Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 113

Figure 4. Surface plots (left and right) and cross-sections (middle) of structures left by toluene drops
deposited by ink jetting on polystyrene with different molar masses, 1-2 h after drying. The height in
the surface plots is automatically rescaled so that height is only in relative units. For the cross-sections
the ticks are spaced 1 µm vertically and 50 µm horizontally. The four height profiles for each molar
mass refer to depositions of 1-2, 5-6, 9-10, and 18-20 drops deposited at intervals of 0.5 s on the same
spot. [Reprinted figure from: Li, G.; Graf, K.; Bonaccurso, E.; Golovko, D. S.; Best, A.; Butt, H. J.,
Macromolecular Chemistry and Physics 2007 , 208, 2134. Copyright (2007) by Wiley-VCH Verlag
GmbH & Co. KGaA. Reproduced with permission.]
lower than 100 kDa (Fig. 4). When the number of drops deposited is small the struc-
ture is crater-like with a ring of material around. The central depression is spherical
cap shaped. Similarly to the crater-like structures in PEMA, these structures show
the predominance of the coffee-stain effect for small drop numbers. In contrast,
when the number of deposited drops is increased, a dot-like elevation is formed in
the centre of the structure. This situation is explained as due to an excess of dis-
solved polymer at the bottom of the drop, impeding the radial flow towards the
outer rim. For high molar masses, the structures become asymmetric.
In summary, evaporation structures left by microscopic solvent drops on poly-
mer surfaces are characteristic of the specific polymer/solvent combination and the
molar mass of the polymer. If the solvent evaporates with a constant contact radius
from the polymer surface, the shape of the evaporation structures is determined by
the processes mentioned above. Depending on which of these processes is domi-
nating, different structures can be generated. For the system polystyrene/toluene,