We study the response to shear deformations of packings of long spherocylindrical particles that interact via frictional forces with friction coefficient $mu$. The packings are produced and deformed with the help of molecular dynamics simulations com
bined 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 $elltoinfty$, even though the density of the packings vanishes, $rhoproptoell^{-2}$. By way of contrast, the frictionless modulus decreases to zero, $g_0simell^{-2}$, in accordance with the behavior of density. Increasing the strain beyond a value $gamma_csim mu$, 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 $sigma_y=g_inftygamma_c$ and the stress behaves as $sigma=sigma_y+g_0gamma$. The interplay between static and sliding friction gives rise to hysteresis in oscillatory shear simulations.
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 un
til a force-equilibrated state is reached. For different particle aspect ratios $alpha=10ldots 40$, 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.
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 Weisenbe
rg number $ text{Wi}sim bdotgamma/u$ and displays Herschel-Bulkley form $sigma = sigma_y+ctext{Wi}^ u$ with exponent $ u=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 $Deltasigmasim T_s^{2/3}$, 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.
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 ($phi$), attraction strengths ($u$) and i
mposed shear-rates ($dot{gamma}$). 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 $phi ll phi_J$. Further, with increasing shear-rates, even at these low $phi$, 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 $phi$, 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.
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.
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 observ
ables 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.
We consider the shear rheology of concentrated suspensions of non-Brownian frictional particles. The key result of our study is the emergence of a pronounced shear-thickening regime, where frictionless particles would normally undergo shear-thinning.
We clarify that shear thickening in our simulations is due to enhanced energy dissipation via frictional inter-particle forces. Moreover, we evidence the formation of dynamically correlated particle-clusters of size $xi$, which contribute to shear thickening via an increase in emph{viscous} dissipation. A scaling argument gives $etasim xi^2$, which is in very good agreement with the data.
