arXiv subjects

Arvin Gopal Subramaniam

Publications and source records attributed to Arvin Gopal Subramaniam.

7 recordsLinked to original sources

arXiv2026

Collective dynamics of chemo-mechanical colloidal chains with active tips

We report a study of the emergent dynamics arising in two-dimensional suspensions of semi-flexible chains whose tip is chemically active, generating a phoretic field. By varying the chain length (number of monomers per chain $N_{pc}$), the area fraction $ϕ$, and the sign of the phoretic coupling $J_0$, we map out a rich non-equilibrium phase diagram in the presence of phoretic interactions. For repulsive phoretic interactions ($J_0 > 0$) between the chains, we find that short chains ($N_{pc} = 2$) develop a transient chaotic flow state that crosses over at long times to a global polar flock with super-diffusive mean-squared displacement and long-ranged velocity correlations. Surprisingly, we find this state to have suppressed density fluctuations, indicating the emergence of hyperuniformity. At intermediate chain lengths ($N_{pc} \sim 4$-$8$), the repulsive chemical field drives chaotic mesoscale flows -- a dry route to active turbulence -- without the need for hydrodynamic interactions or steric alignment interactions. For attractive phoretic interactions ($J_0 < 0$), chains self-organise into hedgehog-like micellar aggregates with heads forming the core and flexible tails radiating outward, in structural analogy with amphiphile micellisation but driven entirely by non-equilibrium self-propulsion. A coarse-grained theory of a tip-emitting active rod predicts the onset of the flocking of dimers, though overestimates the presence of polar order for longer chains. Our results establish phoretic tip activity as a minimal, experimentally realisable mechanism for a spectrum of collective states hitherto attributed to hydrodynamic interactions or steric alignment.

cond-mat.soft↗

arXiv2026

Mechanics and statistics of a solvable model of an autophoretic colloidal chain

Equilibrium statistical mechanics owes much of its analytical tractability to symmetry: detailed balance, gradient flows, and the resulting vanishing of steady-state entropy production follow directly from the structure of the underlying dynamics, not from any smallness of the driving. Exact solutions of this kind are rare away from equilibrium. Here we identify a class of far-from-equilibrium active colloidal chains -- coupled via roto-translational, autophoretic (monopolar) interactions -- that admit an exact quasi equilibrium description: at fixed chain geometry, the orientational equations of motion for every monomer are derivable from a scalar potential, detailed balance holds exactly in the orientational sector, while the positional sector breaks the equilibrium structure. The associated steady-state entropy production rate (EPR) vanishes identically for this sector, even though the full system is manifestly driven and dissipative. We solve this reduced dynamics exactly for dimers and semi-analytically for general $N$-mers, obtain the orientational fluctuations and the full-system EPR in closed form, and show that all dissipation is carried by the translational (center-of-mass) sector. We further examine the effect of dipolar chemical emission -- expected from asymmetric micelle deposition at the monomer scale -- and find that the equilibrium structure holds exactly for dimers, whereas for longer chains no such description is possible. A purely dipolar coupling instead producesa genuinely non-equilibrium state with no static attractor, sustaining non-monotonic drift with no fixed limit, and an EPR that itself never reaches steady state. Monopolar coupling remains necessary and sufficient for the polarized state; dipolar coupling alone breaks the quasi-equilibrium structure without replacing it with a new static one.

cond-mat.soft↗

arXiv2026

Flocking transition in phoretically interacting active particles with pinning disorder

Recent studies in the collective behavior of active colloids have shown that a global polar order may emerge due to long-ranged chemo-repulsive interactions between them. Here, we report the role of pinning disorder in the flocking transition for such a system. To this end, we study the problem of chemically interacting active colloids with some fraction of the colloids randomly pinned over space such that they can only rotate while phoretically interacting with other particles. Using this model, we investigate the sustenance of global polar order in the presence of quenched spatial disorder. We quantify the flocking transition by studying the global polarization, and the role of finite-size effects. We find that in the crystallite flocking phase, even a small fraction of pinning can destroy spatial crystalline order, although polar order in the form of a liquid phase is maintained. It is observed that polar order is sustained in a system with a higher pinning fraction if the long-ranged repulsive force is subsequently increased. However, in absence of chemo-repulsive forces between particles, polar order drastically decreases even with a smaller pinning fraction. Our work suggests that the flocking transition of active colloids can be controlled via "translationally inert" obstacles, that rotate but do not translate whilst interacting with the bulk.

cond-mat.soft↗

arXiv2025

Intracellular phagosome shell is rigid enough to transfer outside torque to the inner spherical particle

Intracellular phagosomes have a lipid bilayer encapsulated fluidic shell outside the particle, on the outer side of which, molecular motors are attached. An optically trapped spherical birefringent particle phagosome provides an ideal platform to probe fluidity of the shell, as the inner particle is optically confined both in translation and in rotation. Using a recently reported method to calibrate the translation and pitch rotations - yielding a spatial resolution of about 2 nm and angular resolution of 0.1 degrees - we report novel roto-translational coupled dynamics. We also suggest a new technique where we explore the correlation between the translation and pitch rotation to study extent of activity. Given that a spherical birefringent particle phagosome is almost a sphere, the fact that it turns due to the activity of the motors is not obvious, even implying high rigidity of shell. Applying a minimal model for the roto-translational coupling, we further show that this coupling manifests itself as sustained fluxes in phase space, a signature of broken detailed balance.

physics.bio-ph↗

arXiv2025

Minimal mechanism for flocking in phoretically interacting active particles

Coherent collective motion is a widely observed phenomenon in active matter systems. Here, we report a flocking transition mechanism in a system of chemically interacting active colloidal particles sustained purely by chemo-repulsive torques at low to medium densities. The basic requirements to maintain the global polar order are excluded volume repulsions and long-ranged repulsive torques. This mechanism requires that the time scale individual colloids move a unit length to be dominant with respect to the time they deterministically respond to chemical gradients, or equivalently, pair colloids sliding together a minimal unit length before deterministically rotating away from each other. Switching on the translational repulsive forces renders the flock a crystalline structure. Furthermore, liquid flocks are observed for a range of chemo-attractive inter-particle forces. Various properties of these two distinct flocking phases are contrasted and discussed. We complement these results with stability analysis of a hydrodynamic model, which admits the transition corresponding to destabilization of the flocking state observed in particle-based simulations.

cond-mat.soft↗

arXiv2024

Emergent dynamics due to chemo-hydrodynamic self-interactions in active polymers

The field of synthetic active matter has, thus far, been led by efforts to create point-like, isolated (yet interacting) self-propelled objects (\emph{e.g.} colloids, droplets, microrobots) and understanding their collective dynamics. The design of flexible, freely jointed active assemblies from autonomously powered components remains a challenge. Here, we report freely-jointed active polymers created using self-propelled droplets as monomeric units. Our experiments reveal that the self-shaping chemo-hydrodynamic interactions between the monomeric droplets give rise to an emergent rigidity (the acquisition of a stereotypical asymmetric C-shape) and associated ballistic propulsion of the active polymers. The rigidity and propulsion of the chains vary systematically with their lengths. Using simulations of a minimal model, we establish that the emergent polymer dynamics are a generic consequence of quasi two-dimensional confinement and auto-repulsive trail-mediated chemical interactions between the freely jointed active droplets. Finally, we tune the interplay between the chemical and hydrodynamic fields to experimentally demonstrate oscillatory dynamics of the rigid polymer propulsion. Altogether, our work highlights the possible first steps towards synthetic self-morphic active matter.

cond-mat.soft↗

arXiv2024

Rigid flocks, undulatory gaits, and chiral foldamers in a chemically active polymer

Active matter systems - such as a collection of active colloidal particles - operate far from equilibrium with complex inter-particle interactions that govern their collective dynamics. Predicting the collective dynamics of such systems may aid the design of self-shaping structures comprised of active colloidal units with a prescribed dynamical function. Here, using simulations and theory, we study the collective dynamics of a chain consisting of active Brownian particles with internal interactions via trail-mediated chemicals, connected by harmonic springs in two dimensions to obtain design principles for active colloidal molecules. We show that two-dimensional confinement and chemo-repulsive interactions between the freely-jointed particles lead to an emergent rigidity of the chain in the steady-state dynamics. In the chemo-attractive regime, the chain collapses into crystals that abruptly halt their motion. Further, in a chain consisting of a binary mixture of monomers, we show that non-reciprocal chemical affinities between distinct species give rise to novel phenomena, such as chiral molecules with tunable dynamics, sustained undulatory gaits and reversal of the direction of motion. Our results suggest a novel interpretation of the role of trail-mediated interactions, in addition to providing active self-assembly principles arising due to non-reciprocal interactions.

cond-mat.soft↗