Generalized Three-Sphere Microswimmers

Among several models for microswimmers, the three-sphere microswimmer proposed by Najafi and Golestanian captures the essential mechanism for the locomotion of a microswimmer in a viscous fluid. Owing to its simplicity and flexibility, the original three-sphere model has been extended and generalized in various ways to discuss new swimming mechanisms of microswimmers. We shall provide a systematic and concise review of the various extensions of the threesphere microswimmers that have been developed by the present authors. In particular, we shall discuss the following seven cases; elastic, thermal, odd, autonomous three-sphere microswimmers; two interacting ones; and those in viscoelastic and structured fluids. The well-known Purcell's scallop theorem can be generalized for stochastic threesphere microswimmers and also for the locomotion in viscoelastic and structured fluids.