Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 112

Figure 3. Physico-chemical processes involved in the evaporation of solvent drops from soluble poly-
mer surfaces.
surface. The solvent diffuses into the material to a certain depth. At the same
time the polymer swells due to the solvent diffusing into it, and dissolves at
the interface with the solvent. After some time, the drop consists of a more or
less viscous polymer/solvent solution, and the polymer underneath the drop is
softened.
(ii) Second, the drop deforms the softened polymer surface. It pulls the surface
upwards at the rim, due to the surface tension at the pinned TPCL. As well,
due to the Laplace pressure, the drop pushes downwards the bottom of the
drop.
(iii) Third, the 'coffee-stain effect'. It is known that the evaporation rate of a drop
which is pinned at the TPCL is higher at the rim than in the middle. Therefore
a radial flow occurs from the centre of the drop outwards. This flow carries
dissolved polymer from the centre to the rim, and eventually deposits it there.
We want to point out that wetting by the solvent, diffusion of solvent and soften-
ing of the polymer, and solvation of the polymer are all processes characterized by
quite different time constants.
When the solvent finally evaporates completely, a structure is generated at the
area where the drop was deposited. Its characteristics are strongly dependent on the
materials used and on the parameters set.
For instance, Li et al. [30] studied the structures produced by depositing toluene
drops on polystyrene (PS) surfaces, and ethyl acetate drops on polyethylmethacry-
late (PEMA) surfaces. The authors varied the molecular weight of the polymer and
the number of deposited drops, and analysed the structures obtained. When ethyl
acetate drops are deposited on PEMA, a crater-like shape with a high rim of de-
posited material is produced. The bottom of the structure becomes deeper and the
rim higher when the number of drops is increased. The bottom of the crater is al-
ways flat. The reproducibility of this shape for all explored molecular masses of
PEMA and number of drops shows that the coffee-stain effect is predominant in the
system. On the other hand, the structures produced by depositing toluene drops on
PS surfaces are much more diverse, especially in the case of PS molecular masses