Dual-tracer laser-induced fluorescence thermometry in microchannels

Laser-induced fluorescence (LIF) thermometry measures liquid temperatures based on changes in fluorescence intensity. In dual-tracer or ratiometric LIF thermometry, liquid temperatures are based on the ratio of the fluorescent signals from two different fluorophores, since this ratio is independent of changes in fluorescence intensity due to variations in the excitation. Recently, a dual-tracer LIF technique using two species with opposite temperature sensitivities, fluorescein 27 and Kiton Red (sulforhodamine B), excited at 532~nm has been reported with temperature sensitivities as great as 7\% per $^\circ$C [Sutton {\it et al.} (2008) {\it Exp. Fluids} DOI 10.1007/s00348-008-0506-4]. We describe here a similar technique using fluorescein and sulforhodamine B, which have intensities that increase by 2.2\% and decrease by 1.3\%, respectively, per $^\circ$C when volumetrically illuminated at 514 nm. The ratio of these two signals gives temperature sensitivities as great as 9\% per $^\circ$C. This LIF technique is used to measure temperature distributions in water flowing through a 500 $\mu$m $\times$ 1000 $\mu$m polydimethylsiloxane (PDMS) microchannel covered with a glass lid with localized heating to create temperature gradients up to about 15 $^\circ$C /mm. The results are compared with FLUENT predictions.