arXiv subjects

Claus Heussinger

Publications and source records attributed to Claus Heussinger.

At least 19 recordsLinked to original sources

arXiv2023

Collapse of columns of granular rods

Simulation results are presented on the collapse of granular columns composed of rod-like particles. Columns can be stable and free-standing if either the friction coefficient is large enough, or the rods long enough. Destabilizing gravitational forces are counteracted by increased frictional forces between the rods. Different to columns made of spherical particles this is possible, because rod contacts can slide along the rod axes to generate the necessary "frictional cohesion".

cond-mat.soft↗

arXiv2021

Start-up shear of spherocylinder packings: effect of friction

We study the response to shear deformations of packings of long spherocylindrical particles that interact via frictional forces with friction coefficient $μ$. The packings are produced and deformed with the help of molecular dynamics simulations combined with minimization techniques performed on a GPU. We calculate the linear shear modulus $g_\infty$, which is orders of magnitude larger than the modulus $g_0$ in the corresponding frictionless system. The motion of the particles responsible for these large frictional forces is governed by and increases with the length $\ell$ of the spherocylinders. One consequence of this motion is that the shear modulus $g_\infty$ approaches a finite value in the limit $\ell\to\infty$, even though the density of the packings vanishes, $ρ\propto\ell^{-2}$. By way of contrast, the frictionless modulus decreases to zero, $g_0\sim\ell^{-2}$, in accordance with the behavior of density. Increasing the strain beyond a value $γ_c\sim μ$, the packing undergoes a "shear-thinning" transition from the large frictional to the smaller frictionless modulus when contacts saturate at the Coulomb inequality and start to slide. In this regime, sliding friction contributes a "yield stress" $σ_y=g_\inftyγ_c$ and the stress behaves as $σ=σ_y+g_0γ$. The interplay between static and sliding friction gives rise to hysteresis in oscillatory shear simulations.

cond-mat.soft↗

arXiv2020

Packings of frictionless spherocylinders

We present simulation results on the properties of packings of frictionless spherocylindrical particles. Starting from a random distribution of particles in space, a packing is produced by minimizing the potential energy of inter-particle contacts until a force-equilibrated state is reached. For different particle aspect ratios $α=10\ldots 40$, we calculate contacts $z$, pressure as well as bulk and shear modulus. Most important is the fraction $f_0$ of spherocylinders with contacts at both ends as it governs the jamming threshold $z_c(f_0)=8+2f_0$. These results highlight the important role of the axial "sliding" degree of freedom of a spherocylinder, which is a zero-energy mode but only if no end-contacts are present.

cond-mat.soft↗

arXiv2019

Unsteady flow, clusters and bands in a model shear-thickening fluid

We analyse the flow curves of a two-dimensional assembly of granular particles which are interacting via frictional contact forces. For packing fractions slightly below jamming, the fluid undergoes a large scale instability, implying a range of stress and strainrates where no stationary flow can exist. Whereas small systems were shown previously to exhibit hysteretic jumps between the low and high stress branches, large systems exhibit continuous shear thickening arising from averaging unsteady, spatially heterogeneous flows. The observed large scale patterns as well as their dynamics are found to depend on strainrate: At the lower end of the unstable region, force chains merge to form giant bands that span the system in compressional direction and propagate in dilational direction. At the upper end, we observe large scale clusters which extend along the dilational direction and propagate along the compressional direction. Both patterns, bands and clusters, come in with infinite correlation length similar to the sudden onset of system-spanning plugs in impact experiments.

cond-mat.soft↗

arXiv2018

Rheology in dense assemblies of spherocylinders: frictional vs. frictionless

Using molecular dynamics simulations, we study the steady shear flow of dense assemblies of anisotropic spherocylindrical particles of varying aspect ratios. Comparing frictionless and frictional particles we discuss the specific role of frictional inter-particle forces for the rheological properties of the system. In the frictional system we evidence a shear-thickening regime, similar to that for spherical particles. Furthermore, friction suppresses alignment of the spherocylinders along the flow direction. Finally, the jamming density in frictional systems is rather insensitive to variations in aspect-ratio, quite contrary to what is known from frictionless systems.

cond-mat.soft↗

arXiv2018

Discontinuous shear-thinning in adhesive dispersions

We present simulations for the steady-shear rheology of a model adhesive dispersion. We vary the range of the attractive forces $u$ as well as the strength of the dissipation $b$. For large dissipative forces, the rheology is governed by the Weisenberg number $ \text{Wi}\sim b\dotγ/u$ and displays Herschel-Bulkley form $σ= σ_y+c\text{Wi}^ν$ with exponent $ν=0.45$. Decreasing the strength of dissipation, the scaling with $\text{Wi}$ breaks down and inertial effects show up. The stress decreases via the Johnson-Samwer law $Δσ\sim T_s^{2/3}$, where temperature $T_s$ is exclusively due to shear-induced vibrations. During flow particles prefer to rotate around each other such that the dominant velocities are directed tangentially to the particle surfaces. This tangential channel of energy dissipation and its suppression leads to a discontinuity in the flow curve, and an associated discontinuous shear thinning transition. We set up an analogy with frictional systems, where the phenomenon of discontinuous shear thickening occurs. In both cases tangential forces, frictional or viscous, mediate a transition from one branch of the flowcurve with low tangential dissipation to one with large tangential dissipation.

cond-mat.soft↗

arXiv2018

Linear rheology of reversibly cross-linked biopolymer networks

We suggest a simple model for reversible cross-links, binding and unbinding to/from a network of semiflexible polymers. The resulting frequency dependent response of the network to an applied shear is calculated via Brownian dynamics simulations. It is shown to be rather complex with the timescale of the linkers competing with the excitations of the network. If the lifetime of the linkers is the longest timescale, as is indeed the case in most biological networks, then a distinct low frequency peak of the loss modulus develops. The storage modulus shows a corresponding decay from its plateau value, which for irreversible cross-linkers extends all the way to the static limit. This additional relaxation mechanism can be controlled by the relative weight of reversible and irreversible linkers.

cond-mat.soft↗

arXiv2016

Athermal rheology of weakly attractive soft particles

We study the rheology of a soft particulate system where the inter-particle interactions are weakly attractive. Using extensive molecular dynamics simulations, we scan across a wide range of packing fractions ($ϕ$), attraction strengths ($u$) and imposed shear-rates ($\dotγ$). In striking contrast to repulsive systems, we find that at small shear-rates generically a fragile isostatic solid is formed even if we go to $ϕ\ll ϕ_J$. Further, with increasing shear-rates, even at these low $ϕ$, non-monotonic flow curves occur which lead to the formation of persistent shear-bands in large enough systems. By tuning the damping parameter, we also show that inertia plays an important role in this process. Furthermore, we observe enhanced particle dynamics in the attraction-dominated regime as well as a pronounced anisotropy of velocity and diffusion constant, which we take as precursors to the formation of shear bands. At low enough $ϕ$, we also observe structural changes via the interplay of low shear-rates and attraction with the formation of micro-clusters and voids. Finally, we characterize the properties of the emergent shear bands and thereby, we find surprisingly small mobility of these bands, leading to prohibitely long time-scales and extensive history effects in ramping experiments.

cond-mat.soft↗

arXiv2016

Friction-induced Shear thickening: a microscopic perspective

We develop a microscopic picture of shear thickening in dense suspensions which emphasizes the role of frictional forces, coupling rotational and translational degrees of freedom. Simulations with contact forces and viscous drag only, reveal pronounced shear thickening with a simultaneous increase in contact number and energy dissipation by frictional forces. At high densities, when the translational motion is severely constrained, we observe liquid-like gear-states with pronounced relative rotations of the particles coexisting with solid-like regions which rotate as a whole. The latter are stabilised by frustrated loops which become more numerous and persistent with increasing pressure, giving rise to an increasing lengthscale of this mosaique-like structure and a corresponding increase in viscosity.

cond-mat.soft↗

arXiv2016

Thermalized connectivity networks of jammed packings

Jammed packings of repulsive elastic spheres have emerged as a rich model system within which elastic properties of disordered glassy materials may be elucidated. Most of the work on these packings have focused on the case of vanishing temperature. Here, we explore the elastic properties of the associated connectivity network for finite temperatures, ignoring the breaking of bonds and the formation of new ones. Using extensive Monte Carlo simulations, we find that, as the temperature is increased, the resulting spring network shrinks and exhibits a rapidly softening bulk modulus via a cusp. Moreover, the shear modulus stiffens in a fixed volume ensemble but not in a fixed pressure ensemble. These counter-intuitive behaviors may be understood from the characteristic spectrum of soft modes near isostaticity, which resembles the spectrum of a rod near its buckling instability. Our results suggest a generic mechanism for negative thermal expansion coefficients in marginal solids. We discuss the consequences of bond breaking and an apparent analogy between thermalization and shear.

cond-mat.soft↗

arXiv2016

Rheological Chaos of Frictional Grains

A two-dimensional dense fluid of frictional grains is shown to exhibit time-chaotic, spatially heterogeneous flow in a range of stress values, $σ$, chosen in the unstable region of s-shaped flow curves. Stress controlled simulations reveal a phase diagram with reentrant stationary flow for small and large stress $σ$. In between no steady flow state can be reached, instead the system either jams or displays time dependent heterogeneous strain rates $\dotγ({\bf r},t)$. The results of simulations are in agreement with the stability analysis of a simple hydrodynamic model, coupling stress and microstructure which we tentatively associate with the frictional contact network.

cond-mat.soft↗

arXiv2016

Viscoelasticity of reversibly crosslinked networks of semiflexible polymers

We present a theoretical framework for the linear and nonlinear visco-elastic properties of reversibly crosslinked networks of semiflexible polymers. In contrast to affine models where network strain couples to the polymer end-to-end distance, in our model strain rather serves to locally distort the network structure. This induces bending modes in the polymer filaments, the properties of wich are slaved to the surrounding network structure. Specifically, we investigate the frequency-dependent linear rheology, in particular in combination with crosslink binding/unbinding processes. We also develop schematic extensions to describe the nonlinear response during creep measurements as well as during constant-strainrate ramps.

cond-mat.soft↗

arXiv2015

Free volume under shear

Using an athermal quasistatic simulation protocol, we study the distribution of free volumes in sheared hard-particle packings close to, but below, the random-close packing threshold. We show that under shear, and independent of volume fraction, the free volumes develop features similar to close-packed systems -- particles self-organize in a manner as to mimick the isotropically jammed state. We compare athermally sheared packings with thermalized packings and show that thermalization leads to an erasure of these structural features. The temporal evolution, in particular the opening-up and the closing of free-volume patches is associated with the single-particle dynamics, showing a crossover from ballistic to diffusive behavior.

cond-mat.soft↗

arXiv2015

Motor-free actin bundle contractility driven by molecular crowding

Modeling approaches of suspended, rod-like particles and recent experimental data have shown that depletion forces display different signatures depending on the orientation of these particles. It has been shown that axial attraction of two rods yields contractile forces of 0.1pN that are independent of the relative axial shift of the two rods. Here, we measured depletion-caused interactions of actin bundles extending the phase space of single pairs of rods to a multi-particle system. In contrast to a filament pair, we found forces up to 3pN . Upon bundle relaxation forces decayed exponentially with a mean decay time of 3.4s . These different dynamics are explained within the frame of a mathematical model by taking pairwise interactions to a multi-filament scale. The macromolecular content employed for our experiments is well below the crowding of cells. Thus, we propose that arising forces can contribute to biological force generation without the need to convert chemical energy into mechanical work.

physics.bio-ph↗

arXiv2014

Rheology near jamming - the influence of lubrication forces

We study, by computer simulations, the role of different dissipation forces on the rheological properties of highly-dense particle-laden flows. In particular, we are interested in the close-packing limit (jamming) and the question if "universal" observables can be identified that do not depend on the details of the dissipation model. To this end, we define a simplified lubrication force and systematically vary the range $h_c$ of this interaction. For fixed $h_c$ a cross-over is seen from a Newtonian flow regime at small strain rates to inertia-dominated flow at larger strain rates. The same cross-over is observed as a function of the lubrication range $h_c$. At the same time, but only at high densities close to jamming, particle velocity as well as local density distributions are unaffected by changes in the lubrication range -- they are candidates for "universal" behavior. At densities away from jamming, this universality is lost: short-range lubrication forces lead to pronounced particle clustering, while longer-ranged lubrication does not. These findings highlight the importance of "geometric" packing constraints for particle motion -- independent of the specific dissipation model. With the free volume vanishing at random-close packing, particle motion is more and more constrained by the ever smaller amount of free space. On the other side, macroscopic rheological observables, as well as higher-order correlation functions retain the variability of the underlying dissipation model.

cond-mat.soft↗

arXiv2014

Impact of attractive interactions on the rheology of dense athermal particles

Using numerical simulations, the rheological response of an athermal assembly of soft particles with tunable attractive interactions is studied in the vicinity of jamming. At small attractions, a fragile solid develops and a finite yield stress is measured. Moreover, the measured flow curves have unstable regimes, which lead to persistent shearbanding. These features are rationalized by establishing a link between the rheology and the inter-particle connectivity, which also provides a minimal model to describe the flow curves.

cond-mat.soft↗

arXiv2014

Elasto-plastic response of reversibly crosslinked biopolymer bundles

We study the response of F-actin bundles to driving forces through a simple analytical model. We consider two filaments connected by reversibly bound crosslinks and driven by an external force. Two failure modes under load can be defined. \textit{Brittle failure} is observed when crosslinks suddenly and collectively unbind, leading to catastrophic loss of bundle integrity. During \textit{ductile failure}, on the other hand, bundle integrity is maintained, however at the cost of crosslink reorganization and defect formation. We present phase diagrams for the onset of failure, highlighting the importance of the crosslink stiffness for these processes. Crossing the phase boundaries, force-deflection curves display (frequency-dependent) hysteresis loops, reflecting the first-order character of the failure processes. We evidence how the introduction of defects can lead to complex elasto-plastic relaxation processes, once the force is switched off. Depending on, both, the time-scale for defect motion as well as the crosslink stiffness, bundles can remain in a quasi-permanent plastically deformed state for a very long time.

cond-mat.soft↗

arXiv2014

Jamming of Frictional Particles: a Non-equilibrium First Order Phase Transition

We propose a phase diagram for the shear flow of dry granular particles in two dimensions based on simulations and a phenomenological Landau-theory for a nonequilibrium first order phase transition. Our approach incorporates both frictional as well as frictionless particles. The most important feature of the frictional phase diagram is re-entrant flow and a critical jamming point at finite stress. In the frictionless limit the regime of re-entrance vanishes and the jamming transition is continuous with a critical point at zero stress. The jamming phase diagrams derived from the model agree with the experiments of Bi et al. (Nature (2011)) and brings together previously conflicting numerical results.

cond-mat.soft↗