Title

Helical Whiskers of Ice

Author(s)Levi, Laura; Gavanovich, Sara

Citation

Physics of Snow and Ice : proceedings, 1(1), 43-50

Issue Date1967

Doc URL

http://hdl.handle.net/2115/20284

Typebulletin (article)

Note

International Conference on Low Temperature Science. I. Conference on Physics of Snow and Ice, II. Conference on
Cryobiology. (August, 14-19, 1966, Sapporo, Japan)

File Information1̲p43-50.pdf

Instructions for use

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAPHelical Whiskers of Ice
Laura LEVI* and Sara GAVANOVICH**Departamento de Meteorologia y Departamento de Fisica, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina

Abstract,
We have observed the formation of ice whiskers in cold solutions of formvar in ethylene
dichloride, usually used as an etchant of ice crystals. Whiskers are supposed to form through aprocess of freezing of small amounts of water contained in the etchant. They are 1-3 p. in diameterand a few microns up to about 1 mm in length.
The formation of helical and spiral whiskers has been specially noted. Helices usually showquite regular pitches. Changes in direction and kinks are evident in both helix and straight whiskers.
THe kink angles generally correspond to the hexagonal symmetry. These features indicate thatwhiskers grow in most cases in the basal plane. The mechanism of growth is related to that ofsimilar whiskers of other substances, the operating dislocation possibly being in the present casein. <1120> directions.
II). a special case also straight whiskers were formed parallel to the c-axis of the ice substrate.
Their formation is related to probably screw <0001> dislocations.

I. Introduction
The crystal habit of ice grown from the vapour and from the melt has been studied
a wide range of different conditions of temperature and supercooling and tentativeinterpretations have been proposed of the features observed. On the other hand, whiskers
may be considered as a special case of dendritical formation, so that the study of theirmorphology and orientation may give useful information about the general process ofgrowth of these crystals.
It is known that ice needles possibly formed from the melt are sometimes observedIn nature.
Ice whiskers have also been grown from the vapour in the laboratory, using
different substrates, such as frozen droplets (Zawidski and Papee, 1962) and freezing nuclei(Serpolay and Toye, 1962); these authors have observed the formation of some helix andspiral whiskers which they relate to defects of the substrate (particles of iron ferrosoferric).
In the present work we investigate the features of ice whiskers grown in cold solutionsof formvar in ethylene dichloride, containing small amounts of water. 'The results are
discussed taking into account the possible Burgers vector and dislocations of the lattice.
InII. Results and Discussion
Whisker formation
It is known that solutions of formvar in ethylene dichloride are a good etchant of* Present address:

Carrera del Investigador, Consejo Nacional de Investigationes, Rivadavia
1917, Buenos Aires, Argentina.
** Present address: Departamento de Fisica, Universidad de Chile, Santiago, Chile.

44L. LEVI AND S. GAV ANOVICH

ice. However, their etching ability is reduced when they contain small amounts ofwater, which at t<oce precipitates forming small crystals or dendrites (Kobayashi, 1955;
Bryant and Mason, 1960).
During recent studies of dislocation behaviour in ice we observed in some plastic
films showing a poor attack, the formation of spirals, helices and more complex lines.
We interpret these as replicas of ice whiskers formed during the process of etching andsolvent evaporation. To confirm this hypothesis, either ice samples or microscope slides
were coated with a thin layer of formvar solution and exposed for a few seconds tosupersaturated water vapour. Immediately after, they were studied under a microscope
placed in a cold chamber (at -15°C). In these conditions we observed whiskers floatingin the liquid solution shaped as needles, spirals and kinked lines. Only exceptionallythese whiskers were fixed to the substrate. In some cases they appeared to grow awayfrom frozen droplets; more frequently, their starting and ending points were similar
and therefore their growth direction could not be determined. Sometimes if agitationof the formvar solution occurred, the curvature of the whiskers could be observed tochange slightly.
Most of the whiskers considered here were obtained by coating ice with formvar
solution. We consider that the substrate only plays a secondary role in the process,which probably consists of a certain controlling of the water content of the solution.

lvlo rph 0 logy
The observations were made on photographs of replicas. We give in Figs. 1 and 2
some examples of helical whiskers. In Fig. 1 a the axis is straight and in Fig. 1 b slightly
curved. In Fig. 2 (a and b) each whisker is formed by two similar branches subtending
angles corresponding to the hexagonal symmetry (90 0 and 150°, respectively). Figure 2 cFig. L

Helical whiskers with straight and curved axis

(a, X 500; b, X 200)

45

HELICAL WHISKERS OF ICE

Fig. 2.

Helical whiskers formed by branches at different angles(a, X 1000; b, X 500; c, X 750)

Fig. 3.

Kinked whiskers(x500)

shows a whisker formed by two branches of different morphology.

In some cases thehelices seem to be reduced to nearly plane sinusoidal lines ; an example is given in Fig. 2 b.
Kinked whiskers have been observed as well, as it is shown in Fig. 3. The angles
correspond to the hexagonal symmetry (30°, 60 and 90°). Examples in Fig. 3 also indicatethat an approximate periodicity exists in the formation of kinks.
0Spiral whiskers are shown in Fig. 4. The spiral of Fig. 4 a has its center in adendrite. The grain boundaries of the substrate and a low chemical etching may beobserved under the whisker. In Fig. 4 b two similar spirals start at the same origin,probably a frozen droplet. The whisker in Fig. 4 c may be described as a non perfectly
plane spiral formed by several turns.
It has been reported previously that helical whiskers of different substances (Edwardsand Svager, 1964), sometimes show sudden changes of their morphology. Such changes
have also been observed in ice whiskers and are shown in Fig. 5.

The whisker of Fig.

L. LEVI 1\1'<"'D S. GAVANOVICH

46

Fig. 4.

Fig. 5.

Spiral whiskers(X 500)

Whiskers showing changes of morphology

5 a has a nearly straight axis, but the helix changes its diameter and pitch. In Fig. 5 b
two whiskers are shown, one of them is a spiral with an helix turn in its center, while
the other is a tapered helix which ends in a nearly straight branch.
Whiskers in Fig. 6 were obtained without ice substrate. Straight branches forming
angles of 120 may be observed in Fig. 6 b; the whisker marked A (Fig. 6 a) is formed
by straight segments, connected by helix turns.
047

HELICAL WI-IlSKERS OF ICE

Fig. 6.

Whiskers obtained without an ice substrate

Eg. 7.

Helical ribbon(x500)(x 200)In Fig. 7 a helical ribbon is shown; a central line is noticed, which might correspondto the original whisker. Ribbons of this type were also observed by De Micheli andLicenblat in experiments of evaporation of ice (unpublished).
Finally, Fig. 8 (a and b) represents one of the rare examples where whiskers havegrown from the edges of the ice substrate. They are parallel to the striations producedby chemical etching of the substrate, i. e., to the c-axis of the crystals. The sample wasa polycrystal and it is seen that both striations and whiskers change their direction indifferent grains.
In all the cases studied here, whiskers have diameters of about 1-3/J.. Their lengthchanges from a few microns to about 1 mm. We give in Table 1 the average values of

48

L. LEVI AND S. GAVANOVICH

Fig. 8.

(0001) whiskers(X 500)

Table 1.
I(,u)

2R(,11)P (,u)

Fig.

----------------------

1 X 102

810

1a7/I

10

17

1b0.35/I

l.5

2.5

2a__ 1.6/I

2.7

2.9

2cl.4/I

38.82bthe external diameter 2R and pitch P and of the total length l of some helical whiskers.
We note that helices are generally open. In most cases the angle between the helix
axis and the tangent to the helix turns

49

HELICAL WHISKERS OF ICE

21CR(J=tan- 1 p- ,fJ

(1)

was
(2)

Only exceptionally, t3<45° has been observed.

Discussion
In the present work we have not determined crystallographically the orientation andstructure of the observed whiskers. However, we have shown that usually either kinks
or different branches of the same whisker form angles of 30°, 60° and 90° (Figs. 2 and 3).
Whiskers showing these features must lie in (or very near to) the basal plane probably
running in low index directions of both types, <1120> and <1010>. Then, whiskers show
the same preferential growth in the basal plane, as known for ice crystals in a wide
range of different conditions. An example was given, however, in which whiskers areformed in a non basal plane (Fig. 8).
On the other hand, the similarity between whiskers of ice and of other substances,suggests that a similar mechanism may be responsible for their formation. Helicalwhiskers of Pd have been particularly studied by Webb (1965). According to this author
their growth is determined by the operation of an axial dislocation with a screw component, the helix shape being caused by climb of the edge component at the tip of thewhiskers. Whiskers of Pd are found to be in <111> directions forming an angle r=35.27°
with the <110> Burgers vectors of this f.c.c. structure. Webb points out that helix
whiskers may be formed as an equilibrium configuration if the angle between their axis
and the tangent to the dislocatio~ line fulfils the condition (3)r.
We have seen that helical whiskers of ice usually have their axis in <1120> and <1010>
directions. The Burgers vector for basal dislocations of these crystals have been shown
by Hayes and Webb (1965) to be of the type! <1120>. Then, the following values forthe angle r are geometrically possible:<1120> whiskers
r=O° or 60° ,<1010> whiskers
r=30° or 90° .

Of those, we must rule out r=90°, because it would lead to a pure edge dislocation,
the other cases being possible. For r = 60° the screw component is relatively low, but
we 'note that according to eq. (2), for the three cases the condition (3)r is fulfilled.
In particular, the whisker of Fig. 2 a is an example where r=60° is expected .. Actually, if a screw component of the same dislocation is supposed to operate in bothbranches at 90° to each other, it must be r=30° for one of them, in a <1010> direction
and consequently r=60° for the other, in a <1210> direction.
On the other hand it does not seem very probable that two different dislocations,one of them perfectly screw, would operate in the two branches with the same external
morphology.
Finally, it is interesting to note that the growth of <0001> whiskers of Fig. 8 havebeen evidently caused by the conditions of the substrate. Their concentration, as determined from the photographs, is about 103 em-I, i.e. it coincides with the linear concentration of dislocations, which may be expected to emerge in the basal plane (Achaval

50L. LEVI AND S. GAVANOVICHet al., 1964). Then it may be supposed that these whiskers have grown by operation ofscrew dislocations, with (0001) Burgers vectors (Levi et al., 1965).
Laura Levi acknowledges the support received from the Consejo Nacional de In-'. vestigaciones Cientificas y Tecnicas (Argentina).

References
1) ACHAVAL, E. M., LEVI, L. and SURASKI, E. 1964 Chemical etching in ice crystals. Physica
Status Solidi, 12, 61-63.
2) BRYANT, G. W. and MASON, B. J. 1960 Etch pits and dislocations in ice crystals. Phil.
Mag., 5, 1221-1227.
3) EDWARDS, P. L. and SVAGER, A. 1964 Spiral copper whiskers. J. Appl. Phys., 35, 421-423.
4) KOBAYASHI, T. 1955 Studies of small ice crystals, III. Some remarks on replica methods.
Contr. Inst. Low Temp. Sci., 8, 75-86.
5) LEVI, L., ACHAVAL, E. M. and SURASKI, E. 1965 Experimental study of non basal dislocations in ice crystals. J. Glacial., 5, 691-699.
6) SERPOLAY, R. and TOYE, M. J. 1963 Un nouveau processus pour la croissance de la glace
sur les noyaux glac;ogenes. J. Recherches Atmospheriques, 1,85-91.
7) WEBB, W. W. 1965 Dislocation mechanism in the growth of palladium whisker crystals.
J. Appl. Phys., 36, 214-221.
8) , HAYES, C; E. and WEBB, W. W. 1965 Dislocation in Ice. Science, 147, 44-45.
9) ZAWIDSKI, T. and PAPEE, H. 1962 Pseudo whiskers of ice grown from douds of supercooled
water in an electric field. NatU1-e, 196, 568.