Laser velocimetry with fluorescent dye-doped polystyrene microspheres

Area of Science:

• Fluid dynamics
• Optical measurement techniques
• Particle imaging velocimetry

Background:

• Accurate flow velocimetry is crucial for understanding fluid behavior, especially near surfaces.
• Traditional methods can be limited by factors like laser flare in confined spaces.
• Doping microspheres with fluorescent dyes offers potential for enhanced flow measurement.

Purpose of the Study:

• To investigate the feasibility of using simultaneous Mie scattering and laser-induced fluorescence (LIF) for flow velocimetry.
• To evaluate the performance of Rhodamine B (RhB) and dichlorofluorescein (DCF) doped microspheres for flow measurement.
• To assess the validity of LIF signals for laser-Doppler velocimetry (LDV) compared to Mie scattering.

Main Methods:

• Simultaneous acquisition of Mie scattering and LIF signals from individual polystyrene latex microspheres (<1 μm diameter).
• Microspheres doped with Rhodamine B (RhB) and dichlorofluorescein (DCF).
• Varying laser excitation power (6.5 W and 1.0 W at 532 nm) to measure fluorescence and scattering signals.

Main Results:

• Fluorescence signals were 10-3 to 10-4 times weaker than Mie scattering signals.
• High validation rates for LIF-based LDV were achieved: 95.5% for RhB and 32.2% for DCF at 6.5 W.
• Even at 1.0 W, RhB maintained a 95.4% valid velocimetry signal rate via LIF.

Conclusions:

• Simultaneous Mie scattering and LIF is a viable technique for flow velocimetry.
• Fluorescently doped microspheres, particularly RhB, provide reliable data for LDV, even with low laser power.
• The method is suitable for wind tunnel measurements near walls where laser flare is a concern and monodisperse particles are required.