Robo-Linked Purcell'S Swimmer

EM Purcell proposed three-linked swimmer 60 years ago as a hypothetical creature capable of swimming in highly viscous fluid. We have constructed a scaled model of this swimmer to investigate reciprocal and non-reciprocal swimming motion. The links were programmatically rotated by the interactive servomotors controlled by a microcomputer. Reciprocal motion of the links produced no net displacement illustrating the effect of scallop theorem. Experiments on non-reciprocal motion captured very important property suggested by theories: small stroke angles cause forward swimming and large stroke angles backward. We have measured non-dimensional displacement per cycle and compared with the theoretical predictions and found consistent. Surprisingly, in low-frequency region, flapping frequency of the links also play a role in determining swimming direction. The low-Reynolds number swimming is characterised by Stokes equation which is linear and time-independent. But frequency dependence in low-frequency region shows the breakdown of Stokes law with inertia.