34
FREEZE—DRYÏNG
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—5°c —10°C
—20°C
-30°c
-40°c
Temperature
FIGURE 17. Growth velocity of an ice layer thickness 250 À at different temperatures.
moleculay smooth in their basal plane, do not contain dislocations and require an appre,
ciable supersaturation to initiate growth. The layers of thickness” 250
to 1 000 À propagate
With a velocity inversely proportional to their thickness. This implies that the layers are
“
‘
growing by collection of molecules which are adsorbed on the surface from some distance
—
on either.side-of the step-. If the thickness of the step is
and xs> hWe may write the
,
‘
‘
_
_
,
2 x A
.
,
veloc1ty of propagaüon of
the step,
as U : h—SP
Where A 1s the net ux to the surface.
-
The variation of xs With teh1ÿerature for constant net ux (controlled independeny by
‘
the temperature difference betWeen crystal and surrounding ice) can therefore be measured
and-shows a complicatçdwaation With temperature (gure
An estimate of the
_
absolute value of .xs
;be obtained
anaobservation;bf
_
between two approaching layers.« This 18.
microns: at—: 6C
enables ascale
_
to be given
to gure 17. This interaction distanceis
thereleaseof latent