arXiv subjects

Ehsan Irani

Publications and source records attributed to Ehsan Irani.

7 recordsLinked to original sources

arXiv2025

Emergent Isotropic-Nematic Transition in 3D Semiflexible Active Polymers

Active semiflexible filament collectives, ranging from motor-driven cytoskeletal filaments to slender organisms such as cyanobacteria and worm aggregates, abound in nature. Yet how activity and flexibility jointly govern their organization, especially Isotropic-Nematic (I-N) transition, remains poorly understood. Performing large-scale Brownian dynamics simulations of 3D active semiflexible polymers with varying flexibility degrees, we show that tangential active forces systematically shift the I-N transition to higher densities, with the shift controlled by the flexibility degree and activity strength. Strikingly, activity alters the nature of the transition: discontinuous at low strengths, continuous at moderate strengths, and ultimately suppressed at high activity levels. The delayed I-N transition originates from enhanced collective bending fluctuations, resulting in chain shrinkage and enlargement of effective confinement tube. At moderate activity levels, these fluctuations can trigger large-scale excitations that stochastically drive temporal transitions between nematic and isotropic states, indicating an activity-induced instability of the nematic field. We summarize this behavior in non-equilibrium state diagrams of density and activity for different flexibility degrees.

cond-mat.soft↗

arXiv2023

Active motion of tangentially-driven polymers in periodic array of obstacles

We computationally investigate the active transport of tangentially-driven polymers with varying degrees of flexibility and activity in two-dimensional square lattices of obstacles. Tight periodic confinement induces notable conformational changes and distinct modes of transport for flexible and stiff active filaments. It leads to localization and caging of flexible polymers inside the inter-obstacle pores, while promoting more elongated conformations and enhanced diffusion for stiff polymers at low to moderate activity levels. The migration of flexible active polymers occurs via hopping events, where they unfold to move from one cage to another. In contrast, stiff chains travel mainly in straight paths within inter-obstacle channels, while occasionally changing their direction of motion. Both the duration of caging and persistent directed migration within the channels decrease with increasing the activity level. As a consequence, at high active forces polymers overcome confinement effects and transport within inter-obstacle pores as swiftly as those in free space. We explain the center of mass dynamics of semiflexible polymers in terms of active force and obstacle packing fraction by developing an approximate analytical theory.

cond-mat.soft↗

arXiv2023

Effects of inertia on conformation and dynamics of tangentially-driven active filaments

Active filament-like systems propelling along their backbone exist across the scales ranging from motor-driven bio-filaments to worms and robotic chains. In macroscopic active filaments such as chain of robots, in contrast to their microscopic counterparts, inertial effects on their motion cannot be ignored. Nonetheless, consequences of interplay between inertia and flexibility on shape and dynamics of active filaments remain unexplored. Here, we examine inertial effects on flexible tangentially-driven active polymer model pertinent to above examples and we determine the conditions under which inertia becomes important. Performing Langevin dynamics simulations of active polymers with underdamped and overdamped dynamics for a wide range of contour lengths and activities, we uncover striking inertial effects on conformation and dynamics at high activities. Inertial collisions increase the persistence length of active polymers and remarkably alter their scaling behavior. In stark contrast to passive polymers, inertia leaves its fingerprint at long times by an enhanced diffusion of the center of mass. We rationalize inertia-induced enhanced dynamics by analytical calculations of center of mass velocity correlations, applicable to any active polymer model, which reveal significant contributions from active force fluctuations convoluted by inertial relaxation.

cond-mat.soft↗

arXiv2022

Dynamics of bacteria scanning a porous environment

It has recently been reported that bacteria, such as E.coli and P. putida, perform distinct modes of motion when placed in porous media as compared to dilute regions or free space. This has led us to suggest an efficient strategy for active particles in a disordered environment: reorientations are suppressed in locally dilute regions and intensified in locally dense ones. Thereby the local geometry determines the optimal path of the active agent and substantially accelerates the dynamics for up to two orders of magnitude. We observe a non-monotonic behavior of the diffusion coefficient in dependence on the tumbling rate and identify a localisation transition, either by increasing the density of obstacles or by decreasing the reorientation rate.

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↗

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↗

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↗