Laser-induced fluorescence temperature mapping in microfluidics: Calibration strategy and application to AC electrothermal microflows

This paper presents a non-invasive laser-induced fluorescence (LIF) approach for temperature mapping in microfluidic chambers and then applies it to an alternating-current electrothermal (ACET) flow considered as a representative electro-microfluidic test case. Temperature mapping remains difficult when the device height is only a few hundred micrometers and when intrusive probes would disturb either the hydrodynamic field or the applied electric field. Rhodamine B dissolved in physiological saline is therefore characterized independently in absorbance and fluorescence before being used as a temperature-sensitive tracer. At a concentration as small as 1 × 10 -4 mol/L, the Beer–Lambert regime is retained over the optical path of the microchannel, absorbance is weakly affected by temperature in the range of interest, and the fluorescence intensity decreases linearly with a sensitivity of approximately 1.78 %/K between 20 °C and 45 °C. The calibrated LIF measurements are complemented by micro-particle image velocimetry measurements performed in separate experimental runs in a saline-filled microchannel equipped with a pair of coplanar electrodes. In this demonstrator, the temperature gradient, not imposed externally, is generated by Joule heating associated with conduction currents, and it is coupled with the non-uniform AC electric field to produce ACET forcing. A three-dimensional electro-thermo-hydrodynamic model, based on quasi-static electrostatics, Navier–Stokes equations, and energy equation with Joule heating and temperature-dependent electrical properties included, is used to interpret and validate the measurements. Both simulations and experiments reveal a quadripolar four-clover velocity pattern above the electrode gap. Experimentally, the peak velocity reaches 350 µm/s for a 20 V peak-to-peak actuation at an AC frequency of 1 MHz, while fluorescence thermometry reveals moderate temperature rise, below about 10 K up to 25 V peak-to-peak. The present methodology thus establishes a calibrated thermal diagnostic and demonstrates its ability to assess heat transfer and flow organization in electrofluidic microsystems, thus confirming its ability to prevent denaturation of biological targets, for instance.