An investigation on a peculiar fluid dynamic phenomenon in the motion of bacteria swimming close to a rigid boundary.

本研究的目的是澄清细菌游泳运动中的有趣现象，这种现象仅在细菌靠近刚性边界时才被观察到。靠近墙壁的轨迹与自由空间中的轨迹明显不同。除了运动轨迹外，游泳速度还受到墙壁的影响。这一点很重要，因为大多数对微生物甚至微型机器运动的观察都是在将它们的悬浮液密封在两个玻璃杯之间进行的。运动和细菌运动都会受到眼镜的显著影响。实验研究了细菌的游动速度与细胞体旋转速率之间的关系。当细菌直线移动时，测量它们向前和向后的运动。每个细胞的速度和旋转速率成正比。向前运动和向后运动的比例常数是相同的。这意味着细胞体或鞭毛的变形并不像它对游泳运动的影响那么大。假设这种特殊运动是由于刚性边界的存在引起的流体动力相互作用引起的，我们对边界元分析进行了改进，考虑了壁面的防滑条件。结果表明，对于俯仰运动，前向运动是稳定的，后向运动是不稳定的。根据实验结果，可以推导出细菌在边界附近运动的图像，该图像与观察到的现象定性一致。我们还试图开发一种能自动追踪细菌的显微镜。这使我们能够获得连续的数据，如游泳速度或位置，而不会丢失视线之外的细菌图像。由图像处理软件发送的信号控制暗场显微镜的电机驱动平台。响应时间不足以顺利跟踪细菌，因此需要改进。

The objective of this study is to clarify the interesting phenomenon in swimming motion of bacteria that has been observed only when the bacteria swim close to a rigid boundary. The trajectory close to a wall is noticeably different from the trajectory in free space. In addition to the trajectory, the swimming speed is affected by the presence of a wall. This is important because most observations of motion of microorganisms or even micro-machines have been done with suspensions of them sealed between two glasses. The motion as well as bacterial motion can be affected significantly by the glasses.The relationships between swimming speeds and rotational rates of the cell bodies of bacteria were experimentally investigated. They were measured in both motions of forward and backward when the bacteria moves straightly. The speed and the rotational rate was proportional for every cells. The proportional constant of forward motion and that of backward motion are the same. This means that the deformation of the cell body or the flagellum does not occur as much as it affects on the swimming motion.Assuming that the peculiar motion is caused by fluid dynamic interaction due to the presence of a rigid boundary, we perfumed a boundary element analysis into account the non-slip condition on a wall. The result shows that as for pitching, forward motion is stable and backward unstable. According to the results, an image of bacterial motion close to a boundary can be deduced and the image qualitatively agrees with the observed phenomenon.We also tried to develop an microscope that automatically tracks a bacterium. This enables us to obtain successive data such as swimming speed or positions without losing the image of the bacterium out of sight. An motor-driven stage equipped to an dark-filed microscope is controlled by the signal sent by an image processing software. The response time is not sufficient to track a bacterium smoothly so that improvement is needed.