Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 114

if the dissolved molecules diffuse a larger distance than covered by evaporation
driven flow, convex structures are formed. If the evaporation driven flow dominates,
concave structures are formed. The amount of dissolved polymer influences this re-
lation by increasing the viscosity and reducing the vapour pressure of the solution,
thus reducing also the evaporation driven flow. In the following paragraph, various
applications of ink jetting solvent microdrops on polymer surfaces will be presented
along with the microstructures obtained.
C. Applications of Evaporating Solvent Drops on Deformable/Soluble
Substrates
1. Microlenses on Polymer Surfaces by Ink Jetting Solvent Drops
By depositing a tiny drop of solvent onto a polymer surface (volume between 0.1
and 1 nL) by, e.g., ink jetting [53, 54] it is possible to fabricate a spherically shaped
microcrater (Fig. 5A). Such concave structures can be used as moulds for another
polymer to fabricate convex microlenses in a second processing step. As seen above,
the solvent has three distinct effects on the polymer: (i) it dissolves part of it, so
that the drop is made of a polymer solution rather than of pure solvent after some
time; (ii) it diffuses into it and makes it swell; (iii) it softens the material, so that it
Figure 5. (A) Technology to fabricate concave and convex lenses by solvent microdrops. (B) Detail of
a micrograph and X and Y profiles of an array of 10 × 10 concave microlenses made by toluene drops
on polystyrene ( M w = 200 kDa). In each line a different number of drops was deposited. (C) Ex-
perimental determination of the focal length of a row of convex PDMS microlenses by LSCM and
intensity profile of the light focussed by one lens (white arrow). (D) Calculated and measured focal
lengths of an array of convex PDMS microlenses. [Reprinted figure from: Pericet-Camara, R.; Best,
A.; Nett, S. K.; Gutmann, J. S.; Bonaccurso, E., Optics Express 2007 , 15, 9877. Copyright (2007) by
the Optical Society of America. Reproduced with permission.]