Simultaneous Three-Dimensional Velocity And Temperature Measurement In Microfluidics Using An LED And A Double-frame Camera

With astigmatism particle tracking velocimetry combined with luminescence lifetime imaging, the temperature and all velocity components of spherical trace particles in a volume of fluid can be simultaneously measured with one optical access. Generally, a pulsed laser is required for this measurement technique. However, it may cause additional heating of a small volume of stagnant fluid, as often present in microfluidics. Here we show that this problem can be avoided by replacing the pulsed laser with an LED. To compensate for the lower power provided by the LED, the illumination time and the exposure time for the image acquisition were vastly extended. In addition, we replaced the typically used high-speed camera with an ordinary double-frame camera. With these modifications, the temperature of the fluid is unaffected, while low uncertainties on all the measured quantities can be achieved. Temperature can be measured with an uncertainty of ∼1°C with a varying location. With a varying temperature covering the range of 20-50°C, measurement uncertainties were found to be lower than 0.2 μm for x- and y-coordinates and lower than 1.1 μm for the z-coordinates for the current setup. The system was applied to measure the location and temperature of particles trapped in an acoustic tweezer setup. No heating on the fluid can be observed using the new technique. Therefore, the heating of the particles could be solely related to the acoustic field. The measurement technique may serve as a powerful tool to investigate the thermodynamic and fluid dynamic phenomena in microfluidic environments. The lower cost on the hardware and the comparable or even lower measurement uncertainties on the measured quantities are further advantages.