The removal of dielectric particles and bacteria from water is an extremely important global issue, particularly, for drinking and sanitation. This work provides a demonstration of optical purification of water using an optical fiber-ring. The size of particles suspended in water for trapping is 2.08 μm in diameter and the wavelength of light used for inducing photothermal effect is 1.55 μm with a power of 97 mW. The fiber, 6 μm in diameter, was formed to a racket-shaped ring with a minimum diameter of 167 μm and a maximum one of 350 μm. Experiment indicates that the particles moved toward the ring with the highest velocity of 4.2 μm/s and are trapped/assembled in the center of the ring once the laser beam of 1.55-μm wavelength was launched into the fiber. With a moving of the fiber-ring, the trapped/assembled particles were moved and the water can be purified by removal of the particles.
© 2011 OSA
Since the pioneering work of Ashkin and associates [1,2], optical trapping and manipulation have become one of the most important tools for research in the fields of biology, physical chemistry and soft condensed matter physics [3,4]. This success largely lies on the noninvasive nature of optical forces, which can be exerted on particles. Optical tweezers have been developed and widely used to control and actuate dielectric microparticles and other microscopic objects by using a tapered fiber  or focusing laser beams [6,7]. Similar work on microsphere manipulation with evanescent fields by Kawata and Sugiura  provides more interests on optical manipulation at surfaces. Particles are attracted to high intensity region by optical gradient force of the evanescent fields around planar photonic devices, such as slot waveguides , planar waveguides , plasmonic waveguide , and microring resonators . Unlike free space optical trapping geometries, evanescent wave propagating along waveguides extends over long distance, and thus is capable of trapping particles over large areas. Subwavelength optical wires, without substrates, have been exploited for three-dimensional particle manipulation with an even higher efficiency . In these works, particles are trapped by utilizing radiation pressure force (optical gradient force) via a transfer of the momentum from photons refracted in transparent particles. Light can also be scattered and absorbed by particles suspended in a fluidic background, and consequently, turned into an uneven heat distribution in the particle volume. The uneven heat distribution can cause a force to drive the particles to move. This phenomenon is called photophoresis [14,15]. Particles with high absorptivity move away from the light source (positive photophoresis), while particles with low absorptivity move toward the source (negative photophoresis) . Photophoretic force is usually orders of magnitude larger than optical gradient force [1,16], and thus photophoresis can be used for large scale and long range manipulation of particles. Besides optical field, temperature gradient can also be used for trapping and manipulation of microscale particles. Strong temperature gradients can result in an isotropic diffusion of particles/molecules in a bulk medium. Particles can move along temperature gradient, typically from hot to cold . This phenomenon, i.e. thermophoresis, has been widely used in molecules and small particles trapping and manipulation [17–20]. In this work, we demonstrate a method for large scale trapping of dielectric particles with the assistance of an optical fiber-ring by combining photophoresis and temperature gradient.
Figure 1(a) depicts the experimental setup including a microscope for observing trapping process, a computer connected charge coupled device (CCD) for capturing images, and an erbium doped fiber amplifier (EDFA) for amplifying laser of 1.55-μm wavelength. A tapered silica fiber is fixed by a microstage and connected with the 1.55-μm laser through the EDFA. The fiber taper was formed to a racket-shaped ring and immersed in a water suspension of silica particles with a diameter of 2.08 μm. Figure 1(b) shows the optical microscope image of the racket-shaped ring. The average diameter of the fiber is r = 6 μm. The maximum diameter of the ring is a = 350 μm while the minimum diameter is b = 167 μm.
To get an effective photothermal effect while inducing little harm to the biophysical circumstances , in this experiment, the 1.55-μm laser was amplified to 97 mW by the EDFA and launched into the fiber-ring. Figure 2(a) shows the optical microscope image without laser launched. It can be seen that the silica particles are randomly suspended in water. In this case, there are a few particles inside the ring. Once the laser is turned on, photothermal effect will be induced by the 1.55-μm laser with a power of 97 mW. As a result, the particles start to move. At the beginning, particles move fast but unsteadily in the vertical direction because of the convection, and so these particles cannot be trapped. In the horizontal direction, the convection becomes much weaker. The particles outside the ring move toward the ring by a force (F P) of negative photophoresis. Meanwhile, a great temperature gradient in water is caused by the strong absorption of light, which consequently induces a thermophoreitc force (F T) larger than F P. The F T and the F P act on the particles in opposite direction. Therefore, the particles slow down and are gradually accumulated, i.e. trapped in the center of the ring. Detailed trapping process is shown in Media 1. Figure 2(b) shows the optical microscope image with the optical power of 97 mW launched into the fiber for 4 mins. It is estimated that about seven hundred particles were trapped in the center of the ring. As time goes by, the number of trapped particles keeps increasing. For examples, when t = 9′00″, more than two thousand particles were trapped (Fig. 2c). As more and more particles were trapped in the ring, particles outside the ring become fewer and fewer, and thus only a few particles move toward the ring. When t = 14′00″, the number of particles trapped in the ring has tended to saturation (Fig. 2d). The profile of the main accumulation region is marked by blue dashed line. It should be noticed that, due to the much stronger scattering light in the sharp bending region of the fiber ring, the accumulation region is closer to left boundary of the ring. It should also be pointed out that on the right boundary, some particles are stopped on the fiber because a cluster of particles is obstructed by the fiber when moving toward the main accumulation region.
We counted that there are about 3250 particles trapped in the main accumulation region. Figure 3(a) shows a relation of the counted number of trapped particles with the time of the optical power applied. The inset of Fig. 3(a) shows the final state of the trapping process. It is obviously that the number of particles outside the ring decreased a lot and the water is much cleaner. The average velocity of particles at different time of optical power applied with the distance from the accumulation position was calculated as shown in Fig. 3(b). Particles moving toward the ring can get the highest average velocity of 4.2 μm/s when the optical power has been applied for 3 mins. During the trapping process, we found that the velocity of the clusters is higher than that of the single particles, which is consistent with the results of Refs [15,18].
To demonstrate the stiffness of the trapping, about 550 silica particles were firstly trapped and then dragged to a new location with the same laser power. Figure 4(a) is the image of the fiber-ring which was moved to a distance of ~50 μm along x direction and ~40 μm along y direction from its original location (i.e. the white-dashed-line ring). In this case, all particles are at their originally trapped locations. With an increase of the dragging time (td) from td = 0′00″ (Fig. 4a) to td = 0′20″ (Fig. 4b), td = 0′40″ (Fig. 4c), td = 1′00″ (Fig. 4d), and td = 1′20″ (Fig. 4e), the trapped particles gradually moved and centered to their new location. When td = 1′40″, almost all of the assembled particles were dragged to the new location (Fig. 4f) with an average distance of ~64 μm. This case is the same with that before the dragging process (i.e., the same with the original case indicated by the white-dashed-line ring of Fig. 4a). Experiments for different moving distances and speeds of the fiber-ring have also been done and concluded that, over 98% trapped particles can be dragged and centered to the new location if the moving distance is less than 70 μm (in one movement of the ring, no limitation to the moving speed) or the moving speed of the fiber-ring is smaller than 0.5 μm/s (no limitation to the moving distance of the fiber-ring) ensures. The results indicate that this method is applicable for the removal of dielectric particles and bacteria from water.
To explain the photothermal trapping of the optical fiber-ring on the dielectric particles, a two-dimensional (2D) finite element simulation (without particles in water) has been carried out. Figure 5(a) shows the power distribution around the fiber (6-μm diameter) in water when a 1.55-μm laser is launched with a power of 97 mW. It indicates that the scattering light at the sharp bend (left boundary) of the ring (see inset I of Fig. 5a) is much stronger than that at the right boundary of the ring (see inset II of Fig. 5a). Stronger scattering light leads to a larger photophoretic force and a greater temperature gradient in the left boundary than those in the right boundary. Therefore, the accumulation region of the particles is closer to the left boundary of the ring, which agrees well with the experimental results observed in Figs. 2 and 4. To estimate the temperature distribution induced by the optical power around the ring, the power flow along X axis (the white line in Fig. 5a) is given in Fig. 5(b). The temperature increment is approximated to ΔT = ΔQ/(cm) = ηSτ/(cρd), where d = 0.5 mm is the effective depth of water, c = 4.2 × 103 J/(kg·K) is the heat capacity of water, ρ = 1 × 103 kg/m3 is the density of water, S is the power flow, η = 1−e− αd = 42% (α = 10.9 cm−1 is the absorption coefficient of 1.55 μm in water ) is the ratio of light absorbed by water, and τ is thermal relaxation time, approximated to τ = d 2/κ = 1.9 s , where κ = 1.4 × 10−3 cm2/s is the thermal diffusivity of water at room temperature . The inset of Fig. 5(b) shows the temperature increment along X axis inside the ring. Far from the ring, temperature difference is negligible, but light leaked out from the fiber and scattered by particles still exits, and thus the low-absorptivity particles (silica) move toward the ring because of negative photophoresis. While near the ring, the large temperature gradient (0.1 K/μm) near the fiber forms a temperature well in the ring, the center of the ring is the temperature minimum position. This traps particles well to the center of the ring, and particles are consequently assembled in the center. It should be clarified that, first, since the size of the water drop used in this experiment is about 0.5 mm thick and 10 mm in diameter, therefore, convection is mainly occurred at the surface and the vertical direction of the drop, while it is much weaker in the horizontal direction of the inner part of the drop. Second, the optical power (97 mW) is not large enough to cause a strong inner convection. Compared to the effect caused by the photophoresis and temperature gradient, convection can be ignored. Therefore, the trapping process is mainly induced by the photophoresis and the temperature gradient. Our experiments confirmed that the trapping is in the inner of the water rather than on the glass substrate. It should be pointed out that if the convection is strong, it cannot be ignored, and the particles should be trapped on the substrate .
To discuss the trapping capability of different size and shape of the fiber-ring on the particles, a series of 2D finite element simulations have been carried out. Figures 6(a) and (b) show the results of the fiber-ring with the same shape (racket shape) as that we used in the experiment but with different size. From the simulation, we can see that, for the ring with larger size (Fig. 6a, the maximum diameter 390 μm, the minimum diameter 210 μm), the power flow S exists around the fiber, leaving a large part without power flow in the center of the ring. As a result, the temperature gradient only exists near the fiber, which traps particles in a narrow girdle-shaped region represented by the yellow area together with dark dots. For the ring with smaller size as shown in Fig. 6(b) (the maximum diameter 190 μm, the minimum diameter 90 μm), the power flow exists in the main region of the ring, which leaves only a small region without temperature gradient, and thus a small number of particles can be trapped in the trapping region (indicated by the yellow area together with dark dots). Further simulations show that when the minimum diameter of the racket-shaped ring is less than 50 μm, power flow from both side of the fiber-ring meets each other. As a result, the particles cannot be trapped in the ring. Figures 6(c) and (d) show the results of the fiber rings with different shapes. For the elliptic ring (Fig. 6c) with the maximum diameter of 320 μm and the minimum one of 100 μm, most of the light is scattered and leaked out from the fiber at the sharp bend. Therefore, only a small number of particles can be trapped in a very narrow region on the left of the ring, as shown in Fig. 6(c). For the fiber ring with a circular shape and a diameter of 170 μm, the power flow is almost uniformly distributed around the fiber ring, and thus a large number of particles can be trapped in a disc-shaped region at the center of the ring, as shown in Fig. 6(d). It should be pointed out that circular fiber-ring is the best one for particle trapping.
We have experimentally demonstrated a method of highly efficient photothermal trapping of dielectric particles suspended in water using an optical fiber-ring. With a laser power of 97 mW, particles were trapped by the ring with the highest average velocity of 4.2 μm/s. Experiment shows that the number of particles assembled in the center of the ring has increased by three orders of magnitude in 14 mins. This method gives a way for removal of bacteria and other particles from water, and thus can be applied in local water purification, particularly, for sanitation.
This work was supported by the National Natural Science Foundation of China (Grants 61007038, 60625404 and 10974261) and China Postdoctoral Science Foundation (Grant 20100470952).
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