## 3 Results and Discussion
### disadvantage of the Diffraction Trap in the Application of Optical Tweezers  
When the optical tweezers are close to the interface, the diffraction may seriously disturb some experiment processes, such as application in the micro-channel. The particles may be seriously gathered by optical tweezers at the bottom surface of the micro-channel. In our experiment, the 2 \(\upmu\)m particle suspension is injected into the micro-channel. The velocities of the particles are measured by tracking the particles' motion in the images. The particles are flowing with the velocity of \(715\pm 157\)\(\upmu\)m/s in the micro-channel. The particles' concentration is \(2.5\times 10^{8}\) cm\({}^{-3}\) and the laser power is 1.5 W. As shown in Figure 7a, the optical tweezers can only trap a single particle, and there is no aggregation in the micro-channel at the beginning. However, more particles are trapped by the diffraction traps, and more and more aggregation of particles settles in the lower part of the flow close to the trap. On the other half side of the micro-channel, the particles do not aggregate due to the absence of the optical tweezers. The experiment shows that the diffraction will seriously affect the flowing of particles in the micro-channel.  
The particles aggregate at the surface of the micro-channel, which can be due to the ability of trapping a large number of particles by the diffraction traps. After the traps confine a lot of particles and form the aggregation, the flow velocity downstream of the aggregation becomes slow, which will induce more particles into further aggregation.  
The aggregation is mainly related to the change of particle flowing. If one solvent with lower viscosity is used to dilute the particles, the flowing ability of particles in the micro-channel can be improved, which may be helpful to reduce the aggregation in the micro-channel. If the particles are diluted by the solvents, which can weaken the particle interaction, we think it might not be effective for reducing the aggregation. On the other hand, if the solvent strengthens the particles interaction, it will lead to more aggregation.  
Figure 6: Images of forming two particle patterns by the diffraction pattern. (**a**,**b**) forming process of the first pattern. (**c**,**d**) forming process of the second pattern. The first pattern has been moved to the left. The 0.5 \(\upmu\)m particles’ concentration is 4 \(\times\) 10\({}^{9}\) cm\({}^{-3}\). The laser power is 820 mW; scale bar, 10 \(\upmu\)m.  
In addition, when optical tweezers are usually used to measure the tiny forces near an interface, the diffraction of trapping beam may seriously interfere with the correct experiment result if the optical trap is close to the surface. Besides the hydrodynamic correction [42; 43], the optical trapping by the diffraction fringes should be avoided. In the experiments, we found that using the total reflect mirror as the bottom surface of sample cell can prevent the optical tweezers from forming diffraction traps for collecting a large amount of particles. The axial forces on the particles from the high order diffraction fringes are equal to axial forces of the total reflection beam, so the traps generated from the high order fringes are unstable traps, and the particles leave the fringes due to the Brownian motion. The optical trapping becomes similar to the optical trapping in water, and the particles can only be trapped on the central spot.