## 3 Micromanipulation by Vibration-Induced Acoustic Wave
### Manipulation by BAW  
#### 3.2.1 UTW Devices (Ultrasonic Acoustic Beam Tweezers)  
As mentioned above, research on such devices is not popular, but some research programs are important. The most prominent feature of in-plane micromanipulation is that this system is suitable for manipulating a large number of particles or cells simultaneously, but cannot control a single particle without affecting the movement of the other particles. The acoustic beam manipulator driven by BAW (or UTW for detail) can compensate for this disadvantage. The ideal acoustic beam tweezers is depicted in Figure 1c. [PERSON] et al. [56] first proposed the concept of ultrasonic acoustic beam tweezers. They followed the idea of optical tweezers and experimentally explored using two counter-propagating focused acoustic beams from 3.5 MHz transducers. Although they trapped the latex particles successfully, no movement of the trapped particles was performed in their experiments. A variant on this simple beam device was reported by [PERSON] et al., who trapped particles in a channel at the focus of two propagating acoustic beams driven out of phase, where the transducers were placed side by side. [PERSON] et al. proved a single acoustic beam is able to trap particles like an optical beam if the particles are small enough and the acoustic field force is relatively large [19; 57]. UTW devices have two manifestations: shear wave and longitudinal wave. This device can generate Bessel vortex in a cross-section. Similar to the sound field pattern in Figure 1c, particles can be trapped in the center of the sound field in the cross-section to achieve the manipulation effect. [PERSON] et al. used a single beam to achieve three-dimensional (3D) control of particles, as shown in Figure 3a [58; 59; 60]. Acoustical beam tweezers can push, pull, and accurately control both the position and the forces exerted on a single particle. The broad spectrum of frequencies covered by coherent ultrasonic sources provides a wide variety of manipulation possibilities from macro- to microscopic-length scales. However, the array of equipment that produces the acoustic beam is too large, which is cumbersome to move and thus provides inflexible manipulation of particles. In response to this problem, [PERSON] et al. proposed a simpler solution [18], as shown in Figure 3b. They unleashed the potential of focalized acoustical vortices by developing the first flat, compact, paired single electrode, and focalized acoustical tweezers. These tweezers rely upon spiraling transducers obtained by folding a spherical acoustical vortex on a flat piezoelectric substrate. The focused vortices acoustically trap in a small range, so produces better performance in the control of a single particle or cell. Due to the simplicity of the technology and its scalability to higher frequencies, this work opened possibilities of individual manipulation and in situ assemblies of physical and biological micro-objects. To strictly demonstrate the 3D trapping function with progressive waves, it is necessary to eliminate any stray standing waves that may occur due to wave reflections on the wall and thus should be performed in a large water tank to simulate free-space conditions. The device shown in Figure 3c, which was proposed by [PERSON] et al. [61], is another kind of acoustic beam tweezer. The main idea is to levitate objects in the air through the acoustic beam generated by a BAW. This method has the advantages of optical tweezing (i.e., single-beam, rotation, holographic control, and multiple particles) with the efficiency and versatility of acoustic levitation, and could lead to the development of powerful tractor beams. Multiple acoustic beam manipulation units are evenly arranged on the arc surface and aligned with the center of the sphere. By changing the phase or switching the unit, different-shaped sound field potential wells are formed at the focal point to achieve  
Figure 3: Ultrasonic acoustic beam tweezers by [PERSON] et al. [58; 59; 60], [PERSON] et al. [18], and [PERSON] et al.