii) the dissipated energy, set by the fracture energy field. The potential energy and its derivatives are evaluated efficiently using the Fast Fourier Transform. The resulting nonconvex box-constrained minimization problem is solved with a matrix-free Newton conjugate gradient algorithm with a trust region and physics-based preconditioning, enforcing irreversibility while resolving energy barriers and long-range elastic interactions. We validate our implementation against newly derived analytical solutions. We then perform 116,000 large-scale simulations of tensile and shear crack propagation in disordered media to quantify the impact of finite-size effects, disorder intensity, and mode mixity. The simulations reproduce the transition from smooth to intermittent crack growth, and show that mode mixity has limited influence on the onset of intermittency but induces quasi-elliptic fronts in mixed II+III loading. They reveal a size-dependent crossover from disorder-induced weakening to toughening controlled by the emergence of depinning instabilities.

cond-mat.mtrl-sci↗

arXiv2026

Resolving Structural Avalanches in Amorphous Carbon with Arclength Continuation

Plastic deformation in amorphous solids is carried by localized shear transformations that self-organize into avalanches. In amorphous carbon modeled with a machine-learned interatomic potential, we find that the energetics and organization of these avalanches can be resolved by systematically following the underlying energy landscape. With a pseudo-arclength numerical continuation framework, we decompose avalanches into constituent shear transformations and determine their strain-dependent energetics. Our analysis shows that, prior to onset, avalanches have a latent structure that consists of well-separated local minima. We further demonstrate that arclength continuation yields an event driven framework for following avalanche dynamics, eliminating time-step effects on statistical avalanche properties such as distributions of stress drops.

cond-mat.mtrl-sci↗

arXiv2025

Jacobi-accelerated FFT-based solver for smooth high-contrast data

The computational efficiency and rapid convergence of fast Fourier transform (FFT)-based solvers render them a powerful numerical tool for periodic cell problems in multiscale modeling. On regular grids, they tend to outperform traditional numerical methods. However, we show that their convergence slows down significantly when applied to microstructures with smooth, highly-contrasted coefficients. To address this loss of performance, we introduce a Green-Jacobi preconditioner, an enhanced successor to the standard discrete Green preconditioner that preserves the quasilinear complexity, $\mathcal{O}(N \log N)$, of conventional FFT-based solvers. Through numerical experiments, we demonstrate the effectiveness of the Jacobi-accelerated FFT (J-FFT) solver within a linear elastic framework. For problems characterized by smooth data and high material contrast, J-FFT significantly reduces the iteration count of the conjugate gradient method compared to the standard Green preconditioner. These findings are particularly relevant for phase-field fracture simulations, density-based topology optimization, and solvers that use adaption of the grid, which all introduce smooth variations in the material properties that challenge conventional FFT-based solvers.

math.NA↗

arXiv2025

Influence of tip materials on the friction force microscopy of epitaxial graphene on SiC(0001): comparison of diamond and silicon tips in experiments and atomistic simulations

Friction force microscopy (FFM) with silicon tips on epitaxial graphene supported by SiC(0001) previously revealed a sharp increase in friction at a threshold normal force, linked to the intermittent rehybridization of graphene and the formation of single-layer diamond above 12 GPa. In this study, the FFM behavior of a diamond tip is compared with that of the silicon tip. The diamond tip exhibited a similar abrupt increase in friction, but the threshold normal force was approximately twice as high for comparable tip radii. Simulations of graphene on SiC(0001) sliding against hydroxylated amorphous carbon (a-C) and silicon oxide (SiO2) slabs show that both systems exhibit low shear stress at low pressures, which increases at higher pressures due to bond formation between the graphene and counter slabs. The pressure dependence differs slightly: SiO2 reaches a shear stress plateau around 6 GPa, while a-C continues to increase gradually. For a-C, the transition threshold shifts to higher pressures, consistent with FFM results. Temperature lowers the transition threshold significantly, while sliding velocity has minimal impact on shear stress. These findings provide insights into the stability of low-friction interfaces between epitaxial graphene and the key materials which come into contact with graphene in current micro-electro-mechanical systems.

cond-mat.mtrl-sci↗

arXiv2025

Active learning for parameter-free multiscale modeling of boundary lubrication

Lubricated friction is a multiscale problem where molecular processes dictate the macroscopic response of the system. Traditional lubrication models rely on semi-empirical constitutive relations, which become unreliable under extreme conditions. Here, we present a simulation framework that seamlessly couples molecular and continuum models for boundary lubrication without fixed-form constitutive laws. We train Gaussian process regression models as surrogates for predicting interfacial shear and normal stress in molecular dynamics simulations. An active learning algorithm ensures that our model adapts in scenarios where common constitutive laws fail, such as near phase transitions. We demonstrate our approach for nanoscale fluid flow over rough and heterogeneous surfaces, paving the way for accurate boundary lubrication simulations at experimental length and time scales.

cond-mat.soft↗

arXiv2024

Fractal structure, depinning, and hysteresis of dislocations in high-entropy alloys

High-entropy alloys (HEAs) are complex alloys containing multiple elements in high concentrations. Plasticity in HEAs is carried by dislocations, but the random nature of their composition pins dislocations, effectively hindering their motion. We investigate the resulting complex structure of the dislocation in terms of spatial correlation functions, which allow us to draw conclusions on the fractal geometry of the dislocation. At high temperature, where thermal fluctuations dominate, dislocations adopt the structure of a random walk with Hurst exponent $1/2$ or fractal dimension $3/2$. At low temperature we find larger Hurst exponents (lower dimensions), with a crossover to an uncorrelated structure beyond a correlation length. These changes in structure are accompanied by an emergence of hysteresis (and hence pinning) in the motion of the dislocation at low temperature. We use a modified Labusch/Edwards-Wilkinson-model to argue that this correlation length must be an intrinsic property of the HEA. This means dislocations in HEAs are an individual pinning limit, where segments of the dislocation are independently pinned by local distortions of the crystal lattice that are induced by chemical heterogeneity.

cond-mat.mtrl-sci↗

arXiv2024

Vibrational lifetimes and viscoelastic properties of ultrastable glasses

Amorphous solids are viscoelastic. They dissipate energy when deformed at finite rate and finite temperature. We here use analytic theory and molecular simulations to demonstrate that linear viscoelastic dissipation can be directly related to the static and dynamic properties of the fundamental vibrational excitations of an amorphous system. We study ultrastable glasses that do not age, i.e. that remain in stable minima of the potential energy surface at finite temperature. Our simulations show four types of vibrational modes, which differ in spatial localization, similarity to plane waves and vibrational lifetimes. At frequencies below the Boson peak, the viscoelastic response can be split into contributions from plane-wave and quasilocalized modes. We derive a parameter-free expression for the viscoelastic storage and loss moduli for both of these modes. Our results show that the dynamics of microscopic dissipation, in particular the lifetimes of the modes, determine the viscoelastic response only at high frequency. Quasilocalized modes dominate the linear viscoelastic response at intermediate frequencies below the Boson peak.

cond-mat.soft↗

arXiv2023

Elastic shakedown and roughness evolution in repeated elastic-plastic contact

Surface roughness emerges naturally during mechanical removal of material, fracture, chemical deposition, plastic deformation, indentation, and other processes. Here, we use continuum simulations to show how roughness which is neither Gaussian nor self-affine emerges from repeated elastic-plastic contact of rough and rigid surfaces on a flat elastic-plastic substrate. Roughness profiles change with each contact cycle, but appear to approach a steady-state long before the substrate stops deforming plastically and has hence "shaken-down" elastically. We propose a simple dynamic collapse for the emerging power-spectral density, which shows that the multi-scale nature of the roughness is encoded in the first few indentations. In contrast to macroscopic roughness parameters, roughness at small scales and the skewness of the height distribution of the resulting roughness do not show a steady-state, with the latter vanishing asymptotically with contact cycle.

cond-mat.soft↗

arXiv2023

Why soft contacts are stickier when breaking than when making them

Insects, pick-and-place manufacturing, engineered adhesives, and soft robots employ soft materials to stick to surfaces even in the presence of roughness. Experiments show that the force required for making contact is lower than for releasing it, a phenomenon known as the adhesion hysteresis. The common explanation for this hysteresis is either contact aging or viscoelasticity. Here, we show that adhesion hysteresis emerges even for perfectly elastic contacts and in the absence of contact aging and viscoelasticity because of surface roughness. We present a crack-perturbation model and experimental observations that reveal discrete jumps of the contact perimeter. These stick-slip instabilities are triggered by local differences in fracture energy between roughness peaks and valleys. Pinning of the contact perimeter retards both its advancement when coming into contact and its retraction when pulling away. Our model quantitatively reproduces the hysteresis observed in experiments and allows us to derive analytical predictions for its magnitude, accounting for realistic rough geometries across orders of magnitude in length scale. Our results explain why adhesion hysteresis is ubiquitous and reveal why soft pads in nature and engineering are efficient in adhering even to surfaces with significant roughness.

cond-mat.soft↗

arXiv2023

Entropic stress of grafted polymer chains in shear flow

We analyze the shear response of grafted polymer chains in shear flow via coarse-grained molecular dynamics simulations. Our simulations confirm that the shear response is dominated by the brush's outermost correlation volume, which depends on shear rate at high Weissenberg number. The system's shear stress can be approximated by the brush's entropic stress. The simulations further reveal that at low Weissenberg number, the entropic shear stress of grafted chains is independent of the Weissenberg number. Increasing the Weissenberg number leads to Wi-dependent behavior: chains first reorient along the shear direction and elongate at higher Wi. The entropic shear stress increases linearly with Weissenberg number in this regime. We relate these calculations to experimental observations on the velocity dependence of brush and hydrogel friction.

cond-mat.soft↗

arXiv2023

Sound waves, diffusive transport, and wall slip in nanoconfined compressible fluids

Although continuum theories have been proven quite robust to describe confined fluid flow at molecular length scales, molecular dynamics (MD) simulations reveal mechanistic insights into the interfacial dissipation processes. Most MD simulations of confined fluids have used setups in which the lateral box size is not much larger than the gap height, thus breaking thin-film assumptions usually employed in continuum simulations. Here, we explicitly probe the long wavelength hydrodynamic correlations in confined simple fluids with MD and compare to gap-averaged continuum theories as typically applied in e.g. lubrication. Relaxation times obtained from equilibrium fluctuations interpolate between the theoretical limits from bulk hydrodynamics and continuum formulations with increasing wavelength. We show how to exploit this characteristic transition to measure viscosity and slip length in confined systems simultaneously from equilibrium MD simulations. Moreover, the gap-averaged theory describes a geometry-induced dispersion relation that leads to overdamped sound relaxation at large wavelengths, which is confirmed by our MD simulations. Our results add to the understanding of transport processes under strong confinement and might be of technological relevance for the design of nanofluidic devices due to the recent progress in fabrication methods.

physics.flu-dyn↗

arXiv2023

Molecular simulations of sliding on SDS surfactant films

We use molecular dynamics simulations to study the frictional response of the anionic surfactant sodium dodecyl sulfate (SDS) monolayers and hemicylindrical aggregates physisorbed on gold. Our simulations of a sliding spherical asperity reveals two friction regimes: At low loads, the films show Amontons' friction with a friction force that rises linearly with normal load. At high loads, the friction force is independent of load as long as no direct solid-solid contact occurs. The transition between these two regimes happens when only a single molecular layer is confined in the gap between the sliding bodies. The friction force at high loads on a monolayer rises monotonically with film density and drops slightly with the transition to hemicylindrical aggregates. This monotonous increase of friction force is compatible with a traditional plowing model of sliding friction. At low loads, the friction coefficient reaches a minimum at intermediate surface concentrations. We attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing.

cond-mat.soft↗

arXiv2023

Yielding under compression and the polyamorphic transition in silicon

We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a discontinuous nonequilibrium transition from a low-density to a high-density structure at a pressure of around $13$-$16$~GPa. Ensemble-averaged density and elastic constants change discontinuously across the transition. Densification of individual glassy samples occurs through a series of discrete plastic events, each of which is accompanied by a vanishing shear modulus. This is the signature of a series of elastic instabilities, similar to shear transformation zones observed during shear yielding of glasses. We compare the structure obtained during compression with a near-equilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to nonequilibrium compression at low and high pressure, but the transition between them occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition is of a nonequilibrium nature, and it has the characteristics of a yield transition that occurs under compression instead of shear.

cond-mat.soft↗

arXiv2023

Analytic elastic constants in molecular calculations: Finite strain, non-affine displacements, and many-body interatomic potentials

Elastic constants are among the most fundamental and important properties of solid materials, which is why they are routinely characterized in both experiments and simulations. While conceptually simple, the treatment of elastic constants is complicated by two factors not yet having been concurrently discussed: finite-strain and non-affine, internal displacements. Here, we revisit the theory behind zero-temperature, finite-strain elastic constants and extend it to explicitly consider non-affine displacements. We further present analytical expressions for second-order derivatives of the potential energy for two-body and generic many-body interatomic potentials, such as cluster and empirical bond-order potentials. Specifically, we revisit the elastic constants of silicon, silicon carbide and silicon dioxide under hydrostatic compression and dilatation. Based on existing and new results, we outline the effect of multiaxial stress states as opposed to volumetric deformation on the limits of stability of their crystalline lattices.

cond-mat.mtrl-sci↗

arXiv2022

Interatomic potentials: Achievements and challenges

Interatomic potentials approximate the potential energy of atoms as a function of their coordinates. Their main application is the effective simulation of many-atom systems. Here, we review empirical interatomic potentials designed to reproduce elastic properties, defect energies, bond breaking, bond formation, and even redox reactions. We discuss popular two-body potentials, embedded-atom models for metals, bond-order potentials for covalently bonded systems, polarizable potentials including charge-transfer approaches for ionic systems and quantum-Drude oscillator models mimicking higher-order and many-body dispersion. Particular emphasis is laid on the question what constraints ensue from the functional form of a potential, e.g., in what way Cauchy relations for elastic tensor elements can be violated and what this entails for the ratio of defect and cohesive energies, or why the ratio of boiling to melting temperature tends to be large for potentials describing metals but small for short-ranged pair potentials. The review is meant to be pedagogical rather than encyclopedic. This is why we highlight potentials with functional forms sufficiently simple to remain amenable to analytical treatments. Our main objective is to provide a stimulus for how existing approaches can be advanced or meaningfully combined to extent the scope of simulations based on empirical potentials.

cond-mat.mtrl-sci↗

arXiv2022

Molecular mechanisms of self-mated hydrogel friction

Self-mated hydrogel contacts show extremely small friction coefficients at low loads but a distinct velocity dependence. Here we combine mesoscopic simulations and experiments to test the polymer-relaxation hypothesis for this velocity dependence, where a velocity-dependent regime emerges when the perturbation of interfacial polymer chains occurs faster than their relaxation at high velocity. Our simulations reproduce the experimental findings, with speed-independent friction at low velocity, followed by a friction coefficient that rises with velocity to some power of order unity. We show that the velocity-dependent regime is characterized by reorientation and stretching of polymer chains in the direction of shear, leading to an entropic stress that can be quantitatively related to the shear response. The detailed exponent of the power law in the velocity dependent regime depends on how chains interact: We observe a power close to $1/2$ for chains that can stretch, while pure reorientation leads to a power of unity. Our simulations quantitatively match experiments and show that the velocity dependence of hydrogel friction at low loads can be firmly traced back to the morphology of near-surface chains.

cond-mat.soft↗

arXiv2022

contact.engineering -- Create, analyze and publish digital surface twins from topography measurements across many scales

The optimization of surface finish to improve performance occurs largely through trial and error, despite significant advancements in the relevant science. There are three central challenges that account for this disconnect: (1) the challenge of integration of many different types of measurement for the same surface to capture the multi-scale nature of roughness; (2) the technical complexity of implementing spectral analysis methods, and of applying mechanical or numerical models to describe surface performance; (3) a lack of consistency between researchers and industries in how surfaces are measured, quantified, and communicated. Here we present a freely-available internet-based application which attempts to overcome all three challenges. First, the application enables the user to upload many different topography measurements taken from a single surface, including using different techniques, and then integrates all of them together to create a digital surface twin. Second, the application calculates many of the commonly used topography metrics, such as root-mean-square parameters, power spectral density (PSD), and autocorrelation function (ACF), as well as implementing analytical and numerical calculations, such as boundary element modeling (BEM) for elastic and plastic deformation. Third, the application serves as a repository for users to securely store surfaces, and if they choose, to share these with collaborators or even publish them (with a digital object identifier) for all to access. The primary goal of this application is to enable researchers and manufacturers to quickly and easily apply cutting-edge tools for the characterization and properties-modeling of real-world surfaces. An additional goal is to advance the use of open-science principles in surface engineering by providing a FAIR database where researchers can choose to publish surface measurements for all to use.

cond-mat.mtrl-sci↗