ON-CHIP LIQUID TUNABLE GRATING USING LAMINAR MICROFLUIDIC CONTROL SYSTEM Z. G. Li, L. K. Chin, H. J. Huang, H. N. Unni and A. Q. Liu† School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 (†Corresponding author: Tel: +65-67904336, E-mail:
[email protected]) ABSTRACT A liquid tunable grating in a micro-optical fluidic system (MOFS) chip is demonstrated in this paper. The on-chip grating is realized through a microfluidic control system comprising of multilayer laminar flow. The liquid grating provides tuning possibility through the modulation of refractive index (RI) and grating pitch. The liquid tunable grating can be employed for accurate measurement of refractive index of biological and chemical fluids (e.g. Phosphate Buffered Saline (PBS)) and therefore has enormous potential applications in Lab-on-a Chip based bioanalysis. KEYWORDS: Liquid tunable grating, Microfluidics, Micro-optical fluidic system (MOFS), Kelvin-Helmholtz instabilities INTRODUCTION The liquid tunable grating is one of the most important optical devices. Recent research approaches have focused on magnetic fluid gratings [1] and the liquid reflection grating using conducting liquid driven by electrodes [2]. However, such systems are not easily amenable to Lab-on-a-Chip integration system. The system presented in this work is easily adaptable to microfluidic integration and the largest component of the microfluidic system measures only 650 µm. In addition, the presented MOFS design offers the possibility of optical measurements of biological and chemical liquid materials. In this paper, two kinds of fluids with different refractive index are injected into a microchannel and the liquid grating is formed. The flow rate is precisely controlled to modulate the width of the fluids in the microchannel, and thereby controlling the Fluid A inlets Detection area
Fluid A
PDMS Microchannel
SLED Light Fluid B inlets Fluid B Outlets
Output Spectra The liquid tunable grating
(a) (b) Figure 1: (a) Schematics of the multifluidic control system for liquid tunable grating; (b) The cross section of the microchannel of the detection area. Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences October 12 - 16, 2008, San Diego, California, USA 978-0-9798064-1-4/µTAS2008/$20©2008CBMS
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grating pitch and period, as shown in Fig 1(a). THEORY Fig 1(a) and Fig 1(b) represent the schematics of the microfluidic control system and the cross section of the detection area. Since the liquid grating structure involves the flow of adjacent binary liquid layers, the interfaces are likely to be subjected to wave like fluctuations arising from Kelvin-Helmholtz (KH) instabilities [3]. The KH instability causes the liquid-liquid interface to form waves of small wavelength. Based on the approximation that only normal viscous forces contribute to the interface fluctuation in the direction perpendicular to fluid flow, the condition for a stable liquid-liquid interface can be derived as 2 ⎧⎪ ⎫⎪ γ k 2 ⎡⎣ μ2 coth ( kw2 ) + μ1 coth ( kw1 ) ⎤⎦ 2 ⎨ ⎬ ≥ (U1 − U 2 ) 2 2 2 2 ρ μ coth kw coth kw ρ μ coth kw coth kw + ( ) ( ) ( ) ( ) 2 1 2 1 1 2 1 2 ⎩⎪ ⎭⎪min
(1)
Where (ρ1, μ1, U1) and (ρ2, μ2, U2) represents the density, dynamic viscosity and average velocity of the first and second fluids respectively. w1 and w2 are the widths of the two fluid streams and γ represents the interfacial tension. k represents the wave number where k = 2π/λ (λ is the wavelength of the interface wave). Based on the fluid properties of CaCl2 solution and the flow rates of the neighbouring fluid layers (5µL/min and 10 µL/min), the stability condition referred to by Eq. (1) is satisfied for the case of short waves with k > 104 (i.e., λ < 2π×10-4). EXPERIMENTAL The MOFS chip is fabricated using polydimethylsiloxane (Sylgard 184 Silicone Elastomer, DOW Corning Corporation) and soft lithography. Two kinds of calcium chloride (CaCl2) solutions with different refractive index (1.425 and 1.465) are injected into the MOFS chip by multi-step motor syringe pumps. Initially, the injected fluids pass through a junction such that they are merged to form a multilayer flow. Using a contraction region (Fig. 1(a)), the width of each flow can be individually reduced. In the detection area, a pair of optical fibres is used to couple the light source and detect the light signal. For the experiments, a 1550 nm SLED broadband source (DenseLight Semiconductors) is used as the light source and the detector is an optical spectrum analyzer (OSA, ADVANTEST Q8384). The number of fluid flow can be controlled, resulting in multilayer flow from 3 layers up to 13 layers realized in the MOFS chip. The length of the grating can be adjusted by changing the width of the microchannel using an expanded part or a contracted part in the channel structure. The length of the grating is fixed at 650 μm. The periods of the liquid grating with different layers are 130 μm (5 layers), 92.8 μm (7 layers), 72.2 μm (9 layers) and 59.1 μm (11 layers), as shown in Fig 2. RESULTS AND DISCUSSIONS Figure 2 represents the images of multilayer flow in the grating structure. It can be observed that liquid-liquid interfaces are stable over the length of the grating. However, interfacial spreading is visible when the number of fluid layers is increased Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences October 12 - 16, 2008, San Diego, California, USA
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1.0 11 Layers 9 Layers 7 Layers 5 Layers
(a)
(b)
Normalized Intensity
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(c) (d) Figure 2. Multi-layer flow (contrast enhanced), (a) 11; (b) 9; (c) 7 and (d) 5 layers.
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Figure 3. Transmission Spectra of the liquid tunable grating with different layers.
above 9. A possible reason for this effect can be attributed to the diffusive mixing at the liquid-liquid interface, and the nanoscale molecular interactions at the liquid interface. Figure 3 represents the normalized output spectra corresponding to different number of fluid layers. The transmission spectra indicate a weak grating effect with increasing number of the fluid layers. One possible reason is that the graded refractive index exists at the interfaces between neighbouring fluids due to diffusion. CONCLUSIONS In this paper, a liquid tunable grating based on a microfluidic control system is designed and fabricated on a microfluidic chip. The liquid grating provides the tuning possibility through RI modulation and grating pitch to fit different light sources and applications. The liquid tunable grating has very high potential applications in biological, chemical analysis and detection. REFERENCES [1] C. Y. Hong, H. E. Horng, I. J. Jang, J. M. Wu, S. L. Lee, W. B. Yeung and H. C. Yang, Magneto-chromatic effects of tunable magnetic fluid grating, Journal of Applied Physics, 83, pp. 6771-6773, (1998). [2] D. D. Ryutov and A. Toor, Renewable liquid reflection grating, U. S. Patents No. 6 631 032. [3] A.Gunther and K. F. Jensen, Multiphase microfluidics: from flow characteristics to chemical and materials synthesis, Lab on a Chip, 6, pp. 1487-1503 (2006).
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