## 4 Micromanipulation by Vibration-Induced Steaming Flow
### Vibrated microbubbles and Microrobots  
Micromanipulation with microbubbles or microrobots is an old idea that was not important until the more recent development of manufacturing technology. With both microbubbles and microrobots, the particles can be manipulated by controlling the vibration and movement of single or multiple microelements in operating space. The manipulation of microbubbles and microrobots is no longer limited to acoustic driving. Optical tweezers, electrophoresis, and magnetic tweezers are also effective methods. Microbubbles and robots can further manipulate other particles using the vibrations generated during the operation. Although this method is intuitively cumbersome, it is flexible, efficient, and highly biocompatible for cell manipulation in a fluid field. Manipulation by microbubbles is a useful method. Although a single stable bubble can generate a fragile flow due to its spontaneous vibration, almost no significant effects are provided for control. Controllable-bubble-based microfluid can be generated by controlled vibration like SAW or a piezo actuator, regular collapse by a high-frequency laser pulse, or acoustic cavitation. In particular, several bubbles that are close to each other and in a vibrating state can produce more complex mechanical phenomena that can be partially solved by applying Bjerknes' circulation theorem. Some related studies were published about the stream around multiple bubbles, such [PERSON] et al. [33], and the influence of stream on bubbles, such as [PERSON] et al. [35]. The microrobot system proposed by [PERSON] et al. showed the potential of Bjerknes force in the field of acoustic manipulation. However, micromanipulation using forces among multiple bubbles requires considerable research. A simple microbubble manipulating model is shown in Figure 1g. Many methods for generating microbubbles are possible. Using a laser pulse to irradiate the substrate and then convert into heat to generate vapor microbubbles is universal [36; 74; 75; 76]. Due to the pulse of light and the instability of vapor, the bubbles collapse periodically, resulting in cavitation in a small range. The original intention of this design was to avoid direct damage to the cells caused by optical tweezers, but the idea of using microbubble vibration and cavitation to control the cells has inspired microbubble manipulators. Photothermally excited bubbles respond quickly and the exciting bubbles can move quickly in the plane. Therefore, a two-dimensional movement of microbubble-driven particles in a channel or a chamber can be achieved according to the method above. Generate bubbles using the photothermal effect or other methods such as acoustic cavitation and chemical catalysis is difficult. Inject bubbles into the flow channel directly to make them adhere to the substrate is a more convenient option, as conducted by [PERSON] et al. [31] (Figure 7a). In their work, the piezo-actuator oscillated the gaseous bubble so that current flows were generated around the bubble, and then the neighboring objects are captured in and orbiting in the current flows. The electrowetting-on-dielectric actuation was used in the meantime to move the bubble following the preset direction. The piezo-actuator remained open during the movement of the bubble so that the captured object would not fall off. Glass particles, fish eggs, and even live daphnia can be captured in this system (1.5 mm diameter bubbles, 4.5-15 kHz drive frequency, 400 V drive voltage to piezoelectric actuators, and voltage of 80 V at 1 kHz are applied for electrowetting-on-dielectric actuation). Another simple method of bubble generation is designing a special cavity so that when liquid enters the channel, bubbles can be automatically formed or captured in it [77; 78]. As shown in Figure 7b, the channel contains linear arrays of rectangular microcavities where microbubbles will be trapped due to the residual gas pressure when the liquid is injected.