Microdrops Evaporating from Deformable or Soluble Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 123

Figure 12. A series of images from a Monte Carlo simulation. The white portion represents the
BSA substrate, dark grey represents the water drop, light grey dots within the drop represent trypsin
molecules, and the black background represents air. The snapshots show the various stages of trypsin
etching the BSA sample that has initially swollen. The times indicated are the iteration step of the
simulation. [Reprinted figure from: Ionescu, R. E.; Marks, R. S.; Gheber, L. A., Nano Letters 2003 ,3,
1639. Copyright (2003) by the American Chemical Society. Reproduced with permission.]
previous works. Carre' et al. [79] demonstrated that the spreading dynamics of a
liquid on a soft elastic surface is modified, due to viscoelastic dissipation induced
by the presence of the 'wetting ridge'. They measured the surface profile close to
the TPCL during the spreading of tricresylphosphate and formamide on a soft sil-
icone elastomer and on natural rubber with a scanning white-light interferometer
microscope. They concluded that the spreading time of a drop can vary as much as
an order of magnitude for various conditions. Extrand and Kumagai [11] studied the
contact angle of sessile drops of ethylene glycol and water on elastomer surfaces.
They observed that the hysteresis between the receding and the advancing contact
angle was enhanced for lower surface elastic moduli. By imaging the wetting ridge
with optical microscopy after fast removal of the sessile drop, they established that
the contact angle hysteresis was not negligible when the ridge height exceeded the
local surface roughness. Pu et al. [14, 17, 18, 81] studied the wetting of acrylic ther-
moplastics surface by sessile drops or wetting fronts. Formation of wetting ridges
at quasi-periodic distances after complete removal of the water from the surface
was observed. The contact line causes a plastic deformation of the surface, as it is
deduced from an enhanced rim height as compared to previous measurements. This
deformation hinders the advancement of the wetting front, until the liquid mass is
too large to be supported by the meniscus, and the drop breaks and flows to establish
a new TPCL. The periodic wetting ridges are corroborated by using the Wilhelmy
plate method to obtain force curves as a function of the plate displacement. The
maximum peak periodicity at the force curves coincides with the spatial periodicity
of the wetting ridges observed on the surfaces.
Pericet-Camara et al. [13, 16] measured the complete deformation of a soft poly-
dimethylsiloxane (PDMS) substrate under a fluorophore-dyed ionic liquid drop. By
using simultaneous fluorescence and reflection laser scanning confocal microscopy
(LSCM) it was possible to obtain three-dimensional images of the fluorescing
drop and the reflection from the solid-gas interface (Fig. 13). The reflection im-
age confirmed the occurrence of a ridge at the TPCL. Moreover, a depression of
the substrate underneath the drop was observed. This depression confirmed the ef-
fect of the capillary pressure of a sessile drop on an elastic surface. Due to the
negligible vapour pressure of ionic liquids, the drop does not evaporate during the
typical experimental time, i.e., it is in thermodynamic equilibrium. The complete