Microphase separation in quorum-sensing active particles with competing sensing scales.

Standard quorum-sensing models in active matter exhibit collective phenomena such as motility-induced phase separation. Here, we show that incorporating competing sensing ranges-a minimal ingredient inspired by microbial communication-qualitatively changes this behavior, replacing macroscopic phase separation with self-organized microphases characterized by an emergent finite length scale. Starting from the microscopic dynamics, we derive a coarse-grained field theory whose coefficients are explicitly related to the moments of the microscopic sensing function. This mapping enables a direct comparison between particle-based simulations and continuum theory, allowing the characteristic modulation and correlation lengths to be predicted directly from the microscopic interaction parameters. Two-dimensional numerical simulations confirm these predictions and reveal a transition from macrophase separation to finite-wavelength density modulations as the competition between sensing scales increases. For stronger competing interactions, the system develops a peculiar cluster phase with an interstitial percolating network, which is captured by a higher-order gradient expansion. Our results identify competing quorum-sensing interactions as a simple microscopic mechanism for generating tunable active microphases.