The development of artificial microswimmers has attracted significant attention for targeted drug delivery and minimally invasive microsurgery. Microorganisms such as bacteria and spermatozoa utilize flagella and the action of molecular motors to swim through their world. However, the locomotion of tiny swimming robots that attempt to mimic this behavior, navigating through blood vessels, intestines, and reproductive tracts, are often affected by confinements and interactions with fluid motion generated by the swimmers.

Gürbüz et al. investigated the propulsion of an artificial microswimmer as the radius of the confining tube and the geometric and kinematic properties of the swimmer are varied.

“We use a simple swimmer model and a simple geometric setup, which enables us to not only predict how the confinement affects swimming but also physically explain why it does so,” said author Abdallah Daddi-Moussa-Ider.

The team analyzed the propulsion speed of a widely studied swimmer, consisting of three spheres connected by two extensible rods, as it traveled along a centerline in a capillary tube.

They found that confinement did not significantly affect the microswimmer’s propulsion speed until the ratio of the radius of the tube to the radius of the sphere reached a definable point. After, the swimmer experienced a substantial reduction in its propulsion speed as the radius of the tube decreased. The confinement also led to higher-order scaling between the net displacement and the contraction length of the swimmer, reducing the effectiveness of propulsion.

“With this improved understanding, we are ready to address more complexities arising in realistic biological environments in our subsequent investigations, including complex behaviors of biological fluids and compliance with the confining boundaries,” said Daddi-Moussa-Ider.

Source: “The effect of axisymmetric confinement on propulsion of a three-sphere microswimmer,” by Ali Gürbüz, Andrew Lemus, Ebru Demir, On Shun Pak, and Abdallah Daddi-Moussa-Ider, Physics of Fluids (2023). The article can be accessed at