[CE CRYSTALS
23
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20
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distilled
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2 —40
.
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lp_
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0.1 cm
lcm
10cm
Drop radios
FIGURE 2. Variation of mean freezing temperature of drops of different radins
_
a.
distüled water
b. almost particle free water
‘
’
THE PHYSICAL BASIS OF NUCLEATION
In the same way as the equilibrium vapourÏ pressure of a liquid drop becomes larger for
»
smaller drops, so the equilibrium mdtingtemperaturebf a crystal (Tm) “becomes lower _for
smaller crystals. This may be expressed
in terms of T
hcmpson’s equation, if
be made that the ice crystal is spherical :
‘i
r
*
'
'
'
.
r
….
z «SL/f
T° LL dT ,;
'
f
*
*
‘
…
° “
Ps
T…,
_
T > ”
pszL1T‘?(—To —Tm)
'
_
where O'SL : mean surface energÿ=betWee ice
in practice,
is ,a'funct—ion
_
of crystallographic orientation.)
—
> ]
-
"
.
’
Ps : density
,
…
__
_
,
.
»
‘
To = equilibriüm'mng
‘îœ ‘$“F‘faœ-‘
1 li]
…
.
’
.
'
‘
LT = latent heat offuSi0‘‘attemperatureT
,
j _ Ï
.
—
,
;
This giVCS
: 10Âat—40°C
__
__
,
,
.
f.
,
,
_
=
À&t -—-4°C
1‘.
‘
ÏÏ’ .Â,ÏÏ
…
.