Biomedical Engineering Reference
In-Depth Information
free volume in the form of a number of channels and holes of molecular dimen-
sions, and the first penetrating solvent molecules fill these empty spaces and start
the diffusion process without any necessity for creating new holes. The next layer is
the solid swollen layer where the polymer/solvent system building up in this layer
is still in the glassy state. Next, the solid swollen layer is followed by the gel layer,
which contains swollen polymer material in a rubber-like state, and a liquid layer,
which surrounds every solid in a liquid.
The dissolution/swelling process has been found to be affected by a large num-
ber of factors, like the molecular weight and the polydispersity of the polymer; the
composition, conformation and the structure of the polymer; the type of solvent and
additives to the polymer; the effects of the environmental parameters, like temper-
ature, stirring of the solvent, etc.; the processing conditions of the polymer. To this
end, several models have been proposed to explain the experimentally observed dis-
solution and swelling behaviour and have been reviewed extensively, among others
by Narasimhan
et al.
[75, 76].
In conclusion, as for the coffee-stain effect, the mechanisms responsible for poly-
mer dissolution and swelling in a solvent have been singled out and are understood
to a large extent.
3. Etching and Collapse of the Substrate
A substrate could also be etched by a solvent, and collapse afterwards. This is
not the case for synthetic polymers, but has been described for the special case of
biopolymers, like proteins. Ionescu
et al.
deposited microdrops of an aqueous so-
lution containing the proteolytic enzyme trypsin onto a thin layer of bovine serum
albumin (BSA), and were able to generate small microcraters in the protein layer
[77, 78]. An enzyme like trypsin is able to cleave proteins at specific sites, thus
a major collapse of the three dimensional structure of the BSA layer is expected
after deposition of the drop. The authors were able to tune the size and depth of
the microcraters by the size of the microdrops and by repeated drop depositions.
The action of the solvent is twofold: the water causes the BSA to swell underneath
the drop, thus permitting the enzyme to diffuse to the cleavage sites and to cause
the local collapse of the protein layer. Monte Carlo simulations confirmed the ex-
perimentally observed process (Fig. 12). This process of etching and considerable
collapsing of the material is less significant for polymer-solvent systems, although
the solvent could as well cause a local rearrangement or compaction of polymer
chains underneath the drop, as has been observed by Bates
et al.
[49].
4. Elastic Deformation of the Substrate
The surface effects described in Section B.1 are mostly negligible for sufficiently
hard materials, such as mineral surfaces. However, the deformation may be of mi-
crometer size for surfaces with a Young's modulus below around 100 kPa. The
ridge as result of the surface tension of the droplet pulling upwards at the contact
line has been observed
in-situ
[13, 16, 79] and
ex-situ
[11, 14, 17, 18, 80, 81] in
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