Biomedical Engineering Reference
In-Depth Information
D. Experimental Procedure—Wetting of Smooth and Superhydrophobic
Surfaces by Pure Liquids and Surfactant Solutions
Contact angles were measured on seven surfaces to investigate the role of differing
topography and/or chemistry on contact angles. Surfaces were treated to present
either a fluorinated or a saturated hydrocarbon chemistry to the liquid. Fluorinated
chemistry was achieved either through exploiting the natural chemistry of the sur-
face, or
via
spin coating with a 5:1 dilute solution of Teflon AF 1600 (DuPont Co.)
in FC-75 (3M Co.). Saturated hydrocarbon chemistry was achieved either through
exploiting the natural chemistry of the surface, or
via
immersion in a 12 mM so-
lution of Octadecyltrichlorosilane (OTS) in extra dry toluene (both from Sigma
Aldrich). See reference [78] for details of the coating and surface preparation. XPS
analysis confirmed the chemical modification, and SEM and AFM were used to
investigate surface texture. See Table 1 and Fig. 3 for the results of analysis.
Four different surface topographies were studied. Silicon wafers (referred to as
control surfaces) were taken as received, diced, and coated with either Teflon AF
or OTS (see reference [78] for details). They were seen to be relatively smooth (Ta-
ble 1, Fig. 3); while smoother films are possible [81], the measured roughness of the
control surfaces should not impact contact angle appreciably compared to the much
rougher SHS. Plasma etched poly(tetrafluoroethylene) surfaces (here called PTFE)
were prepared following the procedure outlined by Minko
et al
. [82]. The resulting
topography is of a series of vertical spikes protruding from the surface (Table 1,
Fig. 3). These surfaces are naturally fluorinated and could not be coated with a sat-
urated hydrocarbon chemistry. Electrochemically etched aluminum samples (here
called Teflon aluminum or OTS aluminum) were prepared following the process
outlined by Hennig
et al
. [83]. The surfaces have a dual-scale, bumpy topography
(Table 1, Fig. 3), and were coated with either Teflon AF or OTS. AKD surfaces were
prepared following a previous study [77], and naturally develop a random topogra-
phy of angled, plate-like structures (Fig. 3). AKD presents saturated hydrocarbon
chemistry at its surface, with low quantities of hydrophilic heterogeneities, such
as esters, confirmed with XPS analysis for coated and uncoated AKD. Teflon AF
coating of AKD was shown (Table 1) to be incomplete; the chemistry is a mixture
of saturated and fluorinated hydrocarbons; these surfaces will still be called Teflon
AKD in this chapter. Wetting data for uncoated AKD (here called AKD) was taken
from a previous study [77], but with corrected receding contact angle values.
SDS, HTAB and n-decanoyl-n-methylglucamine (MEGA 10), each at three or
more submiceller concentrations, were used as probe liquids. Surfactants with dif-
ferent ionic properties were chosen to examine possible effects of charge interac-
tions on the surface. Pure water, ethylene glycol (EG), bromonaphthalene (BN), and
hexadecane (HD), were also tested to provide a comparison of wetting results for
pure and impure liquids of similar surface tension/intrinsic contact angle.
Surface tension was measured by means of bubble tensiometry (SITA) and/or
pendant drop analysis (First Ten Angstroms, Inc.) depending on the location of tests
(Germany or Canada). Contact angle measurements are presented with respect to
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