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The study of soot nanoparticulates using laser induced incandescence in a vacuum.
Vivien Beyer & Doug Greenhalgh
Soot and black carbon present serious health hazards to the respiratory system, in addition they are contributing significantly to climate forcing or the greenhouse effect.
New ways to accurately measure these nanoparticulates are required.
Laser-Induced Incandescence (LII) under high vacuum conditions
o A high vacuum eliminates any conductive heat transfer.
o Therefore it is in an ideal situation to study radiative temperature decays also LII duration is > 50 µs and this considerably enhances the signal.
o Thus should be able to estimate E(m).
LII under a High VacuumLII under a High Vacuum
Calculated temperature decay of a 20 nm carbon primary particle.
Top at 1 atms.
Lower for <10-3 millibar
EquipmentEquipment
Experiment setup
A rolling bottle
Experimental ConditionsExperimental Conditionso Sample: carbon black at 10-4 mbar (Monarch® 900).
o “Near” top-hat laser spatial energy profile.
o Pyrometric time-resolved approach coupled to 2D imaging.
o Limited spectral measurements.
o Intensified CCD Camera capable of PIV (just in case…).
o Absolute light calibration (tungsten strip filament lamp).
Typical High Vac. LII signalsTypical High Vac. LII signals
Long exposure LII picturesLong exposure LII pictures
o The aggregates explode….
0.24 J/cm2, ICCD gate 0-50 us
Time Sequenced LII imagesTime Sequenced LII images0.18 J/cm2 0.24 J/cm2
0-5 μs
Propagation of the Laser beam
5-10 μs
20-25 μs
25-30 μs
Photophoretic effect ?Photophoretic effect ?
o At high fluence (above 0.2 J/cm2), intense sublimation occurs during the first 2 μs after the laser pulse.
o A cloud of ejected and excited matter C2 and or C3 leaves the main particulate and can be observed on the ICCD image.
o The light emission image is strongly asymmetric (C-shape) and suggests that a the gaseous excited cloud is “pushed” by the laser beam.
particulate
cloud
cross-talk
Propagation of the Laser beam
angles
Propagation of the Laser beam
Explosions velocities from dual sequenced Explosions velocities from dual sequenced images images -- Particulate Image Velocimetry (PIV)Particulate Image Velocimetry (PIV)
Dynamic range of the experimentDynamic range of the experiment
o After 30 μs, aggregates are spatially separated and measurable at high fluence.
o So we can size them (results obtained at 2550K).
Light shielding since agglomerates are large Light shielding since agglomerates are large is a likely issue!is a likely issue!
Preliminary thoughts for the future• Work remains on improving E(m) and there is a need to study
this parameter for a matrix of soot sources, ages etc.
• Likewise further work on the morphology issues is required.
• The initial study in a vacuum raise lots of issues:– Does the Rayleigh approximation hold for aggregates and agglomerates?
–What is sublimed, is it C2 or C3?
–How important are large aggregates and agglomerates in real life, especially as most current data is based on mobility methods such as SMPS which are insensitive to variations in the size of large particles beyond 200nm?
–There is a need to develop a instrument similar to an aerosol particle beam system to realise the full capability of this novel, new approach.
