1. Snow Characterization Workshop 2008
3D (Lecture 2)
Martin Schneebeli

2. 3D microstructural analysis
two methods for 3D microstructural analysis:
polycut = serial sectioning
microCT = micro computer tomography
polycut
microCT

3. 2D-section: 3D image: serial

4. 2D-section: 3D image: tomography

5. chloro-naphthalene (three-phase)

6. 3D image: tomography-BET

7. Principle of the new method
working point
-20°C
5×10-2 mbar
Temperature -20°C und Vakuumdruck einzeichnen

8. Principle of the new method
snow
phthalate
new method
raw CT
images
"digital replica"
snow
image processing
segmented images
image inversion

9. Sample preparation: casting
casting technique is identical as for polycut
replace the air in the pores by diethyl phthalate
diethyl phthalate: melting point -3°C, but solidification temperature is much lower (c. -30°C)
freeze with dry ice

10. Sample preparation: trimming
at -20°C to prevent melting of the phthalate
cut and trim samples to cylinders of 18 mm diameter

11. Experimental setup: vacuum
store samples in vacuum desiccator
high vacuum pump: final pressure ~5×10-2 mbar
optional: put drying agent in desiccator

12. Experimental setup: microCT
when sublimation terminated: microCT measurement
same measurement protocol as for snow

13. raw CT-images: Gaussian filtered (3D)
Image processing
raw CT-images: Gaussian filtered (3D)
image segmentation: local minimum of the histogram as threshold
threshold
threshold
phthahlate
air
ice
air

14. Sublimation process

15. Weight loss vacuum pressure is a good indicator of the progress
drying agent significantly accelerates the sublimation process
normalized weight
vacuum pressure

16. measurement of the phthalate after sublimation  digital replica
Validation: samples
special sample holder for validation measurements
CT-measurement of snow before casting
casting and vacuum sublimation in the CT sample holder
measurement of the phthalate after sublimation  digital replica
comparison between digital replica and orginal snow structure

17. Validation: visual comparison
original snow
digital replica
difference image
zusätzliche Vergleiche

18. Validation: structural parameters
trabecular thickness
trabecular spacing
zusätzliche Vergleiche
structural parameters: density, SSA, trabecular thickness, trabecular spacing, etc.
 relative errors are very small (a few percent)

19. anaglyph images of rounded snow
zusätzliche Vergleiche

20. anaglyph images of new snow
zusätzliche Vergleiche