Swimming speed and rotational rates in forward motion as functions of pitch angle. Parameter d/2b is a measure of the distance from the wall (see Fig. 8). These data should be inverted with respect to the abscissa for the case of backward motion, (a) Swimming speed normalized with respect to the swimming speed in free space, |Uoo|. (b) Pitch rate normalized using the angular velocity of the cell body in free space, \Q.Go\. (c) Yaw rate. Qz" is the projection of Hz' onto the z axis as defined in Fig. 8. From Goto et al. (2005) with permission

 from a surface. As the cell approaches a surface, the swimming speed varies with 0. When the cell swims parallel to the wall, namely 9=0°, the closer to the wall the cell is swimming, the slower it swims (the smaller the magnitude). When the cell swims in an orientation such that the flagellum is close to the wall and the cell body is away from the wall, namely 0>O°, the speed in-creases. Thus, the swimming speed increases either when the cell swims at a significant distance from the wall or when the flagellar filament interacts with the wall. The pitch rate is shown in Fig. 9(b). If the cell swims at a significant dis-tance from the surface, the pitch rate is independent of 9 and is negligible. However, if the cell interacts with the wall, the pitch rate decreases almost proportionally with the pitch angle. Because the pitch rate is the time deriva-tive of the pitch angle, the negative sign for the proportional constant indicates that the pitch motion associated with forward swimming has positive damping and is stable. For backward motion, all of the data will be inverted about the horizontal axis. Namely, their signs will invert from positive to negative and from negative to positive. Therefore, the pitch motion is unstable in backward motion.