Two-color laser-induced fluorescent thermometry for microfluidic systems

The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. Temperature-sensitive (Rhodamine B) and temperature-insensitive (Sulforhodamine-101) fluorescent dyes are used in tandem to determine fluid temperature with high accuracy and low noise using a pulsed Nd:YAG laser as an illumination source. While the fluorescence intensity of the temperature-sensitive dye is proportional to temperature, it is also biased by variations in the illuminating intensity. Therefore, a second temperature-insensitive dye is required to compensate for such biases. Calibration of the two-color LIF system using the RhB–SR101 dye combination in ethanol and water yields temperature sensitivities of −1.5% K−1 and −2.7% K−1, respectively, with volumetric illumination from an Nd:YAG laser. The feasibility of this methodology for conducting temperature measurements is explored by measuring a steady-state temperature gradient maintained across a microfluidic channel array by two large hot and cold reservoirs. These measurements reveal that the mean steady-state temperatures in the microchannels are within ±0.4 ◦C and ±0.3 ◦C of the predicted temperatures with ethanol and water as the solvents, respectively, with a spatial resolution of 22.2 × 22.2 μm. The experimental uncertainties in the measurements using the RhB–SR101 dye combination are ±0.48–0.59 ◦C and ±0.41–0.49 ◦C for ethanol and water, respectively.