Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 121

Figure 11. (A) Schematic illustration of the one-dimensional solvent diffusion, swelling, and disso-
lution process; (B) Schematic composition of the surface layer. [Adapted figure from: Miller-Chou,
B. A.; Koenig, J. L., Progress in Polymer Science 2003 , 28, 1223. Copyright (2003) by Elsevier B.V..
Reproduced with permission.]
drop. If there were no flow, the evaporation would alter the height profile (Fig. 10A):
at the perimeter, all the liquid would be removed and the drop would shrink. Since
the radius of the drop cannot shrink, as its TPCL is held pinned, to prevent the
shrinkage the liquid must flow outwards as in Fig. 10B. The height profile must
maintain the spherical cap shape dictated by surface tension for small drops. Thus
during a short time the hashed area must be removed. This area is different from
that in Fig. 10A, so that the radial flow must make up for this difference. The flow
drags the solute along with it, so that at the end of the evaporation nearly all of it is
deposited at the former TPCL.
2. Dissolution and Swelling of the Substrate
The dissolution and the swelling of polymers in solvents is a key area in polymer
and material science. A large number of phenomenological, experimental, and the-
oretical literature has been published on this topic over the years [40, 70-73]. The
topic, and the few papers and reviews cited here are only representative, but by no
means exhaustive. The phenomenon is especially interesting because it is differ-
ent from non-polymeric materials, which dissolve instantaneously and where the
dissolution process is generally controlled by the external mass transfer resistance
through a liquid layer adjacent to the solid/liquid interface. The dissolution of a
polymer into a solvent, on the other hand, involves two transport processes, namely
solvent diffusion and chain disentanglement (Fig. 11A). When an uncrosslinked,
amorphous, glassy polymer is in contact with a solvent, the solvent will diffuse into
it. Due to plasticization of the polymer by the solvent, a gel-like swollen layer is
formed. It has two separate interfaces, one between the glassy polymer and the gel
layer, and the other between the gel layer and the solvent. After a certain time the
polymer dissolves. One of the earliest contributors to the study of polymer disso-
lution was Ueberreiter [74] who outlined the surface layer formation process. He
described the structure of the surface layers of glassy polymers during dissolution
from the pure polymer to the pure solvent as follows: the infiltration layer, the solid
swollen layer, the gel layer, and the liquid layer (Fig. 11B). The infiltration layer is
the first layer adjacent to the pure polymer. A polymer in the glassy state contains