Swimming microorganisms are constantly influenced by the presence of boundary surfaces in their natural habitat, giving rise to rich swimming behaviors. However, despite of the fact that interactions between microswimmers and nearby boundary surfaces are vital to many biological functions and industrial applications, little is known about the mechanism responsible for the surface entrapment of microswimmers. After six decades of active research, the mechanism responsible for the surface entrapment of microswimmers is still under heated debate. 

In a recent theoretical study led by Prof. Xinliang Xu at Beijing CSRC, the dynamical behaviors of flagellar bacteria near surfaces are investigated. By relating surface entrapment to a fixed point in bacterial dynamics, it is proved that a commonly neglected thermodynamic effect is essential for bacterial entrapment. Using Escherichia coli as an example where experimental data are abundant, it is demonstrated that the incorporation of this thermodynamic effect to even a simplified model of each bacterium as two spherical beads is capable to quantitatively reproduce all existing experimental observations, including two key features that previous theories/simulations fail to resolve: The bacterial “nose-down” configuration, and the anticorrelation between the pitch angle and the wobbling angle. Furthermore, the theory makes the following important predictions that motivate future experiments for further verification: boundary entrapment of bacteria is governed by two analytic relationships, which only exists within an entrapment zone dictated by two dimensionless parameters: α₁ the ratio of thermal energy to self-propulsion, and α₂ a bacterial intrinsic shape factor. These results provide guidelines for controlling biological and engineering active swimmers near surfaces, and the manuscript summarizing these results authored by Dr. Premkumar Leishangthem (postdoc at CSRC) and Prof. Xinliang Xu is accepted at Phys. Rev. Lett.