## 4 Micromanipulation by Vibration-Induced Steaming Flow
### Vibrated Needles
Using vibration to stimulate fluids or acoustic field force to keep microparticles or cells from contacting the manipulating end is an alternative approach. In the early work of [PERSON] et al. (2018), they used high-frequency vibration to rotate tiny particles around the tip of a needle. In this work, the frequency of the tip vibration was determined by the mode of resonance; the frequency of vibration in a section of water was about 66.75 kHz. In other words, they used a method of exciting acoustic microstreaming near the tip to trap small particles. The streaming rotates the trapped particles near the tip. The viscosity of the liquid and the density of the particles determine the minimum driving sound pressure separately, which means that the effective range with the same power output is different for different types of particles or liquid media. In the process of rapid translation, the limiting force of acoustic pressure is not sufficient, so particles sometimes fall off. Enhancing the acoustic field is the most effective solution to this problem, so such devices are often inefficient. Some other studies used the high-frequency vibration of the tip to generate acoustic radiation to drive the movement of particles, such as [PERSON] et al. (2018), as shown in Figure 5b, who achieved in-air manipulation. A 40 \(\upmu\)m diameter copper wire with a length of 1-14 mm can be trapped stably by ultrasound and transferred through any 3D path in the air by moving the transducer. For special situations, such as cell manipulation, which needs a fluidic environment in most cases, flow can be generated by low-frequency vibration, called stirring. Compared to exciting an acoustic field in liquid, generating a micro-vortex directly through vibration is more efficient and natural. [PERSON] et al. proposed a multifunctional noncontact micromanipulation system [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018), as shown in Figure 5c. Resonance of a cantilever structure is used to extend the vertical vibration of a single piezo actuator to the 2D circular vibration of a micropipette. The circular vibration in fluids generates the whirling flow featured by low pressure in the core area and a flow velocity gradient. The low pressure can immobilize objects nearby and transport them together with the micropipette, and the flow velocity gradient is used to form a torque to rotate the immobilized object. The advantages of using the low-frequency oscillation (~350 Hz) method are that the low-frequency vibration mode is highly controllable, and such a low frequency easily achieved with a piezoelectric driver, and the cost is low. Although the method is versatile in the current practical application of micromanipulation, which is the operation of micron-sized cells in a liquid environment, it still has certain shortcomings. The microflow induced by low-frequency vibration is hard to control, especially for cell rotation. Such microflows have strict requirements for the medium, and the minimum scale of manipulation is also limited by the nature of the fluid, making it challenging to reach submicron or even nanoscale manipulations. Compared with in-planar micromanipulation, needle-driven manipulation is more flexible when controlling a single particle and provides more motions. 3D operation is easy for needle-driven manipulation but difficult for the in-plane manipulation method. It avoids the complex mathematical models required for control and the many conditions that must be ignored when establishing the model. The transducers used in the in-plane systems are more expensive and experience more manufacturing difficulties, which pose barriers to practical implementation. Compared to the scheme using second-order sound pressure to generate microfluidics, probe vibration is more similar to the effect of agitation, so its action mode is simple. The vibration frequency requirements are much lower, and the non-contact operation mode makes it biocompatible.