Programmable hydrodynamics of active particles.

Abstract

Self-propelled microparticles create flow fields that determine how they interact with surfaces, external flows, and each other. These flow fields fall into distinct classes-pushers, pullers, and neutral swimmers-each exhibiting fundamentally different collective behaviors. In all existing synthetic systems, this hydrodynamic character is permanently set during fabrication. Here we demonstrate that the hydrodynamic identity of microswimmers can be programmed and switched on demand. Using patterned laser heating of surface-bound nanoparticles, we create tailored temperature gradients that drive controllable boundary flows. Real-time illumination control transforms the swimmer's flow field from pusher to puller during motion. Flow measurements confirm quantitative agreement with theory and track changes in power consumption and efficiency across modes. This control over hydrodynamic modes provides an experimental platform to explore active matter behavior under external physical constraints, including adaptive swimmer responses to crowding or confinement and the emergence of collective states from tunable pusher-puller mixtures.