arXiv subjects

Kasra Farain

Publications and source records attributed to Kasra Farain.

10 recordsLinked to original sources

arXiv2026

Collective Asperity Dynamics and the Origin of Static Friction

Solid interfaces resist sliding up to a threshold shear force, called static friction, beyond which they start moving and their resistance drops to the kinetic friction. Static friction at rough interfaces has long been described empirically using system-specific coefficients tabulated in engineering handbooks. Here, through nanometer-resolution sliding experiments, we show that it is set by a friction overshoot during the onset of sliding. We demonstrate that this overshoot originates from the collective configurational evolution of surface asperities under shear, and derive a minimal differential equation governing this evolution. Our theory predicts that such overshoots generically emerge when an athermal frictional system evolves smoothly toward a unique steady-state kinetic friction. These results show that static friction is not an intrinsic material property, but an emergent consequence of collective asperity dynamics.

cond-mat.soft↗

arXiv2025

Structural Relaxation in Simple Yield Stress Materials Influences Their Rheology

Simple yield stress materials are composed of soft particles, bubbles, or droplets with purely repulsive forces. The constituent elements are typically too large to undergo thermal fluctuations, suggesting that the internal structure of the material, and therefore the rheology, should not change over time. We explore the rheology of Carbopol, a prototypical simple yield stress material, and show that gradual structural relaxation of the material results in a small yet significant reduction in the dynamic yield stress. This relaxation process can lead to a non-monotonic creep deformation rate under constant stress, culminating in delayed fluidization of the material. These findings show that the yield stress is not merely a static material property but may be a function of the internal structure of the material.

cond-mat.soft↗

arXiv2025

Anomalous Creep as a Precursor to Failure in Granular Materials

Granular materials, composed of discrete solid grains, can be modeled as simple mechanical systems. However, these materials can undergo spontaneous slow deformation, or creep, even under small forces and while in apparent mechanical equilibrium; a phenomenon central to understanding soil mechanics and the behavior of earthquake faults. We show that creep in granular materials originates from frictional dynamics at the contact points between grains. We reveal that the stability of these materials is governed by the interplay between creep and aging at these frictional contacts. Near the yield threshold, the frictional interactions result in anomalously accelerating creep, eventually leading to the delayed failure of the fragile packing. This behavior may serve as an early warning signal for catastrophic events like earthquakes and landslides.

cond-mat.soft↗

arXiv2024

Thermal properties of athermal granular materials

Dry granular materials consist of a vast ensemble of discrete solid particles, interacting through complex frictional forces at the contact points. The particles are so large that these systems are believed to be completely athermal. Here, we arrest the dynamics of a flowing granular material in a steady-state flow configuration, enabling an isolated examination of aging at the particle contacts without granular rearrangements. Our findings reveal that the evolution of interparticle forces within the arrested athermal granular network results in the spontaneous increase of the system's yield stress. This strengthening process is logarithmic in time with a rate that depends on temperature. We demonstrate that the material's stress relaxation exhibits similar time- and temperature-dependent behavior, suggesting a shared origin for aging and stress relaxation in these systems governed by thermal molecular processes at the scale of the grain contacts.

cond-mat.soft↗

arXiv2023

Ductile-to-brittle transition and yielding in soft amorphous materials: perspectives and open questions

Soft amorphous materials are viscoelastic solids ubiquitously found around us, from clays and cementitious pastes to emulsions and physical gels encountered in food or biomedical engineering. Under an external deformation, these materials undergo a noteworthy transition from a solid to a liquid state that reshapes the material microstructure. This yielding transition was the main theme of a workshop held from January 9 to 13, 2023 at the Lorentz Center in Leiden. The manuscript presented here offers a critical perspective on the subject, synthesizing insights from the various brainstorming sessions and informal discussions that unfolded during this week of vibrant exchange of ideas. The result of these exchanges takes the form of a series of open questions that represent outstanding experimental, numerical, and theoretical challenges to be tackled in the near future.

cond-mat.soft↗

arXiv2023

Perturbation-induced granular fluidization as a model for remote earthquake triggering

Studying the effect of mechanical perturbations on granular systems is crucial for understanding soil stability, avalanches, and earthquakes. We investigate a granular system as a laboratory proxy for fault gouge. When subjected to a slow shear, granular materials typically exhibit a stress overshoot before reaching a steady state. We find that short seismic pulses can reset a granular system flowing in steady state so that the stress overshoot is regenerated. This new feature is shown to determine the stability of the granular system under different applied stresses in the wake of a perturbation pulse and the resulting dynamics when it fails. Using an analytical aging-rejuvenation model for describing the overshoot response, we show that our laboratory-derived theoretical framework, can quantitatively explain data from two fault slip events triggered by seismic waves.

cond-mat.soft↗

arXiv2022

Quantitative understanding of the onset of dense granular flows

The question when and how dense granular materials start to flow under stress, despite many industrial and geophysical applications, remains largely unresolved. We develop and test a simple equation for the onset of quasi-static flows of granular materials which is based on the frictional aging of the granular packing. The result is a non-monotonic stress-strain relation which - akin to classical friction - is independent of the shear rate. This relation suffices to understand the below-threshold deformations of aging granular media, and its solid-to-liquid transition. Our results also elucidate the (flow) history dependence of the mechanical properties, and the sensitivity to initial preparation of granular media.

cond-mat.soft↗

arXiv2022

Predicting frictional ageing from bulk relaxation measurements

The coefficient of static friction between solids generally depends on the time they have remained in static contact before the measurement. Such frictional aging is at the origin of the difference between static and dynamic friction coefficients, but has remained difficult to understand. It is usually attributed to a slow increase in the area of atomic contact as the interface changes under pressure. This is however very difficult to quantify as surfaces have roughness at all length scales, and friction is not always proportional to the contact area. Here, we show that plastic flow of surface irregularities within a polymer-on-glass frictional interface exhibits identical relaxation dynamics as that of the bulk, allowing to predict the rate of frictional aging.

cond-mat.soft↗

arXiv2021

Non-monotonic dynamics in the onset of frictional slip

The transition from static to dynamic friction is often described as a fracture-like instantaneous slip. However, studies on slow sliding processes aimed at understanding frictional instabilities and earthquakes report slow friction transients that are usually explained by empirical rate-and-state formulations. We perform very slow ($\sim nm/s$) macroscopic-scale sliding experiments and show that the onset of frictional slip is governed by continuous non-monotonic dynamics originating from a competition between contact aging and shear-induced rejuvenation. This allows to describe both our non-monotonic dynamics and the simpler rate-and-state transients with a single evolution equation.

cond-mat.soft↗

arXiv2019

A relaxation constant in the folding of thin viscoelastic sheets

If one folds a thin viscoelastic sheet under an applied force, a line of plastic deformation is formed which shapes the sheet into an angle. We determine the parameters that define this angle experimentally and show that, no matter how much load one applies, it is impossible to make angles less than a certain minimum angle in a definite time. Moreover, it is shown that regardless of whether the sheet is released freely afterward or kept under the load, a logarithmic relaxation process follows the first deformation. The slope of this logarithm is the same in both conditions and depends neither on the applied force nor on the thickness of the sheet, which indicates it is directly a probe of the molecular mobility of the material. This intrinsic relaxation constant was measured 0.01 and 5.7 for Mylar and paper sheets, respectively. It is also suggested that the observed minimum angle of folding can be defined as a characteristic index for the plasticity of different materials.

cond-mat.soft↗