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Active phase separation: role of attractive interactions from stalled particles

Dry active matter systems are well-known to exhibit Motility-Induced Phase Separation (MIPS). However, in wet active systems, attractive hydrodynamic interactions mediated by active particles stalled at a boundary can introduce complementary mechanisms for aggregation. In the work of Caciagli et al. (PRL 125, 068001, 2020), it was shown that the attractive hydrodynamic interactions due to active particles stalled at a boundary can be described in terms of an effective potential. In this paper, we present a model of active Brownian particles, where a fraction of active particles are stalled, and thus, mediate inter-particle interactions through the effective potential. Our investigation of the model reveals that a small fraction of stalled particles in the system allows for the formation of dynamical clusters at significantly lower densities than predicted by standard MIPS. We provide a comprehensive phase diagram in terms of weighted average cluster sizes that is mapped in the plane of the fraction of stalled particles ($α$) and the Peclet number. Our findings demonstrate that even a marginal value of $α$ is sufficient to drive phase separation at low global densities, bridging the gap between theoretical models and experimental observations of dilute active systems.