Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 119

depends on a combination of three factors: (i) the spreading of the solvent on the
substrate; (ii) the evaporation rate of the solvent; (iii) the rate of dissolution of the
polymer in the solvent. The size of holes formed in a PS film using a number of
good solvents was investigated. Ethyl acetate, isopropyl acetate, n -butyl acetate,
toluene, anisole, and acetophenone drops were deposited on thin films. The initial
size of the drop and/or the surface tension was found to determine the diameter of
the holes. On the other hand, no clear trend was observed for the vapour pressure:
even if the vapour pressure of anisole ( P 0 =

4 . 7 mbar) is ten times larger than the

vapour pressure of acetophenone ( P 0 = 0 . 46 mbar), it had no effect on the diameter

of the hole. This was a surprising finding, since the vapour pressure was expected to
influence the hole etching process in terms of the very different evaporation speeds
of the solvents. The authors thus conclude that the rate of dissolution of PS in the
solvents used always exceeds the rate of evaporation of the polymer/solvent combi-
nation. Thus the wetting of the polystyrene film by the solvent seems to be the only
factor that determines the size of the microstructures.

3.3. Microlithography
In a way similar as above, more complex microstructures than spherical craters can
be produced also on bulk polymer substrates. Again PS was used with toluene as the
solvent. A two dimensional logo was chosen and the inkjet device was programmed
to redraw it on the polymer surface either as single dots, or with continuous lines
(Fig. 9). As in the previous paragraphs, the initial size of a droplet determines the
minimum line width that can be achieved, and the number of droplets determines
the depth of the structures.

D. Decoupling the Processes

The process of structure formation by inkjet etching of polymer surfaces is a com-
plex one, as is demonstrated by the examples given in the previous paragraph. It
is rather a combination of a series of processes, as described in Section B.3. The
first step towards the comprehension is thus to analyse and understand the single
processes, as we try to do in the following paragraphs.

Figure 9. Reproduction of a logo (A) by inkjet etching lithography. Confocal scanning microscopy
image of the structure etched into a polystyrene surface by single, consecutively deposited toluene
microdrops (B) or by a continuous flow of toluene microdrops. (Picture by courtesy of Ramon Pericet-
Camara and Sebastian Nett.)