36
FREEZE—DRYÏNG
4. Melting and evaporaon
In the same way as growth on a molecularly smooth basal plane of ice requires a denite
supercooling or supersaturation to cause surface nucleation, so evaporaüon or meltmg
'
requires a similar undersaturation or superheating. However, as opposed
to the case of
growth, evaporation of layers can begin easily from the edges of a crystal.
This e"ect can be observed for crystals growing on covellite surfaces when evaporaüon
occurs in layers, which recede from the crystal edge (gure 16 d). At suicrent under—
saturation and a suîciently high crystal temperature, evaporation takes place contmously
from the crystal edges, which become rounded. Ice can be made to -evaporate slowly by
covering the surface with a thin layer of formvar, With a few tiny holes when the ice evapo‘
;“
rates in the form of hexagonal pits (gure 19). A similar eifect occurs when evaporation
TABLE 2. GROWTH MECHANISM OF ICE CRYSTALS
‘
System
SOlution
——
Va.
Water
Gas —— Va our
Low
High
P
pour
Process
VÏSCOSÏÎY
VÏSCOSÎÎY
Growth
rough for all
rough
smooth
rough
0 to —- 2 °C
‘a’ axis
supercooling
—
12 to —— 14 °C
?
direction
at large supersaturation
,
Otherwise smooth
‘c’ axis
Smooth
for
?
smooth
rough —— 4 to -—-5 °C
?
direction
supercooling
critical
large supersaturation
less than
supercoong
Otherwise smooth
: 0.02 °C
'
.
Evaporation
Smooth for small undersaturation and
'
_
low temperature
'
'
'
"
-
‘
Rough f0r large undersaturation
Heat
.
DendteS
As pure water.
air
air…pressure '
or diffusion
grow at an
6
,
,,
1.0 _Atm. …:]:ÿ
Q.1
Atm.
:
,
limited.
angle 6 to ‘c’
,
for
small
,
needle
needle and
axis directi0n
superc00ng
dendritic
6—‘>45°'
f0r'more
‘
'
‘
‘
gr0Wth’is heat growth is
.
.
,
i
"
'
for'large
"cone. solutiOns
,
and di"usion'
‘
limited by
«
:
—
,
_,
_
supermolig
_,
.,
-
limited
heat alone
'
',
““
:*
Heatli‘näîted
»
Heat-*
…
=
,
above
‘
;
_
'
,
_
,
and diffusion:
;
——20,°Ç
_
—
OnœuœnultntbhhdtlgphgPttmthurfœar‘ectly
…
,
'
,
‘
-
-
v
* '
xt
'
“taking 131365 Sü'Ch
…
_i_,uïëloc‘ations:'sor ‘CryStal‘èdges
_
FREEZE—DRYÏNG
4. Melting and evaporaon
In the same way as growth on a molecularly smooth basal plane of ice requires a denite
supercooling or supersaturation to cause surface nucleation, so evaporaüon or meltmg
'
requires a similar undersaturation or superheating. However, as opposed
to the case of
growth, evaporation of layers can begin easily from the edges of a crystal.
This e"ect can be observed for crystals growing on covellite surfaces when evaporaüon
occurs in layers, which recede from the crystal edge (gure 16 d). At suicrent under—
saturation and a suîciently high crystal temperature, evaporation takes place contmously
from the crystal edges, which become rounded. Ice can be made to -evaporate slowly by
covering the surface with a thin layer of formvar, With a few tiny holes when the ice evapo‘
;“
rates in the form of hexagonal pits (gure 19). A similar eifect occurs when evaporation
TABLE 2. GROWTH MECHANISM OF ICE CRYSTALS
‘
System
SOlution
——
Va.
Water
Gas —— Va our
Low
High
P
pour
Process
VÏSCOSÏÎY
VÏSCOSÎÎY
Growth
rough for all
rough
smooth
rough
0 to —- 2 °C
‘a’ axis
supercooling
—
12 to —— 14 °C
?
direction
at large supersaturation
,
Otherwise smooth
‘c’ axis
Smooth
for
?
smooth
rough —— 4 to -—-5 °C
?
direction
supercooling
critical
large supersaturation
less than
supercoong
Otherwise smooth
: 0.02 °C
'
.
Evaporation
Smooth for small undersaturation and
'
_
low temperature
'
'
'
"
-
‘
Rough f0r large undersaturation
Heat
.
DendteS
As pure water.
air
air…pressure '
or diffusion
grow at an
6
,
,,
1.0 _Atm. …:]:ÿ
Q.1
Atm.
:
,
limited.
angle 6 to ‘c’
,
for
small
,
needle
needle and
axis directi0n
superc00ng
dendritic
6—‘>45°'
f0r'more
‘
'
‘
‘
gr0Wth’is heat growth is
.
.
,
i
"
'
for'large
"cone. solutiOns
,
and di"usion'
‘
limited by
«
:
—
,
_,
_
supermolig
_,
.,
-
limited
heat alone
'
',
““
:*
Heatli‘näîted
»
Heat-*
…
=
,
above
‘
;
_
'
,
_
,
and diffusion:
;
——20,°Ç
_
—
OnœuœnultntbhhdtlgphgPttmthurfœar‘ectly
…
,
'
,
‘
-
-
v
* '
xt
'
“taking 131365 Sü'Ch
…
_i_,uïëloc‘ations:'sor ‘CryStal‘èdges
_