Lanthanide-based laser-induced phosphorescence for spray diagnostics

Abstract

Laser-induced phosphorescence (LIP) is a relatively recent and versatile development for studying flow dynamics. This work investigates certain lanthanide-based molecular complexes for their use in LIP for high-speed sprays. Lanthanide complexes in solutions have been shown to possess long phosphorescence lifetimes (∼1-2 ms) and to emit light in the visible wavelength range. In particular, europium and terbium complexes are investigated using fluorescence/phosphorescence spectrometry, showing that europium-thenoyltrifluoracetone-trioctylphosphineoxide (Eu-TTA-TOPO) can be easily and efficiently excited using a standard frequency-tripled Nd:YAG laser. The emitted spectrum, with maximum intensity at a wavelength of 614 nm, is shown not to vary strongly with temperature (293-383 K). The decay constant of the phosphorescence, while independent of ambient pressure, decreases by approximately 12 μs/K between 323 and 373 K, with the base level of the decay constant dependent on the used solvent. The complex does not luminesce in the gas or solid state, meaning only the liquid phase is visualized, even in an evaporating spray. By using an internally excited spray containing the phosphorescent complex, the effect of vaporization is shown through the decrease in measured intensity over the length of the spray, together with droplet size measurements using interferometric particle imaging. This study shows that LIP, using the Eu-TTA-TOPO complex, can be used with different solvents, including diesel surrogates. Furthermore, it can be easily handled and used in sprays to investigate spray breakup and evaporation.

Effect of microbubble-induced cavitation on the dispersion of sprays

“…This technique uses ultraviolet (355-nm) laser pulses to excite ("tag") a small volume of fluid, which can subsequently be tracked by high-speed intensified cameras. Lanthanide-based phosphorescence was shown in previous work [19] to have a large decay constant (≈800 μs in water-based sprays). Furthermore, the excitation energy required is minimal, a fluence on the order of (μJ/cm 2 )/pulse.…”

“…In Sec. III, the contribution of cavitation to spray growth is investigated through dispersion measurements performed using laser-induced phosphorescence of lanthanide tracers [17][18][19] and linked to cavitation events through the measured pressure fluctuations.…”

Decreasing luminescence lifetime of evaporating phosphorescent droplets

“…5 The phosphorescent compounds can be characterized by their lifetime, and the concentration of excited molecules can at any time directly be related to the liquid content. However, in previous work it was found that the luminescence lifetime itself may become time-dependent, likely as a result of evaporation 6 (see Figure 1). Especially when the timescale of the evaporation becomes similar to the timescale of the flow, the role of non-radiative decay processes (quenching) due to molecular interactions become essential for quantitative interpretations.…”

“…All liquid in this spray was excited at the same instant, and the decrease in radially integrated luminescence over distance is more than changes in morphology can account for Ref. 6, and is likely due to evaporation. Published by AIP Publishing.…”

“…It is one of the lanthanide complexes useful for flow tracking and evaporation investigations. 5,6,16,17 The UV light induces phosphorescence, and the emitted light is measured with a Photron SA-X2 camera and Nikon AF micro 70-180 mm lens to obtain a framerate of 15 kHz and resolution of 14.7 lm/pixel, respectively (see Figure 3(a)). The recordings start 5 ls after the excitation, to ensure that all fluorescence has died out.…”