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Rajneesh Kumar

Publications and source records attributed to Rajneesh Kumar.

8 recordsLinked to original sources
arXiv2026

Finite-Time Orientational Relaxation Restructures Collective Motion in Polar Active Matter

We introduce a Langevin formulation of Vicsek-like active particles in which orientations evolve through finite-rate relaxation toward the local mean direction, with alignment strength $J$ and rotational diffusivity $D_r$, thereby combining Vicsek-type local consensus with XY-like orientational dynamics. Using large-scale numerical simulations, we determine the nonequilibrium phase diagram as a function of activity and alignment rate. Increasing the alignment rate drives a sequence of transitions from a homogeneous isotropic state to polar bands, a cross-sea phase of intersecting bands, a homogeneous polar state, and ultimately a micro-clustered regime. The isotropic-to-polar transition is strongly first order, as evidenced by Binder cumulants and bimodal distributions of local polarization and density, indicating coexistence of gas-like and liquid-like regions. Near the onset of collective motion, band size increases with activity but depends non-monotonically on alignment rate. Further increasing the alignment rate drives the system through the cross-sea and homogeneous polar phases before enhanced density fluctuations lead to micro-clustering. Our results demonstrate that finite-time orientational relaxation acts as a control parameter that qualitatively restructures collective behavior in polar active matter.

cond-mat.soft↗
arXiv2026

The two-stream instability generation around Moon: Effect of Interplanetary Magnetic Field during Solar Wind - Lunar Plasma Interaction

The relative motion of two interpenetrating streams of charged particles usually leads to the generation of two stream instability (TSI) and eventually result in the onset of non-linear plasma processes such as the turbulence or the plasma waves. A natural example of such an event is the solar wind interaction with the lunar electron plasma where the interplanetary magnetic field (IMF) is embedded within the solar wind. The inclusion of IMF in the solar wind lunar plasma interaction modifies the dispersion relation of the TSI and an angular cyclotron frequency term appears in the denominator of the leading term and hence leads to the change in the parameters such as the instability growth rate as now it depends on the solar wind electron velocity, solar wind and the lunar electron plasma density, and the IMF magnitude. It is observed that the growth rate increases fast with the increase in the magnetic field initially but the increase slows down on further increasing the magnetic field thereby smoothening the top. From the particle in cell (PIC) simulations, it is observed that during the solar wind IMF interaction with lunar plasma, the non-energetic background electrons make a shield around the solar wind electrons in the vortices formed. It is further observed that those lunar electrons which are not participating either in the vortices formation or in the shielding of solar wind electrons, start moving in the direction of the incoming solar wind. These observations indicate this interaction is capable in getting converted into a non-linear physical process in the lunar plasma environment.

physics.plasm-ph↗
arXiv2024

Generation of Streaming Beam-Plasma Instability in Variable Lunar Plasma around Moon

Two-stream instability (TSI) is studied analytically in the lunar plasma environment. The electrons in the solar wind constitute the electron-beam and the lunar electron plasma constitutes the background plasma with which the electron-beam interacts to trigger the TSI. The lunar plasma is considered to have a variable proportion of the energetic (hot) electrons, 1% to 25% of the total lunar electrons, along with the bulk thermal (cold) population. The analysis shows that the presence of energetic electrons in the lunar plasma environment modify the TSI dispersion relation and can have a significant impact on the triggering of TSI and are capable of triggering nonlinear phenomena by making the lunar plasma system unstable.

physics.plasm-ph↗
arXiv2024

Mechanistic insights into Z-ring formation and stability: A Langevin dynamics approach to FtsZ self-assembly

The tubulin-like protein FtsZ is crucial for cytokinesis in bacteria and many archaea, forming a ring-shaped structure called the Z-ring at the site of cell division. Despite extensive research, the self-assembly of Z-rings is not entirely understood. We propose a theoretical model based on FtsZ's known filament structures, treating them as semiflexible polymers with specific mechanical properties and lateral inter-segment attraction that can stabilize ring formations. Our molecular dynamics simulations reveal various morphological phases, including open helices, chains, rings, and globules, capturing experimental observations in the fission yeast model using FtsZ from different bacterial species or mutants of Escherichia coli. Using our theoretical model, we explore how treadmilling activity affects Z-ring stability and identify a spooling mechanism of ring formation. The active ring produces contractile, shear, and rotational stresses, which intensify as the Z-ring transitions to an open helix at high activity.

physics.bio-ph↗
arXiv2021

Driven translocation of a flexible polymer through an interacting conical pore

We study the driven translocation of a flexible polymer through an interacting conical pore using Langevin dynamics simulations. We find that, for a fixed value of externally applied force and pore polymer interaction strength, the mean residence time of monomers inside the pore shows non-monotonic variations with pore apex angle $\alpha$. We explain this behavior using a free energy argument by explicitly accounting for pore-polymer interactions and external drive. Our theoretical observations are corroborated by the simulation results of the mean translocation times as the pore-polymer interactions and external driving force are varied.

cond-mat.soft↗
arXiv2019

Inverse design of charged colloidal particle interactions for self assembly into specified crystal structures

We study the inverse problem of tuning interaction parameters between charged colloidal particles interacting with a hard-core repulsive Yukawa potential, so that they assemble into specified crystal structures. Here, we target the body-centered-cubic (bcc) structure which is only stable in a small region in the phase diagram of charged colloids and is, therefore, challenging to find. In order to achieve this goal, we use the statistical fluctuations in the bond orientational order parameters to tune the interaction parameters for the bcc structure, while initializing the system in the fluid phase, using the Statistical Physics-inspired Inverse Design (SP-ID) algorithm [1]. We also find that this optimization algorithm correctly senses the fluid-solid phase boundaries for charged colloids. Finally, we repeat the procedure employing the Covariance Matrix Adaptation - Evolution Strategy (CMA-ES), a cutting edge optimization technique, and compare the relative efficacy of the two methods.

cond-mat.soft↗
arXiv2017

Sequencing of semiflexible polymers of varying bending rigidity using patterned pores

We study the translocation of a semiflexible polymer through extended pores with patterned stickiness, using Langevin dynamics simulations. We find that the consequence of pore patterning on the translocation time dynamics is dramatic and depends strongly on the interplay of polymer stiffness and pore-polymer interactions. For heterogeneous polymers with periodically varying stiffness along their lengths, we find that variation of the block size of the sequences and the orientation, results in large variations in the translocation time distributions. We show how this fact may be utilized to develop an effective sequencing strategy. This strategy involving multiple pores with patterned surface energetics, can predict heteropolymer sequences having different bending rigidity to a high degree of accuracy.

cond-mat.soft↗