Speedup of self-propelled helical swimmers in a long cylindrical pipe

Abstract

Abstract Pipe-like confinements are ubiquitously encountered by microswimmers. Here we systematically study the ratio of the speeds of a force- and torque-free microswimmer swimming in the center of a cylindrical pipe to its speed in an unbounded fluid (speed ratio). Inspired by E. coli, the model swimmer consists of a cylindrical head and a double-helical tail connected to the head by a rotating virtual motor. The numerical simulation shows that depending on swimmer geometry, confinements can enhance or hinder the swimming speed, which is verified by Reynolds number matched experiments. We further developed a reduced model. The model shows that the swimmer with a moderately long, slender head and a moderately long tail experiences the greatest speed enhancement, whereas the theoretical speed ratio has no upper limit. The properties of the virtual motor also affect the speed ratio, namely, the constant-frequency motor generates a greater speed ratio compared to the constant-torque motor.