## 4 Results and Discussions
### Particle Rotation and Transportation  
A silica rod with 90 \(\upmu\)m length and 15 \(\upmu\)m diameter is adopted as the manipulation target. The related acoustic parameters of the rod are also listed in Table 1. Figure 8 shows the clockwise rotation of the rod lying on the agar with an interval of 45\({}^{\circ}\). By implementing the excitation variation sequentially, a continuous rotation can be easily obtained. The time interval between each rotation step is 1 s, which is long enough for the rod to reach re-equilibrium. For a better demonstration, the rotation processes from 180\({}^{\circ}\) to 0\({}^{\circ}\) and 0\({}^{\circ}\) to \(-\)180\({}^{\circ}\) are presented in the superimposed photomicrograph Figure 9a,b, respectively. The red dots indicate the center position of the rod and the black dashed lines represent the theoretical angular position. The silica rod almost rotates around its center step by step and only slightly deviates (\(<5^{\circ}\)) from the theoretical predictions. During the rotation, the rod center deviates from its original position with a maximum value 15 \(\upmu\)m, which may be caused by the position fluctuation of the pressure node resulting from the same factors, as mentioned in Section 4.2. Figure 10 shows the detailed rotation process with a smaller angle interval 5\({}^{\circ}\), in which the measured angular positions are extracted from a series of captured photos. The corresponding theoretical angular position of the synthesized pressure node is also illustrated by the solid line for comparison. Due to the maximal angular deviation about 5\({}^{\circ}\), the resolution of the prototype rotation device is 5\({}^{\circ}\). The rotation direction can be easily alternated by inverting the adjustment of the transducer excitation. As shown in Figure 11a-g, the rod with an initial angular position of 105\({}^{\circ}\) is driven to rotate clockwise with a 30\({}^{\circ}\) interval angle. When a half-cycle rotation is performed, the sequence of  
Figure 7: The measured pressure distribution within a \(\lambda\times\lambda\) domain located at the center area of the pedestal. The elongated acoustic potential well is formed with orientation angle 0\({}^{\circ}\) in (**a**) and 45\({}^{\circ}\) in (**b**) and then shifted by 0.5\(\lambda\) in the direction of 45\({}^{\circ}\) in (**c**) and 135\({}^{\circ}\) in (**d**). transducer excitation is inverted. Correspondingly, the rod rotates back to its initial position with the same interval (Figure 11g-m). In addition, the measured angular positions of the particle together with the theoretical value predicted from the field orientation are depicted in Figure 12. The maximal angular deviation is also about 5\({}^{\circ}\). Figure 11: Alternating rotation by inverting the adjustment of the transducer excitation. (**a–g**) show a half-cycle clockwise rotation with a 30\({}^{\circ}\) interval angle and (**g–m**) is the reverse rotation with the same interval. Figure 8: The clockwise rotation of the silica rod lying on the agar from 180\({}^{\circ}\) to \(-\)180\({}^{\circ}\) (**a–i**) with 45\({}^{\circ}\) interval. Figure 10: Rotation with 5\({}^{\circ}\) angle interval. Transportation of the silica rod to an appointed position along arbitrary trajectory can also be achieved within the same device. This may be of help in the application of medicine tests and micro-assembly, where the drugs or components usually needed to be orientated and then shifted to a specific position. Figure 13 shows the process of two dimensional transportation along different paths. In Figure 13a, the particle is moved by 125 \(\upmu\)m along the negative direction of \(x\)-axis. During the transporting process, the rod firmly maintains its orientation. Figure 13b shows another trajectory, where the particle is first transported along the direction of \(-165^{\circ}\), and then changed to the direction of \(-145^{\circ}\).