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We investigate the mechanical response of polyampholyte(PA) chain with distinct charge sequences using molecular dynamics simulations and a theoretical approach based on the generalized random-phase approximation (GRPA). A blocky PA under extensional force undergoes a first-order-like transition from a globule to a coiled state. The effective elastic behavior exhibits four regimes: an initial plateau, stress stiffening, exponential stress-softening behavior, and stress stiffening regime. Both elastic softening and sharp transitions are absent at smaller block lengths. Our findings establish a direct link between electrostatic correlations, charge sequence, and nonlinear elasticity, bridging molecular-scale interactions with emergent macroscopic mechanics.
Rakesh Palariya and Sunil P. Singh, Force-Induced Elastic Softening and Conformational Transitions in a Polyampholyte Chain, Macromolecules, 2026, 59, 5748-5760.
The conformational and rheological properties of active polar linear polymers under linear shear flow are studied analytically. We describe a discrete active polar linear polymer as an inextensible flexible Gaussian bead–spring chain supplemented by active forces along the bonds. The linear, non-Hermitian equations of motion are solved by an eigenfunction expansion in terms of a biorthogonal basis set. The model reveals an intimate coupling between activity and shear flow, which implies activity-enhanced polymer conformational and rheological properties. Compared to a passive polymer, we find a significantly enhanced shrinkage transverse to the flow direction with increasing shear rate, with a power-law exponent of −4/3, compared to the passive value of −2/3.
Panda, A., Singh, S. P., & Winkler, R. G. (2025). Analytical analysis of the conformational and rheological properties of flexible active polar linear polymers under shear flow. The Journal of Chemical Physics, 163(22).
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.
Panda, A., Winkler, R. G., & Singh, S. P. (2025). Activity-enhanced shear thinning of flexible linear polar polymers. Physical Review E, 111(5), 055413.
The role of active stress on the conformational dynamics of a polymer has drawn significant interest due to its potential applications in understanding the energy landscape of protein structures, buckling of bio-polymers, genomic spatial organization and their large-scale coherent dynamics. We present a model of bidirectional active force that acts along the polymer’s tangent, with its direction stochastically reversing between head-to-tail and tail-to-head orientations. The active polymer shows a structural transition from a random coil-like state to a compressed state with variations in the active force, directional (polarity) reversal rate, and their fraction. Furthermore, the polymer reswells and stretches more than its passive limit for a large active force.
Panda A, Singh SP. Folding-Unfolding Transition of Active Polymer on the Reconfiguration of Bidirectional Tangential Active Force. Macromolecules. 2025.(Featured as a cover page of the Journal)
Conformational and dynamical study of flexible and semi-flexible polyampholytes. The sequence of the charged monomer on the backbone leads to a coil-to-globule transition, and the mean-square displacement (MSD) behavior reveals distinct dynamics, specifically for the alternating and charge-segregated sequences. For a semi-flexible PA chain, varying the bending rigidity and electrostatic interaction strength (Γe) leads to distinct, fascinating conformational states, including globule, bundle, and torus-like conformations.
Rakesh Palariya, Sunil P. Singh; Structural transitions of a semi-flexible polyampholyte. J. Chem. Phys. 161 (10): 104903 (2024)
Theoretical study of the conformational and dynamical properties of semiflexible active polar ring polymers under linear shear flow. A ring is described as a continuous semiflexible 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 and rheological properties are equal to those of a passive ring under shear.
Active polar ring polymer in shear flow—An analytical study, RG Winkler and SP Singh, J. Chem. Phys. 161, 064902 (2024)
The collective dynamics of active dumbbells in the presence of a static circular obstacle is shown using Brownian dynamics simulation. The active dumbbells aggregate on the surface of a circular obstacle and display persistent rotational motion with a certain angular speed. Further, the dynamical behavior of a tracer particle in the solution of active dumbbells also studied. Interestingly, the speed of the passive tracer particle displays a crossover from monotonically decreasing to increasing with the size of the tracer particle upon increasing the dumbbells' speed.
Collective dynamics of active dumbbells near a circular obstacle, Chandranshu Tiwari, SP Singh Soft Matter, 20, 4816-4826 (2024)
Brownian dynamics simulation 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.
Characteristic features of self-avoiding active Brownian polymers under linear shear flow
A Panda, RG Winkler, SP Singh, Soft Matter 19 (44), 8577-8586, 2023
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