Biomedical Engineering Reference
In-Depth Information
equal proportions, then the crystallites will be smaller but will retain the same
shape; however, preferential inhibition of some surfaces over others will lead to
both smaller crystallites and a modified shape. In this section, the AFP-induced ice
morphological modification is discussed, focusing on a comparison of various ice
morphologies produced by different types of AFPs.
2.3.2
AFP-Induced Morphological Modification
Crystals, including ice crystals, are bounded by the faces of the lowest growth
rate. Therefore, the morphology of crystals is determined by these crystal faces.
In general, crystals are bounded by faceted faces as these are the slow growth faces.
For a given crystalline material, i.e., ice, if the ratio of the growth rates of these faces
remains unchanged, the morphology of the crystals will be identical. Nevertheless,
once the growth rates of some particular surfaces are reduced due to the selective
adsorption of additives (i.e., AFPs or AFGPs), the shape or morphology of crystals
will be modified (cf. Fig.
2.15
).
Given a crystal system, the prediction of the growth morphology of crystals
is equivalent to the prediction of relative growth rate in different crystallographic
orientations. The primary surfaces can be used to derive the structural morphology
[
70
-
72
]. The simplest one is the Bravais-Friedel-Donnay-Harker law [
73
].
Whether or not a primary surface will actually appear on the growth form, and
to what extent it will dominate the morphology, depends on the relative growth
rates of the neighboring surfaces. According to the BFDH theory, the relative
growth rate of the crystal face (
hkl
)
R
hkl
can be predicted by
1
d
hkl
;
R
hkl
/
(2.29)
where
d
hkl
denotes the lattice spacing of (
hkl
) corrected by the extension condition
(cf. Fig.
2.16
). The physics consideration of this model is that the interaction
between the growth units in the adjacent lattice planes will decrease with the
Fig. 2.15
Illustration of
crystal growth habit
modification
