Microswimmers in suspension self-organize into ordered structures, clusters, and turbulent-like states. Add passive particles — inert spheres that can't swim — and the system was known to phase-separate. But nobody had studied what structures form in the mixed state, because accurately modeling the many-body hydrodynamic interactions is computationally prohibitive.
The paper on self-organised structures in mixed active-passive suspensions (arXiv: 2603.23555) uses Stokesian dynamics in three dimensions to investigate dense mixtures of spherical squirmers and passive spheres. The results depend on swimmer type and density.
Without external orientation, passive spheres generally disrupt orientational order. Initially phase-separated states are metastable for neutral or puller squirmers at high packing. But when the squirmers are bottom-heavy (gravitationally oriented), dynamic phase separation can occur. For neutral squirmers and pullers at medium densities, fibrillar separation appears — thread-like structures of swimmers and passive particles.
The most novel finding: at high densities with strong bottom-heaviness, puller squirmers create lamellar phase separation — sandwich-like structures where a layer of passive particles is pushed by a layer of swimmers, followed by a gap, then the pattern repeats. Swimmers, cargo, gap. Swimmers, cargo, gap.
The through-claim: the structure of the mixed phase depends on the type of swimmer, not just the presence of swimmers. Pushers, pullers, and neutral squirmers produce qualitatively different organizations of the same passive particles. The passive particles don't just get pushed around — they become structural elements in patterns that couldn't exist without them.
2603.23555. Active matter / microswimmers / phase separation / hydrodynamic interactions / lamellar structures.