Biomedical Engineering Reference
In-Depth Information
(ii) tuning the deposition parameters such as the distance between a drop and a
solid surface, the time difference between drop formation on the syringe tip
and drop deposition on the surface, as well as the intensity of a very short
stroke from an electromagnet to release the drop from the tip;
(iii) checking the surface tension of surfactant solutions before wetting measure-
ments.
Some surfactant solutions have a very long relaxation towards equilibrium. Dy-
namic surface tension is the change in surface tension before equilibrium conditions
are obtained. Hence, in addition to static surface tension measurements, dynamic
surface tension measurements were performed. To obtain the static surface ten-
sion, the pendant drop method was used (Fibro DAT1122). The shape of the drop
is a measure of the surface tension. The dynamic surface tension was measured
using the maximum bubble pressure method (SINTERFACE, Golm, Germany) as
described in [50].
It is well known that the state of a surface affects crucially its wettability. There-
fore, controlling the surface properties such as roughness, surface structure and
chemical composition is of great importance. At first, a set of polymer samples
with the same surface chemical composition but different prehistory (sheets, foils
and spin-coated films) were used. Sample surface topography was examined using
a scanning force microscope (Digital Instruments, USA). To obtain the degree of
surface hydrophobicity, advancing contact angles of water drops were measured by
the sessile drop method using a Krüss DSA 10 drop shape analyser (Germany). The
chemical surface composition was investigated by means of X-ray spectroscopic
analysis. For the appreciation of the chosen polymer samples on their ability to be
model surfaces for screening method, the values for water and SDS contact angles
as well as their surface roughness were compared. The comparison between indus-
trial polymer samples (sheets and foils) and spin-coated polymer films proved that:
(i) the wetting results for each set of surfaces are in good agreement;
(ii) due to chemical heterogeneity connected with manufacturing process, the wa-
ter contact angles for sheets are lower than that for foils and films (Fig. 14);
(iii) due to considerable surface roughness of the polymer sheets, the relative error
in the measured contact angles for sheets was larger than those for films and
foils as illustrated in Fig. 13.
Consequently, spin-coated polymer films and industrially manufactured polymer
foils were chosen as model surfaces for screening.
Anionic SDS, cationic DTAB and DTAS and non-ionic C
12
E
5
(all with the same
alkyl chain length) were used as model surfactants, as mentioned above. In addition,
technical surfactants anionic n-C
12
−
C
13
-alkylbenzene sulphonate Na salt (Marlon
ARL) and non-ionic C
12
−
C
14
-fatty alcohol polyethylene glycol ether (E
6
, Marli-
pal 24/60) were used to verify the screening method. For all surfactants used, their
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