A number of microfluidic applications require precise thermal control where in-channel temperature measurements are necessary during the prototyping stage. Rhodamine B based laser induced fluorescence is a common technique used to obtain high resolution measurement of the fluid temperature field. However, PDMS has a tendency to absorb small hydrophobic dyes, such as Rhodamine B, which results in a steady increase in the overall fluorescent signal. This increase in light intensity causes a significant problem that must be overcome to obtain reliable temperature measurements. In this work a simple technique is described to remove the fluorescent signal originating from absorbed Rhodamine B dye particles that does not require surface modification or any significant alterations to the experimental setup. Instead a high power light source is used to photobleach the particles prior to taking images for thermometry analysis. Herein we demonstrate the technique with a conventional fluorescence microscope and a 100W mercury arc lamp and study the temperature field at the intersection of a Y-channel PDMS/glass chip where hot and cold streams merge.

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