Condensed Matter Theory

Ordered phases in active suspensions of polar swimmers are under long-wavelength hydrodynamic mediated instabilities. In this article, we show that chemical molecules dissolved in aqueous suspensions, as an unavoidable part of most wet active systems, can mediate long-range interactions and subsequently stabilize the polar phase. Chemoattractants in living suspensions and dissolved molecules in synthesized Janus suspensions are reminiscent of such chemical molecules. Communication between swimmers through the gradients of such chemicals is the foundation of this stabilization mechanism. To classify the stable states of such active systems, we investigate the detailed phase diagrams for two classes of systems with momentum conserving and non-conserving dynamics. Our linear stability analysis shows that the proposed stabilization mechanism can work for swimmers with different dynamical properties, e.g., pushers or pullers and with various static characteristics, e.g., spherical, oblate or prolate geometries.