Simulation of Flagellated Bacteria Near a Solid Surface: Effects of Flagellar Morphology and Ionic Strength

This study systematically investigates the dynamics of the bacterial transition from free-swimming to surface adhesion, a process characterized by both height $h$ and inclination angle $\Psi$. The surface entrapment process is divided into three main stages. Initially, bacteria swim towards the surface at an approach velocity proportional to the motor rotation frequency. Subsequently, during the reorientation stage, the cotangent of the inclination angle decays exponentially with the product of the motor rotation frequency and time. Finally, the combined effect of near-field hydrodynamic interactions and DLVO forces drives the bacteria to a stable fixed point $(h^{*},\Psi^{*})$ near the surface. Bacteria with left-handed chiral flagella exhibit clockwise circular motion on the surface. The stable height, inclination angle, and radius of curvature of these circular trajectories are collectively determined by the flagellar morphology and ionic strength of the electrolyte solution. More precisely, increasing the contour length of the flagellum decreases the stable inclination angle and increases the radius of curvature. In contrast, decreasing the ionic strength increases the stable height and radius of curvature, while also decreasing the stable inclination angle. Typically, the stable inclination angle falls within $(\pi/2,\pi)$, the stable height ranges from a few nanometers to approximately one hundred nanometers, and the radius of curvature of the circular motion spans several to tens of micrometers.