Biomedical Engineering Reference
In-Depth Information
a very complex phenomenon. What can safely be said, is that the final equilibrium
configuration, in the presence of reactions, should be determined by the formation
of continuous layers of specific reaction products, such as the M
6
X metal-like com-
pounds in metal-oxide systems [60, 62, 63]. Which compound can form depends
on the chemistry of the system: the presence in the liquid phase of minor elements,
such as the active elements used in brazing processes (e.g., Ti, Zr, Cr, V, Nb, Al),
can give rise to interfacial reactions whose type and extent depend on the activity
of these elements in the experimental conditions. But the kinetics of the spreading
process is a much more complex process. From the first contact of the liquid phase
with the solid support, diffusion processes take place, so that diffusion-controlled
reactive wetting is one of the basic mechanisms to be considered [42]. The triple
line velocity in reactive-limited wetting has been shown to depend on the instanta-
neous contact angle, whose variation with time follows the relation:
cos
θ
F
−
cos
θ
=
(
cos
θ
F
−
cos
θ
0
)
exp
(
−
kt),
(8)
where
θ
F
and
θ
0
are the final and 'initial' contact angles [39].
C. Role of the Solid Surface in Modifying the Wetting Behaviour
Wetting of solid surfaces is affected, in addition to the mechanisms which are be-
hind the three wetting typologies already discussed, by 'physical' and 'chemical'
factors intrinsic of the solid surface that are briefly discussed below.
1. Roughness
The solid surfaces used in wetting experiments are not ideal. Especially when work-
ing at high temperature with polycrystalline materials, surfaces are not perfectly
smooth. Ceramic materials are usually produced by sintering; this means that a
residual porosity is always present and that, even after a very accurate polishing
procedure, a residual surface roughness is still present when the liquid phase is put
into contact with the solid. In addition, with temperature rising, micro-faceting and
thermal grooving at grain boundaries takes place, so that it may be even impossible
to evaluate the correct roughness value of the solid surface at the test tempera-
ture. It is well known that roughness affects the contact angle value by various
mechanisms: (a) by increasing the solid-liquid contact area (increase of interfacial
energy), (b) by providing 'pockets' of entrapped vapours, so that forming a com-
posite interface, (c) by offering specific nucleation points for reaction and growth
when reactions can occur, (d) by offering specific 'pinning' points to the movement
of the triple line.
Wenzel [64] proposed, for rough surfaces, the following equation linking the
'macroscopic' contact angle
θ
m
to the 'intrinsic' one:
cos
θ
m
=
r
cos
θ.
(9)
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