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Research Papers: Experimental Techniques

# Measurement of Fluid Temperature Across Microscale Gap Using Two-Color Ratiometric Laser-Induced Fluorescence Technique in Combination With Confocal Microscopy

[+] Author and Article Information
Dong Woon Jeong

Department of Mechanical Engineering, KAIST, Daejeon 305-701, South Koread.w.jeong@kaist.ac.kr

Chi Young Lee, Sang Yong Lee

Department of Mechanical Engineering, KAIST, Daejeon 305-701, South Korea

J. Heat Transfer 131(9), 091601 (Jun 22, 2009) (8 pages) doi:10.1115/1.2976553 History: Received September 13, 2007; Revised May 18, 2008; Published June 22, 2009

## Abstract

In the present work, for noninvasive measurement of the liquid temperature in microchannels, the two-color ratiometric laser-induced fluorescence (LIF) technique was combined with the confocal microscopy. By using this technique, the fluorescent light from the tiny volume around a focusing spot can be selectively detected, and it enables us to measure the local liquid temperatures even at the close vicinity of the walls. To check the general performance of this method, as the preliminary stage, a test section consisting of two horizontal plates in different temperatures, separated by a narrow gap filled with a mixture of rhodamine B (a temperature-sensitive dye) and methanol was made, and the temperature distribution was examined. Based on the relationship between the fluorescence intensity and the temperature, a linear temperature distribution across the gap (by conduction heat transfer) could be confirmed. However, the measured results were subject to external disturbances such as the excitation laser intensity fluctuation and the irregular reflection of the light from the glossy walls. Therefore, in the second stage, rhodamine 110 (a temperature-insensitive dye), having a different emission spectrum peak (520 nm) from the rhodamine B (575 nm), was added to the mixture. In principle, the external disturbance effects cancel out each other when the intensity ratio between rhodamine B and rhodamine 110 is considered (instead of taking data only with rhodamine B). To compensate a substantial reduction in the fluorescence intensity from rhodamine 110 by the re-absorption phenomenon within the liquid, which is inherent in using the two-color thermometry, dependency of the intensity ratio on the depth of the measuring point was examined as well. In summary, the two-color ratiometric confocal-LIF thermometry was found to be a very useful tool in measuring the local temperatures of the liquid flow field in microfluidic devices.

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## Figures

Figure 14

Variation in the intensity ratio with the measuring depth for uniform temperature conditions

Figure 15

Dependence of the normalized intensity ratio on the measuring depth at a constant temperature of 24°C

Figure 16

Insensitiveness of the normalized intensity ratio to the liquid temperature

Figure 4

Absorption and emission peaks of RB and R110

Figure 2

Principles of CLSM

Figure 3

Cross-sectional light intensity distributions computed from the 3D point-spread function

Figure 1

Absorption and emission spectra of RB and dependency of fluorescence (emission) intensity on temperature (2)

Figure 5

Schematic of the experimental setup

Figure 6

Schematic of the test section for a one-color thermometry

Figure 7

Schematic and pictures of the test section for a two-color thermometry

Figure 8

Raw RB fluorescence intensity distributions along the depth direction (sample images were acquired at every 13.23 μm interval along the light-axis)

Figure 9

RI mismatching in multilayer media

Figure 10

Variation in normalized RB fluorescence intensity along the depth direction

Figure 11

Measured temperature distribution along the depth direction using a one-color thermometry

Figure 12

Change in the intensity distribution by re-absorption

Figure 13

Dependence of the normalized intensity on the measuring depth with different concentration ratios between R110 and RB

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