Xiaoshuai Liu,
Jianbin Huang,
Hongbao Xin,
Yao Zhang and
Baojun Li*
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, 510275, People's Republic of China. E-mail: stslbj@outlook.com; Fax: +86-20-8411 2260; Tel: +86-20-8411 0200
First published on 10th January 2014
With the assistance of a particle-decorated fiber probe, optically controlled circling of particles was demonstrated using 3.14 μm diameter silica particles. The method is based on the temperature gradient and thermal convection when a laser beam of 980 nm (power: 108 mW) is injected into the fibre. Specific currents were created by decorating the tip of the drawn fibre probe with specific geometries of silica particles (diameter: 3.14 μm). Thus, the water was being circulated (convective flow driven by heating), and the particles were drowned into the flow. They were circled anticlockwise along a relatively steady trajectory with a period varying from 3 to 7 s. Once the laser switched off, the particles were immediately stopped. Further experiments show that the circular trajectory can be shifted by moving the fiber probe.
ΔT = ηSτ/(cρd) | (1) |
F = (n/c)∫∫ΔSdA | (2) |
Since the particle motion depends on the temperature gradient and thermal convection, particle circling can be realized by changing the power flow distribution in water. The most effective and convenient method is to decorate some particles on the probe to change the optical filed distribution at the probe end. By doing so, the temperature gradient in water will be changed and a convection flow will be caused, which drives the other suspended particles near the probe to move along the trajectory of flow. We chose silica particles to decorate the probe because they have same refractive index with the fiber probe so that the reflection loss between the probe and the particles can be weakened. As shown in Fig. 2, the white and yellow dashed arrows indicate the water flow induced by temperature gradient and thermal convection, respectively. The right panel of each graph shows the power flow distribution along y direction at x = 21.5 μm. It should be noted that to provide a stable motion for particles, the optical power flow distribution in water should be symmetric to avoid extra water flow caused by the asymmetric temperature distributions around the probe. Since the fiber probe and the injected laser beam are all axially symmetric, the optical power flow distribution symmetry can be realized by decorating the particles in the same amount on each side of the probe. Therefore, the number of silica particles decorated on the probe should be even. If two particles are decorated at the probe end (Fig. 2a), the power flow is mainly along x direction. The water will flow along the same direction because of the very slight change of the temperature gradient. When four particles are decorated (Fig. 2b), the power flow begin to spread but the change of the temperature gradient is still not enough to cause a circular flow. Thus, when only two or four particles decorated to the fiber probe, the power flow distribution do not change efficiently and the particle will be pushed away along the fiber probe. While if eight particles attached (Fig. 2e), the power flow would be further diverged. Then the flow trajectory will become more complex and uncontrollable, which was consistent with the experiment result. Moreover, the increasing particles will absorb the outputted laser and weaken the optical field distribution in water, which prevented the generation of convection flow. When six particles are decorated as shown in the right panel of Fig. 2c, three peaks appear near the probe end and over 90% power flow is focused within the region from y = −6 to 6 μm. According to the eqn (1), the temperature gradient is along the direction of power flow with a maximum value of 4.7 K μm−1. The water will flow along the changed direction (as shown by the white dashed arrow 1 in Fig. 2c). Meanwhile, due to the liquid continuity, the water will flow toward the heated location (as shown by the yellow dashed arrows 2 and 3 in Fig. 2c). As a result, the particles will be moved along the flow direction and be circled above the fiber probe as schematically described in the Fig. 2d.
As mentioned with the theoretical analysis, the decorated particle affected the light field distribution and hence the location of circling. Thus, it was very important to realize the particles being decorated correctly. However, it was indeed a big challenge to efficiently control the number and location of decorated particles. After analyzing and trying some methods, we realized it with the following process: firstly the particle density in solution needs to be controlled accurately. The fiber probe was decorated by attaching the particle one by one. So if the particle density is too high, it will be hard to realize single particle decorated. Then we tried different kinds of particle density and chose the 5 × 107 # per mL as the optimal choice. After the particle solution was prepared, the fiber probe was positioned on the six-axis microstage (resolution: 50 nm) and being adjusted to approach one particle. Then the laser was turned on (power: 70 mW), and the particle was moving toward the fiber probe because of the optical force.21 When the particle touched the fiber probe, the laser was turned off and the particle was attached to the fiber probe under the action of Van der Waals' force. Thus, the first particle was decorated to the fiber probe successfully. Then the fiber probe was adjusted by the microstage to approach the other particle. The decorated location can be changed by adjusting the fiber probe with the microstage. Similarly, the other five particles were successfully decorated to the fiber probe as schematically shown in Fig. 3c.
After decorating particles to the fiber probe, particle circling was performed by tuning the 980 nm laser power to 108 mW. Fig. 4 shows the circling of a particle above the six-particle-decorated fiber probe with a circling period of 7 s. The yellow arrows show the moving direction. The white and black dashed curves indicate the circular motion induced by temperature gradient and thermal convection, respectively. It can be seen that the particle was circled above the probe anticlockwise. Detailed process is shown in Movie S1 of the ESI.† Fig. 5 shows the velocity of circling particle. It can be seen that the velocity increases firstly and then decreases. According to the analysis presented in the previous section, the particle circling is mainly driven by the temperature gradient force. Thus, the velocity increases rapidly at the nearest location of circling trajectory to the probe because of the largest temperature gradient. When the particle moves away from the probe, the velocity decreases gradually due to the liquid resistance as well as the decrease of temperature gradient. It should be clarified that the particle circling period varies from 3 to 7 s, which is due to the change of circling trajectory and temperature gradient in water. During the experiment, the water evaporation decreased the effective thickness of solution (d). With the experiment time of 0, 20 and 40 minutes, the values of d are 0.4, 0.35 and 0.3 mm, respectively. According to the eqn (1), the temperature gradient in water became larger so that the particle circling velocity also increased with maximum values of 171, 213 and 239 μm s−1 (the black, red and blue curves in Fig. 5). As a result, the circling periods also changed to 7, 6 and 5 s, respectively.
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Fig. 4 Optical microscopic images for the particle circling at different moments in a circling period of 7 s. |
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Fig. 5 Velocity of particle with the solution effective thickness (d) of 0.4, 0.35 and 0.3 mm. The circling periods were 7, 6 and 5 s, respectively. |
To verify that the circling is induced by the outputted beam from the fiber probe, the laser was switched off for 5 s and then turned on again. The particle trajectory and velocity of this process are shown in Fig. 6 and 7, respectively. Firstly, the particle was circled anticlockwise above the fiber probe with a period of 5 s (Fig. 6a–c). At t = 5 s, the laser was switched off. The particle stopped circling immediately and oscillated slightly around the stopped location (Fig. 6d and e) due to the surrounding fluctuations. Turning on the laser at t = 10 s, the particle began to circle again (Fig. 6f–h). Detailed process is shown in Movie S2 of the ESI.† This result indicates that the particle can be positioned at different locations by turning off the laser. Moreover, the particle circling trajectory and velocity nearly keep constant before switching off the laser and after turning on the laser again (Fig. 7).
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Fig. 7 Velocity of the particle before switching off the laser (the blue curve) and after turning on the laser again (the red curve). |
To investigate the stiffness of circling, the fiber probe was shifted by tuning the microstage. The particle motion trajectory and circling velocity are shown in Fig. 8 and 9, respectively. It can be seen from Fig. 8 that the particle was circled above the fiber probe anticlockwise with a period of about 4.5 s. At t = 4.5 s, the fiber probe was shifted along the −y direction with a distance of 14 μm. The particle was also moved with the shift of the fiber while keeping circling above the probe (Fig. 8d). Fig. 9 shows the velocities of the circling particle before and after fiber shifted. In addition, it was also found that multiple particles can be circled above the fiber probe simultaneously. Fig. 10 shows that seven particles were circled above the probe within a period of about 5 s. The maximum and minimum velocities of the seven particles are measured and shown in Fig. 11. Detailed process is shown in Movie S3 of the ESI.† It can be seen that although the trajectories of seven particles are slightly different from each other, the maximum and minimum velocities exhibit a uniformity.
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Fig. 10 Optical microscopic images for seven particles circling. The yellow circles indicate the particle location. (a) t = 0 s. (b) t = 1 s. (c) t = 2 s. (d) t = 3 s. (e) t = 4 s. (f) t = 5 s. |
Footnote |
† Electronic supplementary information (ESI) available: Video clips (Movies S1, S2, and S3) for detailed processes of particle circling. See DOI: 10.1039/c3ra46822d |
This journal is © The Royal Society of Chemistry 2014 |