In the present work, for non-invasive measurement of the liquid temperature in microchannels the 2-color ratiometric Laser-Induced Fluorescence (LIF) technique was improved by adopting 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 consists 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 excitation laser intensity fluctuation and irregular reflection of light from the glossy walls. Therefore, in the second stage, to eliminate those external disturbances, another fluorescent dye, Rhodamine 110 (a temperature-insensitive dye), having a different emission spectrum peak (525 nm) from the Rhodamine B (575 nm) was added. In principle, the external disturbance effects cancel out each other if the intensity ratio between Rhodamine B and Rhodamine 110 is used (instead of using Rhodamine B only). To compensate substantial reduction of fluorescence intensity by re-absorption of the fluorescence light emission from Rhodamine 110, which is inherent in the present 2-color thermometry, dependency of the intensity ratio on the depth of the measuring point also has been examined. In summary, the 2-color ratiometric confocal-LIF thermometry was found to be very useful tool to measure the local temperatures of the liquid flow field in micro-fluidic devices.

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