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Tubular ice crystals

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Publisher’s version / Version de l'éditeur:

Nature, 215, 509, pp. 271-273, 1967-10-01

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Tubular ice crystals

Krausz, A. S.; Harron, B.; Litvan, G. G.

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(Reprinted froin Nature, Vol. 215, No. 5098, p p . 271-273, July 15, 1967)

Tubular

Ice

Crystals

IT has bcen found t h a t on very rare occasions long, thin ice needles grow ~ n t o the air from the solid surface of water frozen in an open container1-a. I n the course of supercool- ing experiments, Dorsey5 observed a similar phenomeno~~ in the laboratory. H e assumed t h a t growth occurred when, because of the increase in volume during solidifica- tion, water was forced through an opening in the Ice covering the surface. Dorsey also suggested that a tuba formed, through which water flowed and which "grew" a t its tip. Recently, Hayward6 reported a method for growing such spikes and obtained experlmcntal cvidence In support of Dorsey's mechanism.

This communication describes a simple method of growlng hollow tubes or needles of controlled size, s u ~ t a b l c for physical testing; some of the crystallographic charac- teristics are also reported. We present ev~dence in support of Dorsey's mechanism, elaborate on it, and suggest a necessary extension. Finally, we indicatc how these results can be used for growing tubular hi- or tri-crystals other than ice.

The apparatus used in these exper~ments consisted of a cell, a cold trap and a rotary vacuum pump. The cell was cylindrical in shape, with a cold finger protruding from the bottom which contained about 20 cm3 water (Fig. 1). The trap was placed in a rnixturc of dry ice and acetone or in liquid air and the system was evacuated to a pressure of 10-I mm of mercury. Soon after the ccll was immersed in the cold bath (-15" t o -35' C) dendrite branches started to grow on the water surface. Before it was completely covered with a crust of ice thc water surface bccsme hemispherical in shape. Solidifica- tion then continued around the edge of the hemisphere and a short section of the tube was thus formed. The continuous flow of water maintained the droplet a t the tip of the tube resulting in steady growth.

Some of the tubes mere longer than 100 mm and were slightly tapercd toward the tip (Fig. 2a). The average diameter varied between 1 and 3 nun with a wall thickness of about 0.2 mm. I t was frequently found that t h e dia- meter dccreased abruptly once or twice during growth. Both the diameter and the length of the secondary tubes were about one order of magnitude smaller than tho same dimensions of the primary tubes (Fig. 2b).

Crystallographic orientation was determined in polar- ized light and with t h e etch pit tcchnique developed by Hlguchi7. The specimens were found to be single, bi-, or tri-crystals of random orientation. I n all cases investi- gated the grain boundaries in the hi- and tri-crystals were parallel t o the main axis over the entire length of the

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- - -

Fig. I. Diaglalll of tllc cell showing growth of tubular ~ L C clystal.

specimen. Microscopc obsorvations sho~vccl t h a t thc wall o f t h c tube was clear, but the frozen core occasionally colltalrled small bubbles of air or vapour.

IVith this techiiiquc it n7as p'oss~blc t o observc tho process of growth in great dotal. Thc formnt~on of tllc ~ c c col-er, the flow of xvater through t h c tube, and grolvth a t the tip werc obscrved visually. The oxistencc of a high \vator pressurc due to freczing was demonstrated by reversing the process which supplicd t h c water flow. Whcn, during t u l ~ c formation, t h c cell mas lifted from the bath and the ice crust inside the colcl fingcr wall meltcd, tho conse- cluent v o l ~ t n ~ c contractioll drained t h e water from the tube of ice. Incidentally, this technique was used t o

produce hollo3v tubes. These obsorvations support

Dorscy's theory and provide f ~ w t h e r insight into the mechanism. I n general, t h e rate of extraction of heat from the rcscrvoir (controllccl in the present experiments by thc tcmporature and geometry of the bath) detcrmincs tho rato of flow of water through the tube. The water

transported t o t h e t i p of t h e tubo has, howcver, t o bc frozen with a n appropriate speed which is governed by the ovaporatioll rate (pumping speed and trap tempera- ture). The vapour pressurc of the water droplct is greater than t h a t of t h c ice bccausc of its higher tempcratare and

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b

Fig. 2. a , Typical icc nccdle. 6, Typical primary and secondary needle firmation vicwcd in polarized light.

convex surfacc. Coolilig by ovaporat~on is thus concen- trated on the water droplet and this method of heat extraction is preferable.

In steady state the amount of water transported from the rcscrvoir in unit time is

whcrc ri and ro are the inside and outside radii of the tube, respectively; 1 is the tube length grown in unit time; s is the density of ice; and a is the amount of water evaporated from unit area in unit time. The condition of steady growth, not co~lsidcring radiation and conduction, is that

where

L,

and

Lf

are the specific heats of evaporation and freezing, respectively. From equations 1 and 2 the follow-

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ing expression is obtai~lcd

I'

- = rt21

+

2r02a

X ( 3 )

The relatively wide range of conditions under which growth can talrc place shoxvs that the hemispherical drop a t the tip of the tnbe can accommodate significant changes in V and in

a.

Thus whcll V becomes greatcr than t h a t corresponding to t h e cstablished rquilibrium, the surplus amount of water increases the droplet size and groxvth contillues with increasing r , until the steadv eromth condi- "

-

tion described by equat?ons 1 and 2 is again satisfied.

The predictions of equations 1 and 2 xv&-c substantiatccl by cxpcriments. but it is realized that in practice tho process is madc feasible only by the high surface tension of water and the apparently loxv polarity of thc icc surfacca which prevents overflow. I t follov-s from our observations t h a t a mechanism composed of water flow and hcat flow alone is not sufficient to explaill tllc proccss and t h a t the introduction of a third component, the surface tension, is ncccssary. The realization of tho essential fcatnrcs of the mechanism leads to the conclus~on t h a t tubes from substa~lces other than water could be grown if a n experi- mental technique could be devised t o create a suitable mass flow and heat extraction and a balance between them. Indecd, tubcs of beilzenc were grown 11-ith a mcchanically controlled liquid flow a t atmospheric prcssure and - 10" C. I n this instance, mass flow was created by a~tificially forcing benzene through the orificc of a syringe. The surfacc tension of benzenc was high cnough to maintain a droplct a t the tip when growth was directed do~vnmard. The successful growth of benzenc tubcs is considered good evidence in support of the suggested mechanism. This mechailism thus explains all the previously reported tube formations under various conditions (open airl-3, undercooledG) and all t h e prcsent observations. The recognition of the essential mechanism, furthermore, malres it possible t o use this technique to grow tubes of a wide varicty of materials.

A. S. ICRAUSZ

B. HARRON

G. G. LITVAN Division of Building Rcsearch,

National Research Conncil, Ottawa.

Received Apr~l 2 4 ; revised June 5 , 1967.

'

Dorsey, H. G., P h ~ r . R c v . , l 7 , 1G2 (1921).

"ally, O . , Hrlv. Chiua. Acla,18, 475 (1935). Hallett, J . , J. Rlaciol., 3 , G98 (19GO).

Rleyer, J., and Pfaff, JV., Zczl. Anorg. 8110. C l ~ e n ~ . , 2 2 4 , 305 (1935). Dorsey, N. E., T r a ~ l s . Amer. P h i l . Soc., 38, 248 (1948).

a H~tlyward, A. T . J., Nalure,211,172 (1966).

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Hlguchi, I<., Acla illel., 6 , G36 (1958).

Adamson, A. V., and Dormant, L. M., J. Amcr. C ~ C I I Z . Soc., 8 8 , 2055 (1966).

Figure

Fig.  I.  Diaglalll  of  tllc  cell showing growth  of tubular  ~ L C   clystal.
Fig.  2.  a ,   Typical icc nccdle.  6,  Typical primary  and secondary  needle  firmation vicwcd in polarized light

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