arXiv subjects

Martin H. Müser

Publications and source records attributed to Martin H. Müser.

At least 19 recordsLinked to original sources

arXiv2026

Plug Flow and Cavitation in Rough Lubricated Contacts: Molecular Dynamics of Single- vs. Two-Component Fluids

We present non-equilibrium molecular dynamics simulations of lubricated sliding between rough, deformable surfaces under conditions representative of mixed lubrication. One aim is to reduce the gap between highly idealized simulations of smooth interfaces and real, rough, load-bearing contacts. Another aim is to determine whether favorable tribological properties of two-fluid lubrication reported for solvated hydrophilic-hydrophobic polymer-brush interfaces can also be realized in rough contacts without brushes. To this end, we compare aqueous (water), hydrocarbon (n-dodecane), and immiscible two-fluid lubrication under identical geometric conditions representing randomly rough surfaces. For the single-component lubricants, the simulations reproduce established trends: water shows stronger speed dependence but reduced load-bearing capacity than n-dodecane, despite their similar ambient viscosities. Beyond this expected behavior, the simulations reveal that the combination of strong confinement and large height gradients can cause plug flow and thereby cavitation after asperity collisions. For a high-surface-tension liquid like water, cavitation leads to an abrupt shear-stress release on scales much exceeding the size of the cavity. Since plug motion only weakens with single-component lubricants at low speeds, the two-fluid lubricant can only leverage its potential advantages at low sliding velocities. It is also the only system in which folding lips form, occasionally developing into transient, detached clusters at high speeds.

cond-mat.soft↗

arXiv2026

Computing finite--temperature elastic constants with noise cancellation

Elastic constants are central material properties, frequently reported in experimental and theoretical studies. While their computation is straightforward in the absence of thermal fluctuations, finite--temperature methods often suffer from poor signal--to--noise ratios or the presence of strong anharmonic effects. Here, we show how to compute elastic constants in thermal ordered and disordered systems by generalizing a noise--cancellation method originally developed for piezoelectric coupling coefficients. A slight strain is applied to an equilibrated solid. Simulations of both the strained and unstrained (or oppositely strained) reference systems are performed using identical thermostatting schemes. As demonstrated theoretically and with generic one--dimensional models, this allows stress differences to be evaluated and elastic constants to be determined with much reduced thermal noise. We then apply this approach across a diverse set of systems, spanning crystalline argon, ordered silicon as well as amorphous silicon, poly(methyl methacrylate), and cellulose derivatives.

cond-mat.mtrl-sci↗

arXiv2026

Granite sliding on granite: friction, wear rates, surface topography, and the scale-dependence of rate-state effects

We study tribological granite-granite contacts as a model for tectonic faulting, combining experiments, theory, and molecular dynamics simulations. The high friction in this system is not dominated by particulate wear or plowing, as frequently assumed, but by cold welding within plastically deformed asperity junctions. We base this conclusion on the observation that wear is repeatedly high after cleaning contacts but decreases as gouge accumulates, while friction shows the opposite trend. Moreover, adding water reduces wear by a factor of ten but barely decreases friction. Thermal and rate-dependent effects-central to most earthquake models-are negligible: friction remains unchanged between -40°C and 20°C, across abrupt velocity steps, and after hours of stationary contact. The absence of rate-state effects in our macroscopic samples is rationalized by the scale-dependence of pre-slip. The evolution of surface topography shows that quartz grains become locally smooth, with height spectra isotropic for wavelength below 10 microns but anisotropic at longer wavelengths, similar to natural faults. The resulting gouge particles have the usual characteristic sizes near 100 nm. Molecular dynamics simulations of a rigid, amorphous silica tip sliding on α-quartz reproduce not only similar friction coefficients near unity but also other experimentally observed features, including stress-introduced transitions to phases observed in post-mortem faults, as well as theoretical estimates of local flash temperatures. Additionally, they reveal a marked decrease of interfacial shear strength above 600°C.

physics.geo-ph↗

arXiv2025

Finite-temperature properties of the Frenkel-Kontorova model: Relation to tribological systems and fluid rheology

The Frenkel-Kontorova model is a simple yet generic framework for the description of tribological phenomena and processes, including dry solid friction and the motion of adsorbed layers. As revealed in this work, it also reproduces qualitatively various features of complex liquids, such as, power-law sub-diffusion between the ballistic and the diffusive regimes as well as a cross-over from a non-Arrhenius to an Arrhenius dependence of the diffusion coefficient near the temperature, where the specific heat assumes its maximum. The study of these and related thermal and kinetic properties highlights several misconceptions prevalent in the literature. Most notably, shear thinning with a shear-thinning exponent close to zero can be the natural consequence from enforced basin hopping: the energy drops caused by shear-induced instabilities dictate the friction-velocity dependence at medium shear rates rather than the way how shear forces reduce the free energy barriers for directed motion. Thus, even if the rheology is described by semi-empirical theories such as the Eyring model, any agreement with experimental data, whether past, present, or future, may be purely coincidental.

cond-mat.soft↗

arXiv2025

Advanced Langevin thermostats: Properties, extensions to rheology, and a lean momentum-conserving approach

The Langevin equation accounts for unresolved bath degrees of freedom driving the system toward the bath temperature. Because of this, numerical solutions of the Langevin equation have a long history. Here, we recapitulate, combine, and extend existing Langevin-equation based thermostats, scrutinize their properties and demonstrate their superiority over global kinetic-energy controls. Our work includes compact, asymptotic-analysis based derivations of stochastic thermostats, including the highly accurate Grønbech-Jensen scheme. Proposed extensions include a precise, colored and a lean, momentum-conserving thermostat.

cond-mat.stat-mech↗

arXiv2025

Cold self-lubrication of sliding ice

The low kinetic friction between ice and numerous counterbodies is commonly attributed to an interfacial water layer, which is believed to originate from pre-existing surface water or from melt water induced by high contact pressures or frictional heat. However, even the currently leading theory of frictional melting appears to defy direct experimental verification. Here we present molecular simulations of ice interfaces that reveal that ice surfaces liquefy without melting thermodynamically but predominantly by cold, displacement-driven amorphization. Despite effective self-lubrication, very small ice friction is found to require water to slip past a hydrophobic counterface -- or an excess amount of water, produced by, e.g., extreme sliding velocities.

cond-mat.soft↗

arXiv2024

Dissipative split-charge formalism: Ohm's law, Nyquist noise, and non-contact friction

The split-charge equilibration method is extended to describe dissipative charge transfer similarly as the Drude model, whereby the generic frequency-dependent dielectric permitivitties or conductivities of dielectrics and metals can be mimicked. To demonstrate the feasibility of the approach, a resistor-capacitor circuit is simulated using an all-atom representation for resistor and capacitor. The simulated dynamics reproduce the expected charging process and Nyquist noise, the latter resulting from the thermal voltages acting on individual split charges. The method bears promise to model friction caused by the motion of charged particles past metallic or highly polarizable media.

cond-mat.mtrl-sci↗

arXiv2024

Brittleness of metallic glasses dictated by their state at the fragile-to-strong transition temperature

The effect of cooling on the brittleness of glasses in general, and bulk metallic glasses (BMGs) in particular, is usually studied with continuously varying cooling rates; slower cooling rates lead to stiffer, harder, and more brittle glasses than higher cooling rates. These protocols obscure any potential discontinuity that a glass might experience depending on whether its microstructure resembles that of a fragile or a strong glass-forming liquid. Here, we use large-scale molecular dynamics to simulate the nanoindentation behavior of model BMGs (Zr$_{0.6}$Cu$_{0.3}$Al$_{0.1}$) obtained by rapidly quenching equilibrium melts from temperatures above and below the fragile-to-strong transition temperature $T_\textrm{fst}$, leading to fragile and strong glasses, respectively. While the contact modulus deduced from the indentation simulation evolves smoothly with the temperature $T_\text{q}$ from which the melt is quenched, the plastic response changes quasi-discontinuously as $T_\text{q}$ passes through $T_\textrm{fst}$. In particular, strong glasses develop highly asymmetric flow profiles with mature shear bands, in contrast to fragile glasses. Quantitative differences reveal themselves not only through a formal von Mises localization parameter analysis but also through image analysis of flow patterns using pre-trained artificial intelligence models. Moreover, seemingly erratic flow profiles for our indentation geometry produced surprisingly reproducible and, thus, deterministic features. It remains to be determined to what extent other classes of glass formers follow our observation that the degree of brittleness is significantly influenced by whether the melt is fragile or strong when it falls out of equilibrium at the glass transition temperature.

cond-mat.mtrl-sci↗

arXiv2024

Structural Lubricity and Molecular Contamination: Rejuvenation, Aging, and Friction Switches

Using atomic force microscopy experiments and molecular dynamics simulations of gold nanoislands on graphite, we investigate why ultra-small friction commonly associated with structural lubricity can be observed even under ambient conditions. Measurements conducted within a few days after sample synthesis reveal previously undiscovered phenomena in structurally lubric systems: rejuvenation, a drop in kinetic friction of an order of magnitude shortly after the onset of sliding; aging, a significant increase in kinetic friction forces after a rest period of 30 minutes or more; switches, spontaneous jumps between distinct friction branches. These three effects are drastically suppressed a few weeks later. Imaging of a contamination layer and simulations provide a consistent picture of how single- and double-layer contamination underneath the gold nanoislands as well as contamination surrounding the nanoislands affect structural lubricity but not lead to its breakdown.

cond-mat.soft↗

arXiv2023

Significance of Elastic Coupling for Stresses and Leakage in Frictional Contacts

We study how the commonly neglected coupling of normal and in-plane elastic response affects tribological properties when Hertzian or randomly rough indenters slide past an elastic body. Compressibility-induced coupling is found to substantially increase maximum tensile stresses, which cause materials to fail, and to decrease friction such that Amontons law is violated macroscopically even when it holds microscopically. Confinement-induced coupling increases friction and enlarges domains of high tension. Moreover, both types of coupling affect the gap topography and thereby leakage. Thus, coupling can be much more than a minor perturbation of a mechanical contact.

cond-mat.soft↗

arXiv2023

On the shear-thinning of alkanes

The approximate power-law dependence of the apparent viscosity of liquids on shear rate is often argued to arise from a distribution of energy barriers. However, recent work on the Prandtl model, which consists of a point mass being dragged by a damped, harmonic spring past a sinusoidal potential, revealed a similar dependence of the friction on velocity as that of many liquids. Here, we demonstrate that this correlation is not only qualitative but can also be made quantitative over a broad temperature range using merely three dimensionless parameters, at least for alkanes, in particular hexadecane, at elevated pressure p. These and other observations made on our all-atom alkane simulations at elevated pressure point to the existence of an elementary instability causing shear thinning. In addition, the equilibrium viscosity shows power law dependence on p near the cavitation pressure but an exponential dependence at large p, while the additional parameter(s) in the Carreau-Yasuda equation compared to other rheological models turn out justifiable.

cond-mat.soft↗

arXiv2023

Revealing the co-action of viscous and multistability hysteresis in an adhesive, nominally flat punch: A combined numerical and experimental study

Viscoelasticity is well known to cause significant hysteresis of crack closure and opening when an elastomer is brought in and out of contact with a flat, rigid, adhesive counterface. A separate origin of adhesive hysteresis is small-scale, elastic multistability. Here, we study a system in which both mechanisms act concurrently. Specifically, we compare the simulated and experimentally measured time evolution of the interfacial force and the real contact area between a soft elastomer and a rigid, flat punch, to which small-scale, single-sinusoidal roughness is added. To this end, we further the Green's function molecular dynamics method and extend recently developed imaging techniques to elucidate the rate- and preload-dependence of the pull-off process. Our results reveal that hysteresis is much enhanced when the saddle points of the topography come into contact, which, however, is impeded by viscoelastic forces and may require sufficiently large preloads. A similar coaction of viscous- and multistability effects is expected to occur in macroscopic polymer contacts and to be relevant, e.g., for pressure-sensitive adhesives and modern adhesive gripping devices.

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

The surface-topography challenge: Problem definition

We present to the community a surface-definition problem, whose solution we consider to be critical for the proper description of contacts between nominally flat surfaces [1,2]. In 2015, Müser and Dapp issued the Contact Mechanics Challenge, which provided complete topography data for a fictional surface and asked theorists and modelers to compute the expected contact parameters for such a surface. This effort was a success, but exposed one glaring flaw in the community's understanding of the nature of contact: these models require as input a complete description of surface topography, which is rarely or never available for real-world surfaces [3-6]. The present challenge is to experimentalists: we will send you samples of two materials (one smoother and one rougher); you determine the surface topography of these materials. We call on you to measure such surfaces however you wish, using contact-based techniques, light scattering, microscopy, or other techniques. Examples of quantities of interest are: root-mean-square (RMS) parameters; the power spectral density (PSD); or the autocorrelation function (ACF). For the material, we have chosen chromium nitride, a wear- and corrosion-resistant coating used in industrial applications including automotive components, cutting tools, and die-casting. To participate, simply go to: https://contact.engineering/challenge to provide your shipping address and other information, then samples will be shipped out to you. The only requirement of participation is that your raw topography measurements are deposited on the free contact.engineering web app to facilitate data sharing. The purpose of this challenge is for our community to move towards: (a) better agreement on how to describe the multi-scale topography of experimental surfaces; and (b) better understanding of how to apply the well-developed models and theories to real-world surfaces.

cond-mat.mtrl-sci↗

arXiv2022

Structural lubricity of physisorbed gold clusters on graphite and its breakdown: Role of boundary conditions and contact lines

The sliding motion of gold slabs adsorbed on a graphite substrate is simulated using molecular-dynamics. The central quantity of interest is the mean lateral force, i.e., the kinetic friction rather than the maximum lateral forces, which correlate with the static friction. For most set-ups, we find Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb) friction is observed for finite-width slabs sliding parallel to the armchair direction if the bottommost layer of the three graphite layers is kept at zero stress rather than at zero displacement. Although the resulting kinetic friction remain much below the noise produced by the erratic fluctuations of (conservative) forces typical for structurally lubric contacts, the nature of the instabilities leading to Coulomb friction could be characterized as quasi-discontinuous dynamics of the Moiré patterns formed by the normal displacements near a propagating contact line. It appears that the interaction of the graphite with the second gold layer is responsible for the symmetry breaking occurring in the interface when a contact line moves parallel to the armchair rather than to the zigzag direction.

cond-mat.mtrl-sci↗

arXiv2022

Improved cutoff functions for short-range potentials and the Wolf summation

A class of radial, polynomial cutoff functions $f_{\textrm{c}n}(r)$ for short-ranged pair potentials or related expressions is proposed. Their derivatives up to order $n$ and $n+1$ vanish at the outer cutoff $r_\textrm{c}$ and an inner radius $r_\textrm{i}$, respectively. Moreover, $f_{\textrm{c}n}(r \le r_\textrm{i}) = 1$ and $f_{\textrm{c}n}(r\ge r_\textrm{c})=0$. It is shown that the used order $n$ can qualitatively affect results: stress and bulk moduli of ideal crystals are unavoidably discontinuous with density for $n=0$ and $n=1$, respectively. Systematic errors on energies and computing times decrease by approximately 25\% for Lennard-Jones with $n=2$ or $n=3$ compared to standard cutting procedures. Another cutoff function turns out beneficial to compute Coulomb interactions using the Wolf summation, which is shown to not properly converge when local charge neutrality is obeyed only in a stochastic sense. However, for all investigated homogeneous systems with thermal noise (ionic crystals and liquids), the modified Wolf summation, despite being infinitely differentiable at $r_\textrm{c}$, converges similarly quickly as the original summation. Finally, it is discussed how to reduce the computational cost of numerically exact Monte Carlo simulations using the Wolf summation even when it does not properly converge.

cond-mat.mtrl-sci↗

arXiv2021

Crack and pull-off dynamics of adhesive, viscoelastic solids

When quickly detaching an elastomer from a counterface, viscoelasticity dramatically increases the perceived adhesion relative to its adiabatic or equilibrium value. Here, we report simulations on the sticking contact between a rigid cylinder and a viscoelastic half space revealing a maximum in the work of adhesion at intermediate pull-off velocities. Maximum tensile forces yet increase monotonically with the pull-off speed and the crack-tip speed in accordance with the Persson-Brener approach. As predicted theoretically, the fracture mode transitions from interfacial crack propagation to quasi-uniform bond breaking with increasing range of adhesion.

cond-mat.soft↗

arXiv2021

On the viscous dissipation caused by randomly rough indenters in smooth sliding motion

The viscous dissipation between rigid, randomly rough indenters and linearly elastic counter bodies sliding past them is investigated using Green's function molecular dynamics. The study encompasses a variety of models differing in the height spectra properties of the rigid indenter, in the viscoelasticity of the elastomer, and in their interaction. All systems reveal the expected damping linear in sliding velocity $v$ at small $v$ and a pronounced maximum at intermediate $v$. Persson's theory of rubber friction, which is adopted to the studied model systems, reflects all observed trends. However, close quantitative agreement is only found up to intermediate sliding velocities. Relative errors in the friction force become significant once the contact area is substantially reduced by sliding.

cond-mat.soft↗