From Sorting to Motility induced phase separation: Active-Passive Dynamics

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The transport dynamics and phase behavior of active-passive mixtures vary drastically across density regimes, shifting from localized, activity-induced transport in the dilute limit to collective crowding in the dense limit. In low-density systems where spontaneous clustering is absent, introducing passive structures allows for precise control over active particles without external stimuli. A dilute monodisperse mixture can be separated using a tuned elastic meshwork based on a threshold activity level, while passive polymers in a dilute active bath accumulate active particles along their contours. This local crowding drives fast, persistent dynamics and causes three-armed star polymers to undergo topological arm-pairing, accelerating their motion past identical linear counterparts. Conversely, the dense limit is governed by motility-induced phase separation (MIPS), where a passive star polymer shifts from a tracer to a mobile nucleation center. The polymer drastically accelerates active cluster growth around itself, which ultimately traps the macromolecule in a highly restricted, caged motion. This intense compressive environment forces dramatic conformational distortions, including arm shrinkage, short-lived hairpin formations, and transient arm-pairing. Understanding these dual limits provides foundational principles for designing non-equilibrium smart materials, microfluidic sorting devices, and targeted biomedical delivery systems.

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Relevant Publication(s):

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1. Behavior of passive polymeric tracers of different topologies in a dilute bath of active Brownian particles - Ramanand Singh Yadav, Ralf Metzler, and Rajarshi Chakrabarti Phys. Rev. Res., 8, 013053 (2026).

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2. Demixing of active-passive binary mixture through a two-dimensional elastic meshwork - Ramanand Singh Yadav and Rajarshi Chakrabarti Soft Matter, 21, 2242 (2025). (Soft Matter 20th Anniversary Collection). (This article has appeared as the outside back cover of the Journal).

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3. A Passive Star Polymer in a Dense Active Bath: Insights from Computer Simulations - Ramanand Singh Yadav, Sanaa Sharma, Ralf Metzler and Rajarshi Chakrabarti Soft Matter 20, 3910 (2024). (This article has appeared as the outside back cover of the Journal). (Correction: Soft Matter, 20, 5258 (2024)).

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