ICE CRYSTALS —
31
2. Growth from solution
'
Crystals growing in some solutions of low concentration show a slightly increased Velocity
of crystallisation compared With pure water 13. This is apparently caused by increasing
the growth rate in the “0” axis direction (which is the slow direction in pure water) so
that the resultant velocity is larger. F0r larger concentrations, velocities fall signicantly
compared with pure water, and are, typically, le’ss by a factor of 10 for M solution, at the
same supercooling, although the velocity Still increases approximately as the square of the
_
supercooling. For those solutions whoseviscosity is not greatly enhanced over pure water
and for which
the diffusion coefcient is not too small (for example alkali halides) crystal—
lisation still takes place in the form of dendrites which grow at an angle to the nücleating
crystal, the spacing between the branches deCreasing with increased supercooling (gure 10).
For growth in more viscous solutions, however, such as sucrose, the crystals show distinct
'
crystal facets, and grow as solid hexagonal prisms.
At relatively slow freezing rates : 10 p. sec—1 and in the absence of stirring, more ions are
incorporated into the ice than at larger freezing rates. Ions which simulate a water molecule
(F“) or a group of molecules
are more easily tted into the ice lattice (with segregation coeicient as large as 5) than ions of far di‘erent structure (Nat I“) 14. We know
very little about the inuence of growth rate at the high velocities which occur at large
supercooling, which would be determined by the local conditions of sùpercooling ahead
of the dendrite growth front, and any modication of crystal habit brought about by
preferential adsorption of solute molecules on some crystallographic faces—an elÏect
.
which may be of considerable importance with strongly polar substances.
All these
processes could lead to a solid With very irregular distribution of solute molecules. In
the case when the growth is dendritic, liquid trapped between dendrite arms will have a
concentration much greater than the original.
This may SuperS'aturate and eventually
nucleate crystals of solute and form a eutectic shOuld the “temperature fall sufciently
'
low after the initial stage of freezing has taken place. If the liquid be both supercooled and
; f '
supersaturated ice and salt crystals may grow side by side, to give crystal lamellae which
lie parallel to the growth direction 15.
_
,
A further complication arises if the crySte‘tllisatio‘n rate is
rapid that nucleation of
;”ff
solute takes place ahead of the dendrites. This occurs for air in solution in water super—
cooled by more than 8 °C. A large number of air bubbles of radius less than 1 micron
during the initial stage of
the water—ice“ mixture appears cloudy
'
(gure 11).
…
-
‘
if
ïî
.
3. Growth from the vapour
‘The growth of ice crystals in air is most
Of a water vapour
dl‘usron chamber. Crystals grow on a bre
tWO ice sheets
at different temperatures (gure 12)
surface
and a
lower ice
sheet An approximate
exists between
the two Planes.
7
'
,
.
Smce the saturation
the region
:
.
_
between the plates is
on the bre;
«
31
2. Growth from solution
'
Crystals growing in some solutions of low concentration show a slightly increased Velocity
of crystallisation compared With pure water 13. This is apparently caused by increasing
the growth rate in the “0” axis direction (which is the slow direction in pure water) so
that the resultant velocity is larger. F0r larger concentrations, velocities fall signicantly
compared with pure water, and are, typically, le’ss by a factor of 10 for M solution, at the
same supercooling, although the velocity Still increases approximately as the square of the
_
supercooling. For those solutions whoseviscosity is not greatly enhanced over pure water
and for which
the diffusion coefcient is not too small (for example alkali halides) crystal—
lisation still takes place in the form of dendrites which grow at an angle to the nücleating
crystal, the spacing between the branches deCreasing with increased supercooling (gure 10).
For growth in more viscous solutions, however, such as sucrose, the crystals show distinct
'
crystal facets, and grow as solid hexagonal prisms.
At relatively slow freezing rates : 10 p. sec—1 and in the absence of stirring, more ions are
incorporated into the ice than at larger freezing rates. Ions which simulate a water molecule
(F“) or a group of molecules
are more easily tted into the ice lattice (with segregation coeicient as large as 5) than ions of far di‘erent structure (Nat I“) 14. We know
very little about the inuence of growth rate at the high velocities which occur at large
supercooling, which would be determined by the local conditions of sùpercooling ahead
of the dendrite growth front, and any modication of crystal habit brought about by
preferential adsorption of solute molecules on some crystallographic faces—an elÏect
.
which may be of considerable importance with strongly polar substances.
All these
processes could lead to a solid With very irregular distribution of solute molecules. In
the case when the growth is dendritic, liquid trapped between dendrite arms will have a
concentration much greater than the original.
This may SuperS'aturate and eventually
nucleate crystals of solute and form a eutectic shOuld the “temperature fall sufciently
'
low after the initial stage of freezing has taken place. If the liquid be both supercooled and
; f '
supersaturated ice and salt crystals may grow side by side, to give crystal lamellae which
lie parallel to the growth direction 15.
_
,
A further complication arises if the crySte‘tllisatio‘n rate is
rapid that nucleation of
;”ff
solute takes place ahead of the dendrites. This occurs for air in solution in water super—
cooled by more than 8 °C. A large number of air bubbles of radius less than 1 micron
during the initial stage of
the water—ice“ mixture appears cloudy
'
(gure 11).
…
-
‘
if
ïî
.
3. Growth from the vapour
‘The growth of ice crystals in air is most
Of a water vapour
dl‘usron chamber. Crystals grow on a bre
tWO ice sheets
at different temperatures (gure 12)
surface
and a
lower ice
sheet An approximate
exists between
the two Planes.
7
'
,
.
Smce the saturation
the region
:
.
_
between the plates is
on the bre;
«