Survival from the Cold Winter: Freezing and Ice Crystallization Inhibition by Antifreeze Proteins - Bioinspiration: From Nano to Micro Scales - page 78

The growth of crystals occurs at the boundaries/interfaces between the crystal
phase and the fluid phase when the boundaries move toward the fluid phase. In
this sense, the configuration of crystal surfaces will to a large extent determine the
growth kinetics of crystal faces [ 55 - 68 ]. We normally have two types of crystal
surfaces: the surfaces with an essentially flat configuration— so-called flat crystal
surfaces , or with a rough configuration— so-called roughened crystal surfaces
(Fig. 2.14 a) . The growth of crystals will therefore be in one of the two modes: (1)
faceted growth or layer-by-layer growth mode or (2) roughened growth or normal
growth mode, respectively [ 60 - 65 ]. A faceted crystal surface will have a transition
from the flat mode to the rough mode at the temperature higher than the critical
temperature, so-called roughening temperature T R [ 55 - 58 , 66 ].
In general, the growth of crystals can be regarded as a process of delivering
growth units from the bulk to the crystal surface and incorporating these units into
the kinks (cf. Fig. 2.14 b). In the case of faceted growth, the crystal face is atomically
smooth and the kinks occur only at the steps. In this case, the steps can be regarded
as “sinks” for growth units to enter the crystals [ 8 ]. As shown in Fig. 2.14 b, each
advancing step will disappear when it spreads over the surface and reaches the edge
of the surface. In order to continue the growth of the crystal surface, a subsequent
crystal layer needs to be generated on the existing crystal surface. Therefore, the
step source for the creation of new layers will determine the growth rate of the
crystal surface. Due to the presence of a step free energy, the creation of a new layer
on the existing layer of the crystal surface requires overcoming that free energy
barrier, which is the so-called 2D nucleation barrier [ 67 ]. Normally, for the growth
of flat or faceted crystal surfaces, the screw dislocations occurring on the surface
will provide uninterrupted step sources for a layer-by-layer growth [ 68 ], and in such
cases the growth is controlled by screw dislocation mechanisms [ 68 ].
The growth of crystals that are free from screw dislocations is governed by the
so-called 2D nucleation growth mechanism [ 67 ]. This implies that the crystal faces
grow by depositing one crystal layer on top of the previous layer [ 67 ].
Ice crystal growth inhibition is attained by reduction of the growth rates of all
crystal faces occurring on the ice crystallites. One of the mechanisms is to “pin”
the growing steps by adsorbing additive (i.e., AFPs) molecules onto the surfaces
(cf. Fig. 2.14 c). This happens only when the crystal surfaces are flat. The basic
mechanism is that when the average distance between the molecules is smaller than
2r 2D

c (r 2 c : critical radius of 2D nucleation [ 67 ]), the movement of the steps on
the crystal surface will be blocked. Another mechanism is that the additives are
adsorbed at the kink sites so that the kink desolvation free energy barrier is greatly
enhanced (cf. Fig. 2.14 d). Based on ( 2.30 ), the kink integration coefficient ˇ kink will
be significantly reduced [ 61 , 62 ]. The second mechanism is applied to both flat and
rough crystal surfaces.
Note that a substantial curtailment of the growth of ice would amount to a
substantial reduction of the absolute size of the ice crystallites. This is a commonly
occurring phenomenon, as evidenced by the relatively small sizes of the ice crystals
found in the bellies of fish in subzero environments. If all facets are inhibited by