Biomedical Engineering Reference
In-Depth Information
phenomena such as spreading, wetting, capillary and soil release. Three-scale con-
cept includes (i) macro-morphological irregularities of textiles such as folds and
wrinkles; (b) meso-scale of textile materials describing the surface topography pro-
duced by the type of weave and yarn used, without attending previous defined
macro-topographic irregularities and details corresponding to fibres or filaments
and (iii) micro-length scale revealing the influence of filaments and fibres char-
acteristics on the resulting topography. Profile, fineness and natural or machined
texture of these elements or distances between them, are only some of the possi-
ble characteristics that as a whole define the resulting morphology and topometry
at the micro-length scale. Dimensional changes (relaxation/shrinkage) of fabrics at
macro scale influence their meso- and micro-topography due to the modification of
repetitive unit dimensions and therefore the distances between yarns, filaments and
fibres.
To reach the macroscopic details of the liquid wetting behaviour in fibrous media,
various computer simulation techniques have been applied in this filed to accom-
modate more complexity so as to investigate more realistic systems, and to better
understand and explain experimental results.
C. Wetting Dynamics
Contact angle as a thermodynamic equilibrium property, virtually all the published
data for which reproducibility is claimed, are measurements of advancing contact
angle within a minute of TPC line displacement. The second category is that of
truly dynamic contact angles. If the TPC line as a phase boundary liquid-solid
simultaneously moves relative to the adjacent solid surface, a dynamic contact angle
will be observed. Dynamic contact angle means the contact angle as a function of
time, which can significantly differs from the static contact angle.
1. Spreading Theories
The dynamic behaviour of a pure liquid on an ideal solid surface can be successfully
mathematically described by the equilibrium contact angle
, the dynamic (time-
dependent) contact angle
θ(t)
as well as the spreading velocity d
r/
d
t
,where
r
is
the base radius of a spreading drop. The
spreading velocity
or
spreading rate
as a
time-dependent drop radius variation is often an important criterion on which basis
the efficiency of surface-active substances (surfactants) can be estimated.
In the hydrodynamic consideration disturbed capillary equilibrium leads to the
spreading force
γ
LG
(
cos
θ
0
−
cos
θ(t))
,where
θ
0
is initial contact angle. The most
popular hydrodynamic models gave a successful interpretation of experimental data
on complete wetting and propagation at high capillary numbers [3, 20-22]. Work
is necessary to expanding the solid-liquid interface, and energy will dissipate due
to viscose shear in the liquid. Such a consideration assumes a slippage of the liq-
uid with respect to the solid in the TPC line vicinity. This theory is applicable to
the description of a slow spreading near equilibrium. The molecular-kinetic the-
ory [23, 24] assumes, however, particular displacements on a molecular level at the
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