arXiv subjects

Sunil P. Singh

Publications and source records attributed to Sunil P. Singh.

At least 19 recordsLinked to original sources

arXiv2026

Optimal Transport of an Anisotropic Tracer in Dense Active Suspensions

The transport of anisotropic tracers in active fluids exhibits rich dynamical behavior arising from the interplay between particle shape, activity, and steric interactions. We employ Brownian dynamics simulations to investigate the motion of an elliptical tracer immersed in a suspension of active dumbbells. We find that both translational and rotational transport, characterized by the mean-square speed and diffusivity, are enhanced by more than an order of magnitude with increasing area fraction, $ϕ$, of active dumbbells. Notably, tracer motion is enhanced along the major axis relative to the minor axis, with $\mathrm{v}_{\parallel}>\mathrm{v}_{\perp}$ and $D_{\parallel}>D_{\perp}$. Remarkably, both translational and rotational transport exhibit an optimum at an intermediate packing fraction of active dumbbells, with the corresponding transport coefficients decreasing at higher densities. We show that this non-monotonic transport arises from the anisotropic accumulation and aggregation of active dumbbells around the tracer, which control the non-equilibrium force and torque fluctuations. Thus, establish a direct connection between the collective organization of active dumbbells at the tracer surface and its emergent translational and rotational transport.

cond-mat.soft↗

arXiv2026

Virial stress in systems of active Brownian particles in the presence of translational and rotational inertia

We elucidate the stress in a system of active Brownian particles augmented with translational and rotational inertia (ABP+TRI). Stress tensors are derived for periodic systems as well as systems confined between walls by employing Lagrange's equations of motion of the first kind for the rotational motion. Using Langevin simulations of an ideal active gas in two dimensions, we confirm the existence of an equation of state for periodic systems that depends on translational and rotational inertia in general. Confinement implies a strong polarization of the propulsion direction near a wall and an enhanced density, both of which increase with increasing rotational inertia. This affects the local stress tensor normal to the confining walls, leading to a breakdown of the equation of state. Yet the local stress in the bulk part of the confined systems is identical with that of the periodic system. Importantly, for both kinds of boundary conditions, the so-called swim stress is not included in the local stress tensor; thus, in general, the swim stress is not representative of the stress in systems of ABP+TRIs.

cond-mat.soft↗

arXiv2025

Force-induced Elastic Softening and Conformational Transitions in a Polyampholyte Chain

The mechanical response of intrinsically disordered proteins (IDPs) and polyampholyte (PA) chains is vital for understanding their biological functions and designing functional materials. We investigate the force-extension behavior of a PA chain with distinct charge sequences using molecular dynamics simulations and a theoretical approach based on the generalized random-phase approximation (GRPA). A diblock PA chain under extensional force undergoes a continuous coil-to-stretch transition at weak electrostatic coupling, which sharpens into a globule-coil-like transition at stronger coupling. The GRPA theory quantitatively captures these behaviors, including the sharp conformational transition and its dependence on electrostatic strength. Simulations reveal pronounced hysteresis during the force-extension and relaxation processes. Additionally, the elastic modulus exhibits four regimes: an initial plateau, stress stiffening, an exponential stress-softening behavior, and a stress stiffening regime. Using the theoretical model and structural input of the PA chain, we have demonstrated that the elastic modulus in the elastic softening regime decreases exponentially, $E\sim \exp(-α_0 f/Γ_e)$, as a function of $f$, which aligns with the simulation results. The elastic response of the PA chain is further examined across different charge sequences, where both elastic softening and sharp transitions are absent at smaller block lengths. Finally, coarse-grained models of IDPs such as LAF-1 and DDX4 exhibit similar nonlinear elasticity, highlighting the universality of these mechanisms. Our results establish a fundamental link between electrostatic correlations, charge sequence, and nonlinear elasticity, bridging molecular interactions and macroscopic mechanics.

cond-mat.soft↗

arXiv2025

Activity-enhanced shear thinning of flexible linear polar polymers

The rheological properties of tangentially propelled flexible polymers under linear shear flow are studied by computer simulations and are compared with analytical calculations. We find a significant impact of the coupled nonequilibrium active and shear forces on the polymer characteristics. The polar activity enhances shear-induced stretching along the flow direction, shrinkage in the transverse direction, and implies a strongly amplified shear-thinning behavior. The characteristic shear rate for the onset of these effects is determined by the activity. In the asymptotic limit of large activities, the shear-induced features become independent of activity, and for asymptotically large shear rates, shear dominates over activity with passive polymer behavior.

cond-mat.soft↗

arXiv2024

Structural transitions of a Semi-Flexible Polyampholyte

Polyampholytes (PA) are charged polymers composed of positively and negatively charged monomers along their backbone. The sequence of the charged monomers and the bending of the chain significantly influence the conformation and dynamical behavior of the PA. Using coarse-grained molecular dynamics simulations, we comprehensively study the structural and dynamical properties of flexible and semi-flexible polyampholytes'. The simulation results demonstrate a flexible polyampholyte (PA) chain, displaying a transition from a coil to a globule in the parameter space of the charge sequence. Additionally, the behavior of the mean-square displacement (MSD), denoted as $<(Δr(t))^2>$, reveals distinct dynamics, specifically for the alternating and charge-segregated sequences. The MSD follows a power-law behavior, where $<(Δr(t))^2> \sim t^β$, with $β\approx 3/5$ and $β\approx 1/2$ for the alternating sequence and charge-segregated sequence in the absence of hydrodynamic interactions, respectively. However, when hydrodynamic interactions are incorporated, the exponent $β$ shifts to approximately 3/5 for the charge-segregated sequence and 2/3 for the well-mixed alternating sequence. For a semi-flexible PA chain, varying the bending rigidity and electrostatic interaction strength ($Γ_e$) leads to distinct, fascinating conformational states, including globule, bundle, and torus-like conformations. We show that PA acquires circular and hairpin-like conformations in the intermediate bending regime. The transition between various conformations is identified in terms of the shape factor estimated from the ratios of eigenvalues of the gyration tensor.

cond-mat.soft↗

arXiv2024

Active Polar Ring Polymer in Shear Flow -- An Analytical Study

We theoretically study the conformational and dynamical properties of semiflexible active polar ring polymers under linear shear flow. A ring is described as a continuous Gaussian polymer with a tangential active force of a constant density along its contour. The linear but non-Hermitian equation of motion is solved using an eigenfunction expansion, which yields activity-independent, but shear-rate-dependent, relaxation times and activity-dependent frequencies. As a consequence, the ring's stationary-state properties are independent of activity, and its conformations as well as rheological properties are equal to those of a passive ring under shear. The presence of characteristic time scales by the relaxation and the frequency gives rise to a particular dynamical behavior. A tank-treading-like motion emerges for large relaxation times and high frequencies, specifically for stiffer rings, governed by the activity-dependent frequencies. In the case of very flexible polymers, the relaxation behavior dominates over tank-treading. Shear strongly affects the crossover from a tank-treading to a relaxation-time dominated dynamics and suppresses tank-treading. This is reflected in the tumbling frequency, which exhibits two shear-rate dependent regimes, with an activity-dependent plateau at low shear rates followed by a power-law regime with increasing tumbling frequency for large shear rates.

cond-mat.soft↗

arXiv2024

Collective dynamics of active dumbbells near a circular obstacle

In this article, we present the collective dynamics of active dumbbells in the presence of a static circular obstacle using Brownian dynamics simulation. The active dumbbells aggregate on the surface of a circular obstacle beyond a critical radius. The aggregation is non-uniform along the circumference, and the aggregate size increases with the activity and the curvature radius. The dense aggregate of active dumbbells displays persistent rotational motion with a certain angular speed, which linearly increases with the activity. Further, we show the strong polar ordering of the active dumbbells within the aggregate. The polar ordering exhibits a long-range correlation, with the correlation length corresponding to the aggregate size. Additionally, we show that the residence time of an active dumbbell on the obstacle surface grows rapidly with area fraction due to many-body interactions that lead to a slowdown of the rotational diffusion. The article further considers the dynamical behavior of a tracer particle in the solution of active dumbbells. Interestingly, the speed of the passive tracer particle displays a crossover from monotonically decreasing to increasing with the tracer particle's size upon increasing the dumbbells' speed. Furthermore, the effective diffusion of the tracer particle displays the non-monotonic behavior with area fraction; the initial increase of the diffusivity is followed by a decrease for larger area fraction.

cond-mat.soft↗

arXiv2023

Characteristic features of self-avoiding active Brownian polymers under linear shear flow

We present Brownian dynamics simulation results of a flexible linear polymer with excluded-volume interactions under shear flow in the presence of active noise. The active noise strongly affects the polymer's conformational and dynamical properties, such as the stretching in the flow direction and compression in the gradient direction, shear-induced alignment, and shear viscosity. In the asymptotic limit of large activities and shear rates, the power-law scaling exponents of these quantities differ significantly from those of passive polymers. The chain's shear-induced stretching at a given shear rate is reduced by active noise, and it displays a non-monotonic behavior, where an initial polymer compression is followed by its stretching with increasing active force.

cond-mat.soft↗

arXiv2022

Compression of a confined semiflexible polymer under direct and oscillating fields

The folding transition of biopolymers from the coil to compact structures has attracted wide research interest in the past and is well studied in polymer physics. Recent seminal works on DNA in confined devices have shown that these long biopolymers tend to collapse under an external field, contrary to the previously reported stretching. These long folded structures have a tendency to form knots that has profound implications in gene regulation and various other biological functions. These knots have been mechanically induced via optical tweezers, nanochannel confinement, etc., until recently, where uniform field driven compression lead to self entanglement of DNA. In this work, we capture the compression of a confined semiflexible polymer under direct and oscillating fields, using a coarse-grained computer simulation model in the presence of long-range hydrodynamics. Within this framework, we show that subjected to direct field, chains in stronger confinements exhibit substantial compaction, contrary to the one in moderate confinements or bulk, where such compaction is absent. Interestingly, an alternating field within an optimum frequency can effectuate this compression even in moderate or no confinement. Additionally, we show that the bending rigidity has a profound influence on the chains folding favourability under direct and alternating fields. This field induced collapse is a quintessential hydrodynamic phenomenon, resulting in intertwined knotted structures, even for shorter chains, unlike DNA knotting experiments, where it happens exclusively for longer chains.

cond-mat.soft↗

arXiv2021

The explicit characterization of counterion dynamics around a flexible polyelectrolyte

The article presents a comprehensive study of counterion dynamics around a generic linear polyelectrolyte (PE) chain with the help of coarse-grained computer simulations. The ion-chain coupling is discussed in the form of binding time, mean-square-displacement (MSD) relative to the chain, local ion transport coefficient, and spatio-temporal correlations in the effective charge. We have shown that a counterion exhibits sub-diffusive behavior $\langle δR^2 \rangle \sim t^δ$, $δ\approx0.9$ w.r.t. chain's centre of mass (COM). The MSD of ions perpendicularly outwards from the chain segment exhibits a smaller sub-diffusive exponent compared to the one relative to the chain's COM. Further, we confirm that the effective diffusion-coefficient of counterions is strongly coupled with the chain. The effective diffusivity of ion is the lowest in chain's close proximity, extending up to length-scale of the radius of gyration Rg. Beyond Rg at larger distances, they attain diffusivity of free ion with a smooth cross-over from the adsorbed regime to the free ion regime. We have shown that the effective diffusivity drastically decreases for the higher valent ions, while the crossover length scale remains the same. Conversely, with increasing salt concentration the coupling-length scale reduces, while the diffusivity remains unaltered. The effective diffusivity of adsorbed-ion reveals an exponential reduction with electrostatic interaction strength. We further corroborate this from the binding time of ions on the chain, which also grows exponentially with the coupling strength of the ion-polymer duo. Moreover, the binding time of ions exhibits a weak dependence with salt concentration for the monovalent salt, while for higher valent salts the binding time decreases dramatically with concentration.

cond-mat.soft↗

arXiv2021

A phase separation of active colloidal suspension via Quorum-sensing

We present the Brownian dynamics simulation of active colloidal suspension in two dimensions, where the self-propulsion speed of a colloid is regulated according to the local density sensed by it. The role of concentration-dependent motility on the phase-separation of colloids and their dynamics is investigated in detail. Interestingly, the system phase separates at a very low packing fraction ($Φ\approx 0.125$) at higher self-propulsion speeds ($\text{Pe}$), which coexists with a homogeneous phase and attains long-range crystalline order beyond a transition point. The transition point is quantified here from the local density profiles, local and global-bond order parameters. We have shown that the phase diagram's characteristics are qualitatively akin to the active Brownian particle (ABP) model. Moreover, our investigation reveals that the density-dependent motility amplifies the slow-down of the directed speed, which facilitates phase-separation even at low packing fractions. The effective diffusivity shows a crossover from quadratic rise to a power-law behavior of exponent $3/2$ with $\text{Pe}$ in the phase-separated regime. Furthermore, we have shown that the effective diffusion decreases exponentially with packing fraction in the phase-separated regime while linear decrease in the single phase regime.

cond-mat.soft↗

arXiv2020

Conformation and dynamics of a self-avoiding active flexible polymer

We investigate conformations and dynamics of a polymer considering its monomers to be active Brownian particles. This active polymer shows very intriguing physical behavior which is absent in an active Rouse chain. The chain initially shrinks with active force, which starts swelling on further increase in force. The shrinkage followed by swelling is attributed purely to excluded-volume interactions among the monomers. In the swelling regime, chain shows a cross-over from the self-avoiding behavior to Rouse-behavior with scaling exponent $ν_a \simeq 1/2$ for end-to-end distance. The non-monotonicity in the structure is analysed through various physical quantities specifically, radial distribution function of monomers, scattering time, as well as various energy calculations. The chain relaxes faster than the Rouse chain in the intermediate force regime, with a cross-over in variation of relaxation time at large active force as given by a power-law $τ_r \sim Pe^{-4/3}$ (P e is Péclet number).

cond-mat.soft↗

arXiv2019

Beating to rotational transition of a clamped active ribbon-like filament

We present a detailed study of a clamped ribbon-like filament under a compressive active force using Brownian dynamics simulations. We show that a clamped ribbon-like filament is able to capture beating as well as a rotational motion under the compressive force. The nature of oscillation is governed by the torsional rigidity of the filament. The frequency of oscillation is almost independent of the torsional rigidity. The beating of the filament gives butterfly shape trajectory of the free-end monomer, whereas rotational motion yields a circular trajectory on a plane. The binormal correlation and the principal component analysis reveal the butterfly, elliptical, and circular trajectories of the free end monomer. We present a phase diagram for different kinds of motion in the parameter regime of compressive force and torsional rigidity.

cond-mat.soft↗

arXiv2019

Behavior of active filaments near solid-boundary under linear shear flow

The steady-state behavior of a dilute suspension of self-propelled filaments confined between planar walls subjected to the Couette-flow is reported herein. The effect of hydrodynamics has been taken into account using a mesoscale simulation approach. We present a detailed analysis of positional and angular probability distributions of filaments with varying propulsive force and shear-flow. Distribution of centre-of-mass of the filament shows adsorption near the surfaces, which diminishes with the flow. The excess density of filaments decreases with Weissenberg number as $Wi^{-β}$ with an exponent $β\approx 0.8$, in the intermediate shear range ($1 < Wi < 30$). The angular orientational moment also decreases near the wall as $Wi^{-δ}$ with $δ\approx 1/5$; the variation in orientational moment near the wall is relatively slower than the bulk. It shows a strong dependence on the propulsive force near the wall, and it varies as $Pe^{-1/3}$ for large $Pe\ge 1$. The active filament shows orientational preference with flow near the surfaces, which splits into upstream and downstream swimming. The population splitting from a unimodal (propulsive force dominated regime) to bimodal phase (shear dominated regime) is identified in the parameter space of propulsive force and shear flow.

cond-mat.soft↗

arXiv2019

Structure and dynamics of a self-propelled semiflexible filament

We investigate structural and dynamical properties of a self-propelled filament using coarse-grained Brownian dynamics simulations. A self-propulsion force is applied along the bond vectors, i.e., tangent to the filament and their locations are considered in two different manners. In case one, force is applied to all beads of the filament, which is termed as homogeneous self-propulsion. Here, we obtain a monotonic decrease in the flexibility of the filament with Péclet number. Hence, radius of gyration also displays the same trend. Moreover, the radius of gyration of the filament shows universal dependence for various bending rigidities with flexure number. The effective diffusivity of the filament shows enhancement with the active force and it increases linearly with force and bending rigidity. In case two, self-propulsion force is applied only to few bond vectors. The location of active forces is chosen in a periodic manner starting from the tail of the filament and leaving the front end without force. In this case, filament acquires various structures such as rod-like, helical, circular, and folded states. The transition from several states is understood in terms of tangent-tangent correlation, bending energy and torsional order parameter. The helical state is identified through a crossover from exponential to oscillatory behavior of the tangent-tangent correlation. A sudden increase in the bending energy separates a helical to a folded states of the filament.

cond-mat.soft↗

arXiv2018

Steady state sedimentation of ultrasoft colloids

The structural and dynamical properties of ultra-soft colloids - star polymers - exposed to a uniform external force field are analyzed applying the multiparticle collision dynamics approach, a hybrid coarse-grain mesoscale simulation approach, which captures thermal fluctuations and long-range hydrodynamic interactions. In the weak field limit, the structure of the star polymer is nearly unchanged, however in an intermediate regime, the radius of gyration decreases, in particular transverse to the sedimentation direction. In the limit of a strong field, the radius of gyration increases with field strength. Correspondingly, the sedimentation coefficient increases with increasing field strength, passes through a maximum and decreases again at high field strengths. The maximum value depends on the functionality of the star polymer. High field strengths lead to symmetry breaking with trailing, strongly stretched polymer arms and a compact star polymer body. In the weak field linear response regime, the sedimentation coefficient follows the scaling relation of a star polymer in terms of functionality and arm length.

cond-mat.soft↗

arXiv2012

Quasi-Probability Distribution Functions for Optical-Polarization

Cahill-Glauber C(s)-correspondence is employed to construct Quasi-Probability Distribution Functions (QPDFs) for optical-polarization in phase space following equivalent description of polarization in Classical Optics. The proposed scheme provides pragmatic insights as compared to obscure SU (2) quasi-distributions on Poincare sphere. QPDF (Wigner function) of bi-modal quadrature coherent states is evaluated and numerically investigated to demonstrate the application.

quant-ph↗

arXiv2012

Higher (2nd)-order polarization-Wigner function for `even' entangled bi-modal coherent states

Higher (2nd)-order Wigner distribution function in quantum phase space for entangled bi-modal coherent states, a representative of higher (2nd)-order optical-polarization, is introduced by generalizing kernel (transiting) operator in Cahill-Glauber C(s)-correspondence rule. The nature is analyzed which reveals the occurrence of oscillating three peaks: 'two' for individual bi-modes and third for interference between modes. Also, the graphics of 2nd-order polarization-Wigner distribution function, incisively, demonstrates that it is of non-Gaussian nature attaining non-negative values in quantum phase space.

quant-ph↗