EMPI - Reactive Fluids

Laser-induced incandescence (LII) is a powerful diagnostic technique used to characterize nanoparticles in combustion environments and particle-laden gases. It provides both time-resolved and imaging capabilities, allowing for detailed analysis of particle size, concentration, and spatial distribution. In our group, we utilize and develop advanced LII methods and maintain the LIISim software for data analysis, which can be accessed at http://liisim.com/.

LII works by heating soot particles to incandescence using a high-energy laser pulse. The particles absorb the laser energy, rapidly heating to high temperatures (around 4000 K in case of soot), and emit thermal radiation as they cool down. This broadband thermal emission is detected and analyzed to determine the properties of the particles. By measuring the emission at multiple wavelengths, we can accurately determine the temperature decay of the particles. The main components of an LII system include a pulsed Nd:YAG laser, an optical system to direct and focus the laser beam, and a detector to capture the incandescence signal. The detected signals are processed to extract information about particle size, concentration, and other properties.

In time-resolved LII, the incandescence signal is measured as a function of time after the laser pulse. This approach provides information about the cooling rate of the particles, which is related to their size. Larger particles cool more slowly than smaller ones, allowing for size determination based on the temporal decay of the signal. Time-resolved LII also enables precise temperature profiles of the particles during their cooling phase, capturing rapid changes in particle properties in dynamic environments. While time-resolved signals are often detected via an array of 2–4 fast photomultipliers equipped with bandpass filters, fundamental studies also exploit the capabilities of a streak-camera system that provides full emission spectra with nanosecond time resolution.

Imaging LII involves capturing spatially resolved images of the incandescence signal, providing detailed information about the spatial distribution and morphology of soot particles. High-speed cameras or intensified CCD cameras (ICCDs) are used to capture the emitted light from the particles across a plane or volume. This allows for visualization of soot particle distribution, examination of particle aggregation, and real-time monitoring of changes in particle formation and distribution.

A key part of our research involves the development and maintenance of the LIISim software, a tool for simulating LII signals and interpreting experimental data. LIISim models the laser heating and cooling of soot particles, providing insights into the influence of various parameters on the LII signal. It can simulate the emission at different wavelengths, enabling accurate determination of particle temperature decay profiles. Features of LIISim include modeling of time-resolved and spatially-resolved LII signals, optimization of experimental parameters, and comprehensive documentation and tutorials. More information about LIISim can be found at http://liisim.com/.

LII is widely used in combustion research, environmental monitoring, and aerosol science. In combustion research, it helps to understand soot formation and oxidation processes in flames and engines. In environmental monitoring, it measures particulate emissions from industrial sources and vehicles. In aerosol science, LII characterizes atmospheric aerosols and their impact on air quality and climate. In the context of our activities to synthesize nanoparticles from gas-phase processes, we systematically expand LII to non-soot materials, such as silicon, iron, and metal oxides, where also the transition from LII to laser-induced breakdown spectroscopy (LIBS) can provide additional information about the particle composition.

In summary, laser-induced incandescence (LII) is a versatile and powerful technique for characterizing soot particles. By measuring the emission at different wavelengths, LII can determine the temperature decay of particles, providing comprehensive insights into particle size, concentration, and distribution. Our development and maintenance of the LIISim software further enhance the utility and accuracy of LII measurements, supporting advanced research and industrial applications.