A Two-Color Fluorescent Thermometry Technique for Microfluidic Systems

The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. A temperature-sensitive fluorescent dye (Rhodamine B) and a temperature-insensitive fluorescent dye (Sulforhodamine-101) 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 utilized in order to compensate for such biases. Calibration of the two-color LIF system reveals that the two-dye mixture in water yields a temperature sensitivity of 2.7\%/K with volumetric illumination from the pulsed Nd:YAG laser. Additionally, the feasibility of this methodology for conducting temperature measurements is explored by measuring a steady-state temperature gradient generated across a microfluidic channel array by two large hot and cold reservoirs. These measurements yielded mean steady-state temperatures in the microchannels within $\pm0.3\,^{\circ}$C of the predicted temperatures, with experimental uncertainties in the range $\pm0.48\,^{\circ}$C to $\pm0.56\,^{\circ}$C. Finally, this technique is applied to study the thermal transport characteristics of laminar and transitional flow within a heated rectangular copper microchannel.