We study collective ordering in confined trains of passive deformable particles driven by Poiseuille flow. An isolated particle adopts a slipperlike shape with two mirror-related orientations with respect to the flow direction, its tail tilting either upward or downward. We call a train polarized when all particles share the same inclination state. When oppositely inclined neighbors approach closely enough, the downstream particle can switch to match the upstream state, while the upstream particle remains unchanged. This directional switching arises from the fore-aft asymmetry of the slipper shape and its disturbance flow. Repeated along the train, this pairwise event generates a downstream-propagating switching front. A minimal bistable model with unequal upstream and downstream couplings captures both front propagation and arrest. In the propagating regime, interactions between fronts drive domain coarsening and can ultimately yield uniform polarization. When the interparticle coupling becomes too weak, fronts arrest, leaving persistent polarized domains. These results reveal a hydrodynamic route to collective polarization in externally driven suspensions of passive deformable particles, without self-propulsion or externally imposed asymmetric interactions.

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• Capsule & cell dynamics
• Collective behavior
• Fluid-particle interactions
• Collective dynamics
• Suspensions
• Immersed boundary methods
• Lattice-Boltzmann methods