arXiv subjects

Hossein Vahid

Publications and source records attributed to Hossein Vahid.

9 recordsLinked to original sources

arXiv2026

Self-organization and cyclic positioning of active condensates

Cohesive active assemblies are often regulated by spatially heterogeneous nonequilibrium driving, such as gradients in motility, biochemical turnover, or mechanical activity. Such heterogeneous driving can influence where condensates or cell collectives accumulate, how stable they are, and how they exchange material with their surroundings. However, the minimal physical mechanisms by which activity gradients control the positioning and turnover of cohesive active matter remain unclear. Here, we address this question using a model of attractive active Brownian particles (ABPs) in a spatially varying activity field. Using Brownian dynamics simulations, we show that these particles undergo liquid-gas phase separation, and spatially varying activity fields induce striking emergent dynamics. Attractive active droplets migrate up activity gradients, and at sufficiently high activity, they can fragment or evaporate into a dilute phase. For finite clusters, evaporated ABPs can redistribute through the simulation box, reassemble into new clusters in lower-activity regions, and migrate again toward higher activity, giving rise to cyclic positioning through repeated nucleation, migration, evaporation, and reassembly.

cond-mat.soft↗

arXiv2025

Collective Dynamics in Active Polar Polymer Assemblies

Tangentially driven active polymers (TDAPs), model systems for motor-driven filaments, have been extensively studied in uniform activity fields. Here, we show that an activity gradient breaks fore-aft symmetry, generating net body forces that steer dimers, asters, and larger assemblies toward high-activity regions. Including temporal stochasticity softens the chains, allowing them to bend and wind around other filaments. Once several contacts are established, steric interlocking arrests relative motion and stabilizes the assembly into a hierarchically entangled cluster. These clusters persist for times far exceeding single-chain relaxation and do not appear under deterministic, temporally constant activity. Remarkably, such activity-induced gelation occurs even at polymer concentrations substantially lower than those typically required for passive chains. Our results reveal a new mechanism for activity-induced aggregation, providing new strategies for designing autonomous and reconfigurable microfluidic systems.

cond-mat.soft↗

arXiv2025

Active Transport of Cargo-Carrying and Interconnected Chiral Particles

Directed motion up a concentration gradient is crucial for the survival and maintenance of numerous biological systems, such as sperms moving towards an egg during fertilization or ciliates moving towards a food source. In these systems, chirality - manifested as a rotational torque - plays a vital role in facilitating directed motion. While systematic studies of active molecules in activity gradients exist, the effect of chirality remains little studied. In this study, we examine the simplest case of a chiral active particle connected to a passive particle in a spatially varying activity field. We demonstrate that this minimal setup can exhibit rich emergent tactic behaviors, with the chiral torque serving as the tuning parameter. Notably, when the chiral torque is sufficiently large, even a small passive particle enables the system to display the desired accumulation behavior. Our results further show that in the dilute limit, this desired accumulation behavior persists despite the presence of excluded volume effects. Additionally, interconnected chiral active particles exhibit emergent chemotaxis beyond a critical chain length, with trimers and longer chains exhibiting strong accumulation at sufficiently high chiral torques. This study provides valuable insights into the design principles of hybrid bio-molecular devices of the future.

cond-mat.stat-mech↗

arXiv2024

Adsorption of polyelectrolytes in the presence of varying dielectric discontinuity between solution and substrate

We examine the interactions between polyelectrolytes (PEs) and uncharged substrates at conditions corresponding to a dielectric discontinuity between the aqueous solution and the substrate. To this end, we vary the relevant system characteristics, in particular the substrate dielectric constant $\varepsilon_{\rm s}$ under different salt conditions. We employ coarse-grained molecular dynamics simulations with rodlike PEs in salt solutions with explicit ions and implicit water solvent with dielectric constant $\varepsilon_{\rm w} = 80$. As expected, at low salt concentrations, PEs are repelled from the substrates with $\varepsilon_{\rm s} < \varepsilon_{\rm w}$ but are attracted to substrates with a high dielectric constant due to image charges. This attraction considerably weakens for high salt and multivalent counterions due to enhanced screening. Further, for monovalent salt, screening enhances adsorption for weakly charged PEs, but weakens it for strongly charged ones. Multivalent counterions, on the other hand, have little effect on weakly charged PEs, but prevent adsorption of highly charged PEs, even at low salt concentrations. We also find that correlation-induced charge inversion of a PE is enhanced close to the low dielectric constant substrates, but suppressed when the dielectric constant is high. To explore the possibility of a PE monolayer formation, we examine the interaction of a pair of like-charged PEs aligned parallel to a high dielectric constant substrate with $\varepsilon_{\rm s} = 8000$. Our main conclusion is that monolayer formation is possible only for weakly charged PEs at high salt concentrations of both monovalent and multivalent counterions. Finally, we also consider the energetics of a PE approaching the substrate perpendicular to it, in analogy to polymer translocation.

cond-mat.soft↗

arXiv2023

Non-monotonic electrophoretic mobility of rod-like polyelectrolytes by multivalent coions in added salt

It is well established that when multivalent counterions or salts are added to a solution of highly-charged polyelectrolytes (PEs), correlation effects can cause charge inversion of the PE, leading to electrophoretic mobility (EM) reversal. In this work, we use coarse-grained molecular dynamics simulations to unravel the less understood effect of coion valency on EM reversal for rigid DNA-like PEs. We find that EM reversal induced by multivalent counterions is suppressed with increasing coion valency in the salt added and eventually vanishes. Further, we find that EM is enhanced at fixed low salt concentrations for salts with monovalent counterions when multivalent coions with increasing valency are introduced. However, increasing the salt concentration causes a crossover that leads to EM reversal which is enhanced by increasing coion valency at high salt concentration. Remarkably, this multivalent coion-induced EM reversal persists even for low values of PE linear charge densities where multivalent counterions alone cannot induce EM reversal. These results facilitate tuning PE-PE interactions and self-assembly with both coion and counterion valencies.

cond-mat.soft↗

arXiv2022

Interactions between polyelectrolytes mediated by ordering and orientation of multivalent non-spherical ions in salt solutions

Multivalent ions in solutions with polyelectrolytes (PE) induce electrostatic correlations that can drastically change ion distributions around the PEs and their mutual interactions. Using coarse-grained molecular dynamics simulations, we show how in addition to valency, ion shape and concentration can be harnessed as tools to control like-charged PE-PE interactions. We demonstrate a correlation between the orientational ordering of aspherical ions and how they mediate the effective PE-PE attraction induced by multivalency. The interaction type, strength, and range can thus be externally controlled in ionic solutions. Our results can be used as generic guidelines to tune the self-assembly of like-charged polyelectrolytes by variation of the characteristics of the ions.

cond-mat.soft↗

arXiv2022

Modified Poisson-Boltzmann theory for polyelectrolytes in monovalent salt solutions with finite-size ions

We present a soft-potential-enhanced Poisson-Boltzmann (SPB) theory to efficiently capture ion distributions and electrostatic potential around rodlike charged macromolecules. The SPB model is calibrated with a coarse-grained particle-based model for polyelectrolytes (PEs) in monovalent salt solutions as well as compared to a full atomistic molecular dynamics simulations with explicit solvent. We demonstrate that our modification enables the SPB theory to accurately predict monovalent ion distributions around a rodlike PE in a wide range of ion and charge distribution conditions in the weak-coupling regime. These include excess salt concentrations up to 1 M, and ion sizes ranging from small ions, such as Na or Cl, to softer and larger ions with size comparable to the PE diameter. The work provides a simple way to implement an enhancement that effectively captures the influence of ion size and species into the PB theory in the context of PEs in aqueous salt solutions.

cond-mat.soft↗

arXiv2022

Interaction between two polyelectrolytes in monovalent aqueous salt solutions

We use the recently developed soft-potential-enhanced Poisson-Boltzmann (SPB) theory to study the interaction between two parallel polyelectrolytes (PEs) in monovalent ionic solutions in the weak-coupling regime. The SPB theory is fitted to ion distributions from coarse-grained molecular dynamics (MD) simulations and benchmarked against all-atom MD modelling for poly(diallyldimethylammonium) (PDADMA). We show that the SPB theory is able to accurately capture the interactions between two PEs at distances beyond the PE radius. For PDADMA positional correlations between the charged groups lead to locally asymmetric PE charge and ion distributions. This gives rise to small deviations from the SPB prediction that appear as short-range oscillations in the potential of mean force. Our results suggest that the SPB theory can be an efficient way to model interactions in chemically specific complex PE systems.

cond-mat.soft↗

arXiv2021

Designing and manufacturing of interference notch filter with a single reflection band

Rugate, HL, and hybrid methods are used to design Notch Filters. Achieving a design that fits the notch characteristics of the filter and reducing transmission band ripples are the most influential factors in this topic. The ability of easy fabrication and choosing the right manufacturing method to have a notch filter with better characteristics should be noted. The main idea of this work is to design a notch filter structure with a narrow notch width and maximum reflection while reducing fabrication challenges. In addition, using anti-reflection layers in the outermost part of the designed structure, the pass-band ripples are reduced. In designed systems, various quarter-wave factors were used, which have less challenge in the manufacturing step. In addition, the stability of the deposition conditions and the density of the layers affect their quality and consequently the result of environmental tests. Therefore, the sputtering method with RF and DC sources is used to construct the designed structure. Finally, the samples were subjected to environmental tests such as abrasion, humidity, temperature, and adhesion. Some of the significant results of the manufactured single-band notch filter are presented.

physics.app-ph↗