Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 118

that the drop is made of a polymer solution rather than of pure solvent; (ii) it dif-
fuses partially into the polymer making the thin polymer film swell slightly; (iii) the
solvent also softens the material, making it prone to be deformed by small surface
forces; (iv) the drop evaporates, and faster at the TPCL than at the centre, causing
a net flow of solvent towards the rim of the drop; (iv) the fluid flow is faster than
diffusion, so that a net amount of polymer is transferred to the rim by the end of
the evaporation. The physical phenomena of hole formation via inkjet printing of
solvents onto a polymer film are very different to conventional photolithography,
soft-lithography and etching processes. In this case the etched material is not re-
moved from the substrate by a bulk flow of solvent, but it is transferred locally from
the central region of the drop to the rim. It is thus rather a re-arrangement than an
etching.
3.2. Microstructure-Etching
De Gans et al. used a technology similar as in the paragraph above to etch mi-
crostructures in thin polymer films spincoated on hard substrates, like glass or
silicon [47]. They used a XYZ-controlled inkjet device to generate and deposit
microdrops of various solvents onto thin PS films (thickness of around 100 nm).
They did not simply fabricate round holes, but also lines and even more complex
patterns (Fig. 8). These geometries were a result of the type of etchant, of the type
of polymer, and of the deposition pattern formed by the solvent droplets. The au-
thors obtained the different geometries mainly by varying the spacing between the
drops and by using different solvents. They assume that the size of a hole or a line
Figure 8. (A) Confocal scanning microscopy image of a hole etched into a PS film by a 100 µm
drop of isopropyl acetate (shown to scale as the white circle in the upper right). (B) Groove etched
in poly(benzyl methacrylate) film by a line of 30 µm n -butyl acetate droplets (circle, upper right)
with a spacing of 120 µm. (C) Hexagonal holes etched into a PS film by a hexagonal array of 30 µm
isopropyl acetate droplets. [Reprinted figure from: de Gans, B. J.; Hoeppener, S.; Schubert, U. S.,
Advanced Materials 2006 , 18, (7), 910. Copyright (2006) by Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission.]