## 3 Micromanipulation by Vibration-Induced Acoustic Wave
### Manipulation by BAW
#### 3.2.2 USW Devices (Ultrasonic Transducers Array)  
The ultrasonic transducers array manipulating devices, as shown in Figure 1d, are also called ultrasonic standing wave (USW) devices. For a 1D USW device, the transducers are evenly arranged on the substrate in a specific direction, where silica reflector covers particle-manipulating channels. Therefore, a stable standing wave field can be generated in the longitudinal direction of the flow channel, achieving particle levitation (Figure 4). Since acoustic waves are transmitted inside the medium, this type of device belongs to the BAW category. With the orderly switching of the transducer array below, particles can move freely in the direction of the array arrangement, which is similar to the principle of a magnetic levitation train. Based on the same principle, [PERSON] et al. [27] expanded the array to a two-dimensional scale, constructing an operation method to control the movement of particles in 3D space. Related research is much older, such as the research of [PERSON] et al. [15], but it is not particularly popular. This configuration is not innovative in mechanics. The aggregation and propagation of particles at the nodes produce a variety of effects using the ARFs, as mentioned above [14]. Many studies have not specifically emphasized the longitudinal movement of particles because it is easily achieved. Similar to the application of SSAW, the instantaneous change in the phase of the transducers can change the position of the standing wave node in the longitudinal direction, thereby controlling the movement of the particles in the longitudinal direction. [PERSON] et al. [62] recently proposed a special hologram display device. A phased array of ultrasonic emitters was used to realize a volumetric acoustophoretic display in which a millimetric particle is held in mid-air using ARF and moved rapidly along a 3D path. Synchronously, a light source illuminates the particle with the target color at each 3D position. The response of the particle operation in this system is fast and the rendering speed can reach 10 Hz.  
In general, USW devices have some of the advantages of in-plane manipulation, including easy establishment, easy control, and stable performance in particle cluster operation. USW devices extend the operation dimension of in-plane operation equipment. However, they have some limitations, mainly including the high driving frequency (\(\sim\)10 MHz) and high cost. This kind of equipment only performs well in the translation of particles or cells, and some complex movements still cannot be achieved.  
Figure 4: Ultrasonic transducers array by [PERSON] and [PERSON] et al. [14; 26; 27], [PERSON] et al. [62], and [PERSON] et al. [15]: (**a**) in the x,z plane, particles can be suspended at the standing wave node formed by resonance between the actuator and the reflector. (This figure is republished reference [27] of [PERSON] et al. and licensed under CC BY 4.0. Copyright © 2015, Elsevier) (**b**) Structure of a pseudo-one-dimensional acoustic driver array. (This figure is republished reference [27] of [PERSON] et al. and licensed under CC BY-NC-SA 3.0. Copyright © 2014, The Authors)