microswimmers

Microswimmers are micro- to nanoscale devices designed to move through fluid environments. They typically operate in low Reynolds number regimes, where viscous forces dominate and inertia is negligible. They may propel themselves autonomously or be guided by external fields or chemical fuel.

Biological microswimmers include bacteria that swim with rotating flagella, sperm cells that use flagellar beating, and ciliates that propel themselves with coordinated cilia. Synthetic microswimmers include Janus particles with asymmetric surface properties that catalyze chemical reactions to create self-propulsion, catalytic nanomotors, magnetically actuated rods, light-driven swimmers, and acoustically driven devices.

Because of the low Reynolds number, propulsion relies on non-reciprocal motions or gradients to overcome viscosity, as described by Purcell's scallop theorem. The small scale also means Brownian motion can randomize orientation, and fluid-structure interactions with boundaries strongly influence trajectories. Design and control strategies aim to achieve directed motion, speed, and stability in complex fluids, often leveraging chemical, magnetic, optical, or acoustic driving forces.

Applications of microswimmers span targeted drug delivery, diagnostics, microscale assembly, microfluidic mixing, and environmental sensing or remediation. Research seeks robust control, cargo transport, and autonomous navigation in heterogeneous environments, with contributions from physics, chemistry, and engineering.

The field remains largely in the research and development stage, with laboratory demonstrations and progress toward practical use. Key challenges include reliable steering in complex media, biocompatibility and safe fuels for biomedical contexts, scalable fabrication, and seamless integration with microfluidic systems.