Biomedical Engineering Reference
In-Depth Information
Figure 13. (A) Schematic of the working principle of the measurement: one wavelength (light grey)
is reflected at the solid-gas or liquid-gas interface, the other wavelength (dark grey) excites the fluo-
rophore. (B) Experimental image of a section of an ionic liquid droplet labelled with a fluorophore on
a soft PDMS surface obtained with LSCM. The grey region displays the fluorescing droplet, the grey
solid line is the reflection from the solid-gas interface, and the white dashed line is the deformation
profile according to Rusanov's model. [Reprinted figure from: Pericet-Camara, R.; Best, A.; Butt, H.
J.; Bonaccurso, E., Langmuir 2008 , 24, 10565. Copyright (2008) by the American Chemical Society.
Reproduced with permission.]
strain profile fitted very well with the theoretical one plotted from Rusanov's model
(Eqs. (6a-c)). By using PDMS substrates with various cross-linking densities it was
possible to deduce that substrates with lower Young's moduli bear a more intense
strain.
In summary, in this last paragraph the multifaceted evaporation process of mi-
crodrops from soluble and/or deformable surfaces has been tentatively described,
with the aim of demonstrating that it is a much more complex phenomenon than
the evaporation from undeformable surfaces. Some of the individual processes in-
volved have been presented and briefly discussed. Some of them are understood to
a fair extent, qualitatively as well as quantitatively. For some others experimental
and modelling efforts have still to be made. The same holds for the understanding
of the entire process.
E. Brief Summary
In this chapter we have reported on the state-of-the-art research on evaporating
droplets on deformable and/or soluble polymer surfaces along with the physico-
chemical processes involved. Since in recent times a number of inkjet technologies
based on the deposition of solvent microdrops on soluble polymer substrates have
been presented, an understanding of the phenomenon is required. This will allow
for an improved knowledge of the technology and a better control of the resulting
microstructures. As we pointed out the evaporation of a solvent droplet from a solu-
ble and deformable polymer surface is a very complex process during which several
independent physico-chemical phenomena occur. Three main actions occur at dif-
ferent time scales: (i) the solvent wets the polymer surface; (ii) it diffuses into the
polymer and swells and softens it; and (iii) it dissolves the polymer. The drop can
deform the softened polymer by the action of capillary forces. Or due to evaporation
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