1. Production of secondary ice particles and splintering of freezing droplets as a potential mechanism of ice multiplication
Annika Lauber, Mona Schätzle, Patricia Handmann, Alexei Kiselev, Thomas Leisner

2. Three mechanisms potentially responsible for ice multiplication:
Mechanical breakup
Riming –Splintering
Drizzle droplets shattering at freezing
80 µm
300 µm
0.5 m·s-1 to 3 m·s-1
d > 25 µm
adapted from Lohmann, Lüönd, and Mahrt, An Introduction to Clouds, Cambridge University Press, 2016

3. Observation of the frozen drizzle droplets in a mixed phase cloud (AS-NS)HM
6000 m
5000 m
4000 m
3000 m
Korolev et al., JAM 2004, 3d Canadian Freezing Drizzle Experiment, over Lake Ontario.
Instruments: CPI, Nevzorov probe.

4. Experimental apparatus
Metal rod cooled
by liquid N2
High speed video camera
Up to fps
Tiny ice crystals trickle into the trap
Electrodynamic trap, connected to cryostat
Levitated supercooled water drop, pure water or PSL suspension, d > 300 µm

5. Freezing of a water drop
Freezing stage I: t ~ 2 ms
300 µm
258 K
Initiation of freezing on contact  ice dendrite grows through the droplet  droplet warms up to the melting point  ice growth stops
At the end of first freezing stage ~ 60% of the droplet is still liquid!

6. Freezing of a water drop
Freezing stage II: 1 ms ~ 1 s
Complete freezing, droplet deformation
Pressure can be released via gas bubbles escaping through noses and spicules, bursting
Pressure released via fracturing or fragmentation

7. The pressure inside a freezing droplet can reach 1000 bar
D = 300 µm
1 kbar
Pressure, kbar
Fraction of freezing time t / tf
Pressure release time compared to the time of complete droplet freezing tf
Pressure rise with freezing time for different viscoelastic factor (hard shell n=0)
King and Fletcher, J Phys D, 1973.

8. Previous results for 30 µm - 80 µm droplets (suspension of PSL particles, 10 to 1000 per drop) No shattering of pure water droplets
T. J. Pander, PhD thesis, Heidelberg Univ., 2015, P. Handmann, diploma thesis 2015, Karslruhe

9. Complete and incomplete shattering with particle ejection
Droplet size now > 300 µm
Complete and incomplete shattering with particle ejection
400 µm
400 µm

10. Bubble breakup at -11°C and -13°C2
300 µm

11. Jetting and particle ejection: drop shape does not change after ejection. Detection of secondary particles hardly possible!

12. Recoil-based detection of ejected particles: no need for visual search of events
Droplet charge: ~14.7 pC
Charge loss: 0.6 pC
Particle velocity: 1 to 7 m/s
Particle size: 5 – 30 µm
ejection
E_el = 1.4*1e-10
E_kin = 1.3*1e-9 Δq y x

13. Shattering and recoil frequencies for D = 300 µm
Pure water droplets
PSL suspension drops
Max freq.
D = 80µm
Max freq.
D = 80µm
Shattering of pure water droplets (larger droplets – more inclusions?)
2x higher fragmentation probability for PSL-containing drops
Average number of fragments per freezing droplet: Nf =
Shift towards higher temperature as compared to smaller drops (d = ~80 µm)

14. Big surprise… SSA drop Drizzle drop D = 300 µm Collision-coalescence
Cloud drop
d=10 µm
300 µm
SSA, d=1µm
0.1 volume% sea salt solution (Instant Ocean) droplet freezing at 259K

15. Inclusion of salt in ice increases its viscoelasticity?
1 kbar
Pressure, kbar
Pressure rise with freezing time for different viscoelastic factor (hard shell n=0)
King and Fletcher,
J Phys D, 1973.
Fraction of freezing time t / tf

16. Summary
New recoil-based method of splintering detection in 300 µm drops
Drop size matters, non-soluble inclusions enhance the mechanism
Higher probability of shattering for larger droplets, up to 40%
Frequency maximum shifted towards higher temperature
Sea salt solution drop are too elastic to shatter

17. Thank you for your attention!