A new computational study has revealed that microscopic swimming organisms mixed with passive particles in suspension can spontaneously organize into unexpected structural patterns, including a novel 'lamellar' or sandwich-like arrangement never previously described. The findings, posted as a preprint on arXiv (arxiv.org/abs/2603.23555) and not yet peer-reviewed, could advance the design of microrobotic systems and deepen understanding of how microorganisms behave in complex biological fluids.

Researchers used a computational technique called Stokesian dynamics to simulate dense, three-dimensional mixtures of spherical microswimmers — modeled as 'squirmers' — and inert obstacle spheres. Crucially, the simulations accounted for many-body hydrodynamic interactions, meaning the study captured how the fluid disturbances created by each particle influence all surrounding particles simultaneously. The authors note this level of physical accuracy has historically been difficult to achieve, leaving the behavior of mixed active-passive suspensions largely unexplored.

The study tested several swimmer types, including 'neutral' squirmers, 'pullers' (which generate thrust from their front), and 'pushers' (which push from the rear), as well as variants weighted toward the bottom — so-called bottom-heavy squirmers that naturally reorient upward in a gravitational field.

Key findings include that passive particles generally disrupted the tendency of swimmers to develop coordinated, orientationally ordered motion. Phase-separation — the spontaneous sorting of active and passive particles into distinct regions — was found to be metastable when starting from a pre-separated state for neutral or puller squirmers at high packing densities. However, when swimmers were bottom-heavy, dynamic phase-separation emerged in some scenarios. Notably, bottom-heavy neutral squirmers and pullers at medium densities produced elongated, fibrillar separation patterns.

The most striking result was observed for bottom-heavy pullers at high densities: a lamellar phase-separation in which a layer of passive particles was sandwiched between an advancing layer of swimmers and an empty gap — a structure the authors describe as 'sandwich-like' and identify as a novel phenomenon.

The researchers conclude that the type of swimmer significantly determines how microstructure and particle transport evolve in mixed suspensions, underscoring the importance of including full hydrodynamic interactions in such models. They suggest the findings could help scientists use external torques to control active particle behavior in complex fluids.

Limitations of the study include its reliance on idealized spherical particle geometries, which may not fully capture the behavior of real biological microswimmers that have irregular shapes and more complex swimming strokes. The simulations also represent a specific range of packing densities and bottom-heaviness parameters, and experimental validation has not yet been reported. As a preprint, the work has not undergone formal peer review.