Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 116

Figure 6. Microvessels on a polystyrene substrate ( M w = 200 kDa) with corresponding height pro-
files across the dashed lines. Structures are obtained with deposition of drops of (A) acetophenone,
(B) ethylacetate, and (C) 5:1 mixture (Vol) of ethylacetate and acetophenone. [Reprinted figure from:
Karabasheva, S.; Baluschev, S.; Graf, K., Applied Physics Letters 2006 , 89, 031110. Copyright (2006)
by the American Institute of Physics. Reproduced with permission.]
is accumulated at the rim as well as in the centre of the structure (Fig. 6C). The
effect occurred not only for microstructures, but also for structures with diameters
ranging from tens of micrometers to several millimetres. The authors tentatively ex-
plain the concave/convex structures stating that the solvent is transported from the
centre to the rim to replenish the loss due to evaporation. As AP evaporates much
slower than EA, the concentration of AP at the pinned TPCL increases, leading to
a concentration gradient of AP that is directed opposite to the concentration gra-
dient of EA. Thus, differences in the evaporation rates might cause an additional
flow from the rim to the centre. Additionally, a gradient of surface tension occurs,
adding a surface flow of Marangoni type to the system. Both are leading to a re-
distribution of dissolved polymer into the centre of the drop. As a consequence, an
increased EA content should enhance the accumulation in the centre. The author
verified this by varying the mixing ratios of AP and EA: for mixing ratios from 2:1
to 6:1 the accumulation of material in the centre becomes more pronounced. For
higher mixing ratios, the final structure becomes more complicated to interpret, due
to the formation of a second crater in the middle.
3. Microlithography and Other Applications
3.1. Via-Hole Etching
Kawase et al. first employed inkjet etching of polymers with solvent drops to fab-
ricate via holes through a 500 nm thick polyvinylphenol (PVP) film. They were
looking for fast and low-cost alternatives to conventional microlithography for mak-
ing a connection between two conducting polymer films, or between two electrodes
more in general, across an insulating layer [22, 56]. They deposited small ethanol
drops onto the insulator with an inkjet printing head. The drops dissolve the in-
sulator locally and evaporate within a second. Here too, when a drop dried, the