Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 115

becomes easier to be deformed by small surface forces. At the same time the drop
evaporates. After evaporation of the solvent, a concave microcrater is left behind
[25]. This crater is formed according to the physico-chemical processes described
above.
Using an automatised XYZ-controlled inkjet head one can make two dimen-
sional arrays of microcraters (Fig. 5B). Diameter and depth of the craters can be
controlled by the number of microdrops, as demonstrated in the surface profiles in
Fig. 5B: along the X -axis the number of drops was similar and thus also the depth of
the craters (see horizontal dashed line connecting the bottoms); along the Y -axis the
number of drops was continuously increased and thus also the depths went deeper
(see inclined dashed line connecting the bottoms). This array was used as a master
for moulding with the elastomer polydimethylsiloxane (PDMS) and making plano-
convex microlenses. The focal length of such lenses was directly measured with a
laser scanning confocal microscope (LSCM) (Fig. 5C). The lenses are irradiated
with a parallel laser beam from the flat side, which is focussed by the lenses. The
intensity of the light is maximum in the focus, as shown by the intensity profile of
the transmitted light. The distance between the point of highest intensity and the flat
surface yields the focal length. The focal length can also be calculated from geo-
metrical parameters, like the refractive index and the radius of curvature of the lens.
Both methods yield similar values (Fig. 5D). Microlens arrays fabricated with this
technique possess extremely good optical qualities, small aberration, and a surface
roughness in the nanometer scale [31].
2. Microstructures on Polymer Surfaces by Mixtures of Solvents

If instead of using pure solvents mixtures of solvents are used, the shape of the
produced microstructures can be additionally controlled by the mixing ratio. This
is also known for droplets of polymer solutions dissolved in two solvents evap-
orating from hard, inert surfaces [46, 55]. Not only diameter and depth can be
controlled, but also the shape, e.g., the sphericity/asphericity of the microcraters.
So did Karabasheva et al. [27] by using a mixture of acetophenone (AP) and ethyl
acetate (EA) and ink jetting drops of the two pure solvents and of mixtures of them
on PS surfaces with a molecular weight M w = 200 kDa. The two solvents are fully

miscible in any proportion. They have different surface tensions, which affect the
contact angle of the deposited drops on the surface, and vapour pressures, which
affect the evaporation rate. AP, e.g., evaporates much slower than EA. In Fig. 6 are
shown micrographs of the structures resulting from the evaporation of the drops,
together with representative profiles. Despite the differences in volatility, in both
cases a concave shape remains (Figs. 6A, B), as was seen in the previous para-
graph for toluene. The crater from the relatively fast evaporating EA shows a lower
depth-to-width ratio than AP. This was explained by the fact that less polymer can
be dissolved by the drop if it evaporates faster, and thus less polymer is accumulated
at the rim. When the authors used a mixture of solvents the shape of the microstruc-
tures drastically changed. With a mixture of EA and AP (5:1 ratio by volume), PS