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

单鞭毛细菌，溶藻弧菌，通过改变其旋转马达的旋转方向向前和向后移动。在自由空间中，它们以之字形的方式沿着几乎相同的轨迹移动。然而，在刚性边界附近，轨道只有在向后运动时才会变成圆形轨道，并且向后的速度大于向前的速度。在本研究中，利用边界元分析计算了弧菌细胞在刚性边界附近游动的运动。游动速度和轨迹取决于细胞的俯仰角。俯仰运动在向前运动时是稳定的，在向后运动时是不稳定的。已经画出了一组运动的图解。根据它，向前游动的细胞由于其髓的运动是稳定的，往往会与边界保持一定的距离。细胞的游动速度受到边界的影响而降低。由于俯仰运动是不稳定的，向后游动的细胞不是离开边界就是接近边界。在分离运动中，细胞受边界的影响较小，因此细胞像在自由空间中一样运动。在接近运动中，鞭毛与边界之间的相互作用变得显著。因此，轨道变圆，速度增加。有几个鞭毛的细菌细胞也沿着圆形轨道向前运动。这不同于单个鞭毛细菌的运动。细胞体的大小与鞭毛的大小之比被认为是导致这种差异的重要因素。

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.