Biomedical Engineering Reference
In-Depth Information
Figure 3.64
Left: contact on a smooth surface; right: contact on a rough surface.
The Wenzel law states that
*
cos
θ
=
cos
θ
(3.84)
r
which changes the Young law to the following form
*
(
γ
cos
θ γ γ
=
-
SL
r
)
(3.85)
LG
SG
Taking into account that r > 1, relation (3.84) implies that
*
(3.86)
cos
θ
>
cos
θ
We can deduce that if
q
is larger than 90° (hydrophobic contact), then
q
*
>
q
and
the contact is still more hydrophobic due to the roughness (Figure 3.64). If
q
is smaller
than 90° (hydrophilic contact), then
q
*
<
q
and the contact is still more hydrophilic.
In conclusion, surface roughness increases the wetting character (Figure 3.65).
Figure 3.65
Contact of a liquid drop on a rough surface.
