## 3 Micromanipulation by Vibration-Induced Acoustic Wave
### Manipulation by SAW
#### 3.1.2 SSAW Devices  
The device shown in Figure 1b is another kind of in-plane manipulation method, a SSAW manipulating device, which places the IDTs symmetrically to create a stable SSAW field inside the chamber or channel, according to the review of [PERSON] et al. [3]. The chamber or the channel should have a simple shape, which is typically has 1D properties. A simple setting method of the two-dimensional (2D) symmetrical distribution is shown in Figure 1b. The placement of the transducer can be designed according to other distributions, such as an annular array. Compared with the TSAW field described above, SSAW better manipulates large numbers of particles. Particles in the standing wave field are easily concentrated at the standing wave node (1D) or in the potential well (2D or 3D). This means that, in most cases, \(R_{p}\) should be relatively much smaller than the wavelength, which satisfies the optimal operating conditions based on acoustic field forces. The devices based on SSAW have different operation modes. These systems can be driven by sinusoidal signals that require only a few volts of input voltage or less than 1 W of input power. Since the condition of resonance needs to be satisfied in a state where the system is stable, and the particle size, mentioned later, should have an additional limitation, the frequency of generating the ARF is approximately larger than 1 MHz (affected by the medium). The use of SSAW equipment is difficult. The particle transport of SSAW requires constant changes in the position of the nodes or potential wells by changing the frequency or phase of the IDT, for which further optimization is required. Single SSAW devices create a static acoustic field, which means that the standing wave field has a fixed 1D shape. If the system has a low input damping, the resonant operation model can provide high efficiency in terms of force applied for given input power. However, if the system does not work in resonant conditions, higher input power may not be accompanied by higher output efficiency. Researchers have to design different resonant devices, which may be infinite but cannot be used in different conditions, to adapt to different applications. A solution was provided soon after people realized this problem. Researchers have tried to overcome the traditional method of operating in a chamber or channel, in other words, the resonant devices, but using a dynamic field. By controlling the input frequency of the transducer, the distribution and strength of the acoustic field can be changed dynamically [41]. Compared to a static field, SSAW manipulation using a dynamic field is not limited to a simple switching operation. In contrast, the complexity and cost of dynamic field devices have also increased dramatically. A single dynamic device might be able to perform multiple functions, such as reconfiguration, oriented to multiple kinds or sizes of particles, which require multiple static acoustic field devices to complete. In general, dynamic fields can be generated using two methods: moving the transducer to change the focus or structures of the field and using a non-resonant device that sets the arrangement of the different transducers and their frequency and phase to change its Gor'Kov field. The non-resonant manipulation methods have a lower efficiency due to the existence of damping. The efficiency can be evaluated, according to [PERSON] et al. [40], by multiplying the efficiency of the corresponding resonance field by the reciprocal of a quality factor (Q factor). Q is strongly correlated with the damping generated by the system (material, medium, etc.). The dynamic field manipulation devices proposed by [PERSON] et al. [13; 52] shown in Figure 2c have indicated the tendency of in-plane micromanipulation, which is the purpose of controlling particle motion by continuously changing the acoustic field. In this work, the transducers were set symmetrically and emitted acoustic waves with the same frequency, generating a 2D standing wave field in the middle chamber.