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Research Papers

Study of Liquid-Metal Based Heating Method for Temperature Gradient Focusing Purpose

[+] Author and Article Information
L. Gui

e-mail: lingui@mail.ipc.ac.cn

J. Liu

Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences,
Beijing 100190, China

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received June 19, 2012; final manuscript received February 15, 2013; published online July 26, 2013. Guest Editors: G. P. “Bud” Peterson and Zhuomin Zhang.

J. Heat Transfer 135(9), 091402 (Jul 26, 2013) (8 pages) Paper No: HT-12-1296; doi: 10.1115/1.4024426 History: Received June 19, 2012; Revised February 15, 2013

Temperature gradient focusing (TGF) is a highly efficient focusing technique for the concentration and separation of charged analytes in microfluidic channels. The design of an appropriate temperature gradient is very important for the focusing efficiency. In this study, we proposed a new technique to generate the temperature gradient. This technique utilizes a microchannel filled with liquid-metal as an electrical heater in a microfluidic chip. By applying an electric current, the liquid-metal heater generates Joule heat, forming the temperature gradient in the microchannel. To optimize the temperature gradient and find out the optimal design for the TGF chip, numerical simulations on four typical designs were studied. The results showed that design 1 can provide a best focusing method, which has the largest temperature gradient. For this best design, the temperature is almost linearly distributed along the focusing microchannel. The numerical simulations were then validated both theoretically and experimentally. The following experiment and theoretical analysis on the best design also provide a useful guidance for designing and fabricating the liquid-metal based TGF microchip.

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Figures

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Fig. 1

Schematic of PDMS/PDMS microfluidic chip embedded with liquid-metal microheater for temperature gradient focusing purpose. L, W, and h are the chip length, width, and height, respectively. l and d are the liquid-metal channel length and width, respectively. s is the distance between the liquid-metal channel and the focusing channel in the x-direction.

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Fig. 2

Schematic of four typical possible chips (plane at z = 0) for numerical simulations. (a) Design 1, (b) design 2, (c) design 3, and (d) design 4.

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Fig. 3

Isothermal diagrams (plane at z = 0) of numerical solutions to four typical possible chips

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Fig. 4

Temperature curve of focusing channel centerline (plane at z = 0) from numerical solutions for each design of four typical possible chips

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Fig. 5

Three-dimensional thin layer with 2ε thickness (ranging from z = −ε to z = ε, 2ɛ≪h)

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Fig. 6

Schematic of two-dimensional analytical model describing temperature field of plane at z = 0. The width of the micro heater d is d = x2 − x1 and the length l is l = y2 − y1.

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Fig. 7

Comparisons of focusing channel centerline temperature curves among numerical results (from numerical model), analytical results (from two-dimensional analytical model), and experimental results

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Fig. 8

Experimental setup for measuring focusing channel temperature distribution of PDMS/PDMS chip

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Fig. 9

Temperature distribution of chip obtained from two-dimensional analytical solution. The black lines show the position of the two microchannels.

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Fig. 10

Temperature distribution curves of focusing channel centerline at various current densities i for d = 50 μm, l = 15 mm, and s = 2 mm

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Fig. 11

Temperature distribution curves of focusing channel centerline at various heater widths d for i = 60 μA/μm2, l = 15 mm, and s = 2 mm

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Fig. 12

Temperature distribution curves of focusing channel centerline at various heater lengths l for i = 60 μA/μm2, d = 50 μm, and s = 2 mm

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Fig. 13

Temperature distribution curves of focusing channel centerline at various distance between heater channel and focusing channel in the x-direction s for i = 60 μA/μm2, l = 15 mm, and d = 50 μm

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