Wetting Dynamics of Aqueous Surfactant Solutions on Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 76

Figure 7. Coordinate system used for surface topography representation.
diagnose, monitor and control the manufacturing process. From an engineering per-
spective, the ultimate objective of a surface topography measurement, as a mean of
control and knowledge, is to establish a correlation between an engineering sur-
face transformation (e.g., wear, chatter, soiling, cleanability, permeability, etc.) and
its topographical characteristics—waviness, roughness, porosity, fractal dimension,
etc. Surface topography measurement, therefore, serves as a link between manufac-
turing, functional performance analysis and prediction, and surface design.
The main topographic characteristics are surface morphology and surface topom-
etry. Surface morphology qualitatively describes the form and structure of a surface
disregarding of fine details. Surface topometry means the exact coordinates of each
single point. A “topography measurement” is the determination of x -, y -and z -
coordinates for a set of representative points of the surface, as shown in Fig 7.
The topography analysis allows filtering measured total profiles of a surface
to split them into two analytical representations for displaying surface features—
roughness and waviness. The first information represents the shorter spatial wave-
lengths, whereas the second one represents the longer wavelength features of the
surface. Both filtered profiles as well as the schematic of a complex fabric structure
are illustrated in Fig. 8.
Some measurements possibilities—both conventional and modern—as well as
topogaphic characterisation on different length scales are described in detail in [6].
2. Different Wetting Regimes
In general, surfaces can be either randomly rough or periodically rough. A liquid
drop placed on a rough surface can sit on peaks or wet grooves depending on the
geometry of surface roughness as well as on surface tension of a liquid.
The apparent static contact angle θ on a rough surface depends on the intrin-
sic Young contact angle Y . Existing theories for pure liquids can be taken into
account in case of different wetting regimes (i) homegeneous surfaces according
to Wenzel [7]; (ii) heterogeneous surfaces according to Cassie and Baxter [8];
(iii) composite surfaces according to Dettre and Johnson [9]; (iv) TPC line approach