_
Nucleati0n and growth of ice crystals _
_
J. HALLETT Ph. D.
DEPARTMENT OF PBYSICS, IMÏPERIAL COLLÈGE, LONDON, ENGLAND
INTRODUCTION
'
The procedure for freeze drying any biological specimen must be preceded by a period
of cooling.
The solutions which are contained within and between;the cell twalls will
rst supercool. Eventually, ice crystals will nucleate at specic sites, and will growthrough—
out the specimen, leading to the formation of a Varietÿ of euteCtic mixtures as the tem—
perature falls.
The ice crystals will subsequently evaporate thr0ugh the cellular matrix
until the specimen is dry. All these processes 'are very complicated. Before We can “hope
to understand the mechanism of freeze drying completely we mustpossess an adequate
knowledge of the fundamental physical processes which are taking place. rn this ai‘tiCle’ I
shall examine the present state
our knowledge of nuClèation and crystàllisaonof ice
and shall discuss experimental work which elucidates these processes in
any
surrounding matrix. It ShOuld be made clear that this matrixmust play an important rOle
in the nal structure of the frozen specimcän; we shall not howeverbe able to interpret
this structure unless we are able rst to understand the 'behaViom of the liqUid-ice phases
in isolation.
‘
"
‘
Ÿ
‘
;_
SUPERCOOLING AND
_
…
During recent years many studies have
of WatÇfëdëother
,
—.
..
substances to supercool below their cquibumgmèl‘tig 19—0ïÏîm8- -ç.Ïiî_l.g?1ïhe
.
’
«
'
,
experiments show that the maximum
,
"
:
on the origin of the
its volume
cation has been carried out freeZingcan beinterÿreedg as…bcingccaused by,the presence
-
‘
of small insoluble partiCulate impurities, ,diameterîafractionof a micron, which are
suspension in the liquid,”
water and
…l
may be readin observed
concentration may exceed 106
,
;
;
‘
perm] It
into a large number. dsmadropsandcountmgthenumberWchfreezeswhmSü°°€SSNB
-“
temperature 5increment‘sasthedfopsaæ°0°led Typlcallyawo m1beakemfŒtled
…“ Â
…