## 4 Micromanipulation by Vibration-Induced Steaming Flow
### Vibrated microbubbles and Microrobots
Although the bubbles cannot move at will, their array can generate microfluidics in the flow channel through the vibration excited by the acoustic piezo-actuator, thereby driving the particles (_Caenorhabditis elegans_ in their work) to move. Some microrobots also use a method similar to microbubbles to manipulate particles through their vibrations [79; 80]. Many microrobots are a kind of particle, and also urgently requiring micromanipulation. However, compared with some objects sensitive to manipulating methods such as cells, these microrobots have stronger adaptability. Therefore, this compromise method to achieve cell translation, rotation, etc. is also feasible. In addition, microrobots have strong scalability. A robot cluster composed of multiple microrobot units can be regarded as an individual, which can enable the precise control of a single cell in a large field. Figure 7c depicts the research of [PERSON] et al. [81; 82]. A microparticle can be transported using three methods of manipulation, achieved through microrobot rotation and vibration, including pushing, steering, or pulling. Compared with microbubbles, microrobots are more stable and their mechanical properties are more prominent, but their manufacturing costs are higher and their use involves some difficulties. Some schemes combine microbubbles and microrobots, such as [PERSON] et al. [83] (Figure 7d). A microbubble was enclosed in a semi-capsule-shaped robot, and the secondary Bjerknes force formed between the bubble and the actuator was used to drive the microrobot motion. Figure 7: Vibrated microbubble and microrobot methods by [PERSON] et al. [31], [PERSON] et al. [77; 78], [PERSON] et al. [81; 82], and [PERSON] et al. [83]. (**a**) Particles are manipulated by an individual bubble, in which vibration is actuated by piezo-actuator or heat. (Republished with permission of IOP Publishing Ltd from reference [31]. Copyright © IOP Publishing Ltd; permission conveyed through Copyright Clearance Center, Inc.) (**b**) Particles are manipulated by bubbles trapped in channel caves; the vibration is usually actuated by SAWs. (Republished with permission of The American Association for the Advancement of Science from reference [77]. Copyright © 2019, The American Association for the Advancement of Science; permission conveyed through Copyright Clearance Center, Inc.) (**c**) A kind of microrobot that can catch and release particles by micro steam from their self-rotation or vibration. (Republished with permission of John Wiley and Sons from reference [82]. Copyright © 2019, John Wiley and Sons; permission conveyed through Copyright Clearance Center, Inc.) (**d**) A typical example of fusion among bubbles, microrobots, magnetic fields, and acoustic waves. (This figure is republished reference [83] of [PERSON] et al. and licensed under CC BY 4.0. Copyright © 2019, The Authors)  
The reason why microbubbles and microrobots are classified into one category is that they both perform secondary micromanipulation, which means that their motion requires manipulation, and their motion characteristics drive the micromanipulation for other particles. This mode of manipulation is gentle enough for particles, especially cells that are not easily manipulated directly by other external forces. However, with the current level of technology, the result produced of this method is not perfect, the control cost is high, and the efficiency is very low. This field has more significance in research than in practical applications.