Dielectrophoretically assembled particles: feasibility for optofluidic systems
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- Khoshmanesh, K., Zhang, C., Campbell, J.L. et al. Microfluid Nanofluid (2010) 9: 755. doi:10.1007/s10404-010-0590-7
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This work presents the dielectrophoretic manipulation of sub-micron particles suspended in water and the investigation of their optical responses using a microfluidic system. The particles are made of silica and have different diameters of 600, 450, and 250 nm. Experiments show a very interesting feature of the curved microelectrodes, in which the particles are pushed toward or away from the microchannel centerline depending on their levitation heights, which is further analyzed by numerical simulations. In doing so, applying an AC signal of 12 Vp–p and 5 MHz across the microelectrodes along with a flow rate of 1 μl/min within the microchannel leads to the formation of a tunable band of particles along the centerline. Experiments show that the 250 nm particles guide the longitudinal light along the microchannel due to their small scattering. This arrangement is employed to study the feasibility of developing an optofluidic system, which can be potentially used for the formation of particles-core/liquid-cladding optical waveguides.
Micro and nano particles can be manipulated in microfluidic systems using dielectrophoretic (DEP) forces. A DEP force is generated when a neutral particle is suspended in a non-uniform electric field. This electric field induces electrical charges within the particle to establish a dipole. If a non-uniform electric field is applied, the ends of the dipole experience unequal Columbic forces, which result in a total non-zero, imposed force. If the particle is less polarizable than the suspending medium, it is repelled from the regions of higher electric field and the motion is called negative dielectrophoresis, while the opposite case is referred to as positive dielectrophoresis.
Already, DEP systems are used in applications such as separation, filtering, and permanent localization of micro/nano particles (Cummins et al. 2006; Lao et al. 2006, Ofir, et al. 2008; Webb and Li 2005; Zhang et al. 2009). However, the possibility of forcing particles into forming suspended three-dimensional (3D) objects using dielectrophoresis is yet to be investigated. The advantages of such systems stems from the fact that while suspended particles in liquid can be brought into intimate contact with each other, they can still remain unbounded. Such suspended particles can then be employed to form suspended devices with exceptional functionalities due to the unique properties of the micro/nano materials.
Using DEP forces, suspended particles can be pushed into distinct regions and later be scattered from those regions by applying electric fields. Hence, DEP forces can be employed for the ‘forced-assembly’ of particles, positioning them to establish 3D optical devices within a liquid. The accessibility of a variety of micro, sub-micron, and nanoscale particles can allow us to form such optofluidic systems with various optical scattering and guiding properties. As a result, particles of different dimensions can be used as the building blocks of devices for targeted applications. For instance, it is possible to make suspended optical waveguides that scatter certain wavelengths while allowing others to pass through, using the nanoparticles with dimensions comparable to the wavelengths. Furthermore, a mixture of various materials with distinctly different optical and electrical properties can be mixed and manipulated to create new optical materials suspended in microfluidic systems.
In this article, we describe the DEP patterning of silica sub-micron particles using an array of curved microelectrodes. The magnitude and frequency of the AC signal, which is applied across the microelectrodes, as well as the medium flow rate, are selected such that the microelectrodes act as a funnel, smoothly forming a narrow band of particles along the centerline of the microchannel. The width of the patterned particles remains constant and can be tuned along the microchannel due to the curved configuration of the microelectrodes.
The silica particles have a higher refractive index than the surrounding flow and can confine the coming light. Therefore, our system is a platform to create a particles-core/liquid-cladding optical waveguide, in which the light is confined inside the high refractive index patterned particles (the core) by the low refractive index liquid (the cladding). Optical waveguides with solid-core/liquid-cladding (Schmidt et al. 2007; Yang and Erickson 2008), liquid-core/solid-cladding (Wang and Fang 2005) and liquid-core/liquid-cladding (Bernini et al. 2008; de Matos et al. 2007; Wolfe, et al. 2004) configurations have already been reported. It has also been shown that the characteristics of these optical systems can only be tuned by changing the flow rate and the composition of the liquids, while the solid component remains unchanged (Bernini et al. 2008; Conroy et al. 2005; de Matos et al. 2007; Wolfe et al. 2004, 2005). However, in our waveguides the optical characteristics can be controlled on demand to a much higher degree by changing the composition and size of sub-micron particles as well as the applied signal and frequency.
4 Results and discussions
The experiments showed that applying a combination of a high electric potential across the microelectrodes and a low flow rate within the microchannel pushed the particles toward the sidewalls, producing a particle-free region along the centerline (Khoshmanesh et al. 2009). By contrast, the combination of a low potential and a high flow rate funneled the particles, producing a particle-rich region along the centerline.
In our simulations, we have applied the single-particle model of DEP force throughout the microchannel including the centerline, where we expect a high concentration of particles. Although this model is reasonably consistent with the experimental observations (Doh and Cho 2005; Kang, et al. 2008), the modified models of DEP force can be applied in calculations, in which the particle-particle interactions are taken into account (Khusid and Acrivos 1995; Kumar et al. 2007).
The sedimentation force applied on particles is 8.2 × 10−17, 4.8 × 10−16, and 2.2 × 10−15 N, respectively for 230, 450, and 600 nm particles, as obtained from Eq. 4. The levitation height of the particles is obtained after the DEP-z and sedimentation forces reach a balance along the z-axis. Applying a low potential across the microelectrodes decreased the DEP-z force and levitated the particles at lower heights. For example, applying an AC signal of 12 Vp–p and 5 MHz most particles were levitated at z = 15–35 μm nearby the tips, as observed by an inverted microscope (Nikon, TE2000-U) while increasing the signal magnitude to 20 Vp–p pushed the particles upward and levitated most of them at z = 35–50 μm. The experimental results are consistent with the predicted DEP-z forces obtained from simulation (not shown). The effects of other forces induced by Joule heating effect, fluid heating by light, and optical radiation pressure are ignored due to their negligible influence, as given in Appendix 2.
In our experiments, the combination of a low potential and a high flow rate (12 Vp–p and 1 μl/min) was employed to concentrate the silica particles along the centerline for the purpose of producing a silica particle-rich region (path-3) in DI water medium. These silica particles have a higher refractive index (nd = 1.475 at 20°C) (Khlebtsov et al. 2008) than DI water (nd = 1.333 at 20°C). Therefore, this configuration can establish a particles-core/liquid-cladding optical waveguide, as shown in Fig. 2. The refractive index contrast (nd core − nd cladding) of our system is 0.142, which is higher than that of 0.11 obtained with aqueous solution of CaCl2 and DI water (Wolfe et al. 2004), and 0.017 obtained by ethylene glycol and ethylene glycol–water mixture (Wolfe et al. 2005) used to develop diffusion controlled optical waveguides. We also apply silica particle of different diameters to investigate the influence of light scattering.
Figure 4a–c shows the response of 600, 450, and 250 nm silica particles to the DEP field while a 1 mW light was applied perpendicular to the substrate. The magnitude and frequency of the applied signal were 12 Vp–p and 5 MHz, respectively while the flow rate of the suspension was 1 μl/min (equivalent to an average velocity of 0.2 mm/s) using a syringe pump (Harvard Apparatus, PHD 2000). These conditions were applied to levitate the particles at z = 15–30 μm nearby the tips, as described before. All particles exhibited negative DEP behavior and were repelled from the microelectrodes. The particles formed a funnel before the first microelectrode tips and subsequently a narrow band along the centerline afterwards. The width of the band, Δ could be tuned by varying the applied voltage and flow rate. For example, in the case of 450 nm particles, the maximum Δ was obtained at 12 Vp–p and 1 μl/min as ~20 μm. The Δ was very sensitive against voltage variations and decreased to ~10 μm by varying the voltage between 10–14 Vp–p; however it was more resilient against flow rate variations and only decreased to ~15 μm by varying the flow rate between 0.8–1.5 μl/min. Interestingly, the width of the band remained largely constant along the centerline within those ranges. This is another unique feature of curved microelectrodes, in which the magnitude of DEP force increases smoothly over the length of electrodes and abrupt motions of particles are avoided. In contrast, using oblique microelectrodes the width of the band could not remain constant along the centerline due to the abrupt increase of DEP forces over the microelectrode tips. Increasing the applied signal to 20 Vp–p pushed the particles toward the sidewalls, which was not of interest in this work for forming an optical waveguide. Further increasing of flow rate to 2 μl/min destabilized the funneling of particles and also caused the width of the narrow band to change between the consequent pairs, as shown in Appendix 3.
In order to further assess the feasibility of developing a particles-core/liquid-cladding optical waveguide, we conducted a series of experiments by placing a broadband light source horizontally at the inlet side of the microchannel, while the vertical light was turned off. This provided a longitudinal beam of light along the microchannel, as shown in Fig. 2. Results are shown in Fig. 4d–f for different dimensions of particles.
It was seen that the 600 nm particles were sufficiently large to scatter the horizontal light, and hence the narrow band of particles along the centerline was observed brighter than the surrounding medium, as shown in Fig. 4d. A significant degree of scattering was also observed near the sidewalls where the image looked brighter. A similar response was also seen for 450 nm particles, as shown in Fig. 4e. However, the response of 250 nm particles to the horizontal light was quite different. The 250 nm particles guided the light, due to their small diameters and their intimate contacts, along the centerline. As a result, the light passed through this narrow band with much less extinction and the band was observed darker than the surrounding medium, as seen in Fig. 3F.
Alternatively, when the light was applied along the microchannel (Fig. 4d–f), the large 450 and 600 nm particles scattered the light within the particle-rich and particle-depleted regions while the small particles of 250 nm guided the light through the particle-rich (centerline) region. This region was enclosed by liquid streams on both sides that had a lower refractive index (Fig. 6c, d—top view). Under these conditions, the small particles confined the light to form a multimode waveguide of less than 20 μm width and 40 μm height, while the large particles scattered the light.
In summary, the optical properties of a DEP-activated system were demonstrated using silica submicron particles. The particles were focused along the microchannel centerline by energizing the microelectrodes at 12 Vp–p and 5 MHz, and applying a flow rate of the 1 μl/min. The system was applied to analyze the possibility of establishing a particles-core/liquid-cladding optical waveguide when 250 nm silica particles were used. Additional optics (optical fibers, lens and camera) are necessary at the outlet to analyze the output light profile. The performance of the system can be optimized continuously by providing feedback to change in real-time the magnitude and frequency of the AC signal, as well as the flow rate of the medium. We believe that DEP-activated optical waveguides have a great potential to be integrated into lab-on-a-chip systems to facilitate the optical excitation and detection of target samples.