Fluid dynamic effect of a rigid boundary on swimming motion of self-propelled microorganisms

单鞭毛细菌，溶藻弧菌，通过改变旋转马达的旋转方向向前和向后移动。在自由空间中，它们沿着几乎相同的轨迹以之字形移动。然而，在刚性边界附近，弧菌细胞的运动轨迹仅在向后运动时才变为圆形，且向后速度大于向前速度。游泳速度和航迹取决于电池的俯仰角。俯仰运动向前运动稳定，向后运动不稳定。已经画出了一套运动的图解。根据它，向前游的细胞倾向于与边界保持一定的距离，因为它的髓运动是稳定的。细胞的游动速度被边界减慢。由于俯仰运动是不稳定的，向后游动的单元格要么离开边界，要么接近边界。在离开运动中，细胞受边界的影响较小，因此细胞的运动就像在自由空间中一样。在接近运动中，鞭毛和边界之间的相互作用变得重要。结果，细菌的运动轨迹由圆形变为圆形，速度加快，有多个鞭毛的细菌细胞也沿圆形轨迹向前运动。这与单鞭毛细菌的运动不同。细胞体的大小与鞭毛的大小之比被认为是造成差异的重要因素。

Singly flagellated bacteria, Vibrio alginolyticus, move forward and backward by changing the rotational direction of their rotary motor. In free space, they move along almost the same tracks in a zigzag pattern. Near a rigid boundary, however, the track changes to a circular one only in backward motion and the backward speed is bigger than the forward speed.In this study, the motion of a Vibrio cell swimming close to a rigid boundary has been calculated by use of the boundary element analysis. The swimming speed and the track depend on the pitch angle of the cell. The pitching motion is stable in forward motion and unstable in backward motion. A set of diagrammatic representation of the motion has been drawn. According to it, a cell swimming forward tends to keep a certain distance from the boundary since its pith motion is stable. The swimming speed of the cell is reduced by the boundary. A cell swimming backward moves either departing from the boundary or approaching the boundary since the pitch motion is unstable. In departing motion, the cell is less affected by the boundary so that the cell moves like it is in free space. In approaching motion, the interaction between the flagellum and the boundary becomes significant. As a consequence, the track changes circular one and the speed increases.Bacterial cells with several flagella also move along circular tracks in forward motion. This is different from the motion of singly flagellated bacteria. The ratio of the size of the cell body and the size of the flagella is considered as an important factor introducing the difference.