arXiv subjects

Fabian Coupette

Publications and source records attributed to Fabian Coupette.

9 recordsLinked to original sources

arXiv2025

Relevance of Aggregate Anisotropy in Sheared Suspensions of Carbon Black

Carbon Black is a filler frequently used in conductive suspensions or nanocomposites, in which it forms networks supporting electric conductivity. Although Carbon Black aggregates originate from a presumably isotropic aggregation process, the resulting particles are inherently anisotropic. Therefore, they can be expected to interact with shear flow, which significantly influences material properties. In this study, we investigate sheared suspensions of Carbon Black aggregates to elucidate the impact of aggregate anisotropy on the rheological properties. We aim at concentrations below and above the conductivity percolation threshold and comprehensively characterize particle behavior under flow conditions. Aggregates assembled by a diffusion-limited aggregation process are simulated with Langevin dynamics in simple shear flow. The simulations reveal a clear alignment of the aggregates' long axis with the flow direction, an increase in tumbling frequency with higher shear rates, and a shear-thinning response. This behavior closely parallels that of rod-like particles and underlines the significance of the anisotropic nature of Carbon Black aggregates. These findings will facilitate the optimization of nanocomposite precursor processing and the tailoring of Carbon Black-based conductive suspensions.

cond-mat.soft↗

arXiv2025

A universally applicable approach to connectivity percolation

Percolation problems appear in a large variety of different contexts ranging from the design of composite materials to vaccination strategies on community networks. The key observable for many applications is the percolation threshold. Unlike the universal critical exponents, the percolation threshold depends explicitly on the specific system properties. As a consequence, theoretical approaches to the percolation threshold are rare and generally tailored to the specific application. Yet, any percolating cluster forms a discrete network the emergence of which can be cast as a graph problem and analyzed using branching processes. We propose a general mapping of any kind of percolation problem onto a branching process which provides rigorous lower bounds of the percolation threshold. These bounds progressively tighten as we incorporate more information into the theory. We showcase our approach for different continuum problems finding accurate predictions with almost no effort. Our approach is based on first principles and does not require fitting parameters. As such it offers an important theoretical reference in a field that is dominated by simulation studies and heuristic fit functions.

cond-mat.stat-mech↗

arXiv2025

Polydispersity in Percolation

Every realistic instance of a percolation problem is faced with some degree of polydispersity, e.g., the pore-size distribution of an inhomogeneous medium, the size distribution of filler particles in composite materials, or the vertex degree of agents in a social network. Studies on different classes of systems have independently found very similar conceptual results for the percolation problem, i.e., that the percolation threshold is insensitive to the particular distribution controlling the polydispersity. Rather, the percolation threshold depends only on the first few moments of the distribution. In this article, we explain this frequently observed pattern using branching processes. The key observation is that a reasonable degree of polydispersity effectively does not alter the structure of the network that forms at the percolation threshold. As a consequence, the critical parameters of the monodisperse system can be analytically continued to account for polydispersity.

cond-mat.stat-mech↗

arXiv2024

Flow-induced anisotropy in a carbon black-filled silicone elastomer: electromechanical properties and structure

Carbon black (CB)-elastomers can serve as low-cost, highly deformable sensor materials, but hardly any work exists on their structure-property relationships. We report on flow-induced anisotropy, considering CB-silicone films generated via doctor blade coating. Cured films showed slight electrical anisotropy, with conductivity parallel to the coating direction being lower than perpendicular to it. Furthermore, piezoresistive sensitivity was much larger for stretch perpendicular to the coating direction than for parallel stretch. Structural analysis for length scales up to the CB agglomerate level yielded only weak evidence of anisotropy. Based on this evidence and insight from CB network simulations, we hypothesize that shear flow during coating fragments the CB network and then induces a preferential aggregate alignment, as well as increased inter-particle distances, parallel to the coating direction. As a practical conclusion, already weak anisotropic structuration suffices to cause significant electric anisotropy.

cond-mat.mtrl-sci↗

arXiv2023

Anomalous Underscreening in the Restricted Primitive Model

Underscreening is a collective term for charge correlations in electrolytes decaying slower than the Debye length. Anomalous underscreening refers to phenomenology that cannot be attributed alone to steric interactions. Experiments with concentrated electrolytes and ionic fluids report anomalous underscreening, which so far has not been observed in simulation. We present Molecular Dynamics simulation results exhibiting anomalous underscreening that can be connected to cluster formation. A theory that accounts for ion pairing confirms the trend. Our results challenge the classic understanding of dense electrolytes impacting the design of technologies for energy storage and conversion.

cond-mat.soft↗

arXiv2021

Nearest-neighbor connectedness theory: A general approach to continuum percolation

We introduce a method to estimate continuum percolation thresholds and illustrate its usefulness by investigating geometric percolation of non-interacting line segments and disks in two spatial dimensions. These examples serve as models for electrical percolation of elongated and flat nanofillers in thin film composites. While the standard contact volume argument and extensions thereof in connectedness percolation theory yield accurate predictions for slender nanofillers in three dimensions, they fail to do so in two dimensions, making our test a stringent one. In fact, neither a systematic order-by-order correction to the standard argument nor invoking the connectedness version of the Percus-Yevick approximation yield significant improvements for either type of particle. Making use of simple geometric considerations, our new method predicts a percolation threshold of $ρ_c l^2 \approx 5.83$ for segments of length $l$, which is close to the $ρ_c l^2 \approx 5.64$ found in Monte Carlo simulations. For disks of area $a$ we find $ρ_c a \approx 1.00$, close to the Monte Carlo result of $ρ_c a \approx 1.13$. We discuss the shortcomings of the conventional approaches and explain how usage of the nearest-neighbor distribution in our new method bypasses those complications.

cond-mat.stat-mech↗

arXiv2020

Bayesian unsupervised learning reveals hidden structure in concentrated electrolytes

Electrolytes play an important role in a plethora of applications ranging from energy storage to biomaterials. Notwithstanding this, the structure of concentrated electrolytes remains enigmatic. Many theoretical approaches attempt to model the concentrated electrolytes by introducing the idea of ion pairs, with ions either being tightly `paired' with a counter-ion, or `free' to screen charge. In this study we reframe the problem into the language of computational statistics, and test the null hypothesis that all ions share the same local environment. Applying the framework to molecular dynamics simulations, we show that this null hypothesis is not supported by data. Our statistical technique suggests the presence of distinct local ionic environments; surprisingly, these differences arise in like charge correlations rather than unlike charge attraction. The resulting fraction of particles in non-aggregated environments shows a universal scaling behaviour across different background dielectric constants and ionic concentrations.

cond-mat.soft↗

arXiv2019

Continuum percolation expressed in terms of density distributions

We present a new approach to derive the connectivity properties of pairwise interacting n-body systems in thermal equilibrium. We formulate an integral equation that relates the pair connectedness to the distribution of nearest neighbors. For one-dimensional systems with nearest-neighbor interactions, the nearest-neighbor distribution is, in turn, related to the pair correlation function g through a simple integral equation. As a consequence, for those systems, we arrive at an integral equation relating g to the pair connectedness, which is readily solved even analytically if g is specified analytically. We demonstrate the procedure for a variety of pair-potentials including fully penetrable spheres as well as impenetrable spheres, the only two systems for which analytical results for the pair connectedness exist. However, the approach is not limited to nearest-neighbor interactions in one dimension. Hence, we also outline the treatment of external fields and long-ranged interactions, and we illustrate how the formalism can applied to higher-dimensional systems using the three-dimensional ideal gas as an example.

cond-mat.stat-mech↗

arXiv2018

Screening Lengths in Ionic Fluids

The decay of correlations in ionic fluids is a classical problem in soft matter physics that underpins applications ranging from controlling colloidal self-assembly to batteries and supercapacitors. The conventional wisdom, based on analyzing a solvent-free electrolyte model, suggests that all correlation functions between species decay with a common decay length in the asymptotic far field limit. Nonetheless, a solvent is present in many electrolyte systems. We show using an analytical theory and molecular dynamics simulations that multiple decay lengths can coexist in the asymptotic limit as well as at intermediate distances once a hard sphere solvent is considered. Our analysis provides an explanation for the recently observed discontinuous change in the structural force across a thin film of ionic liquid-solvent mixtures as the composition is varied, as well as reframes recent debates in the literature about the screening length in concentrated electrolytes.

cond-mat.stat-mech↗