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In their experiment they used two impact velocities: high and low.
For the low-impact velocity (0.44 m/s) case, the droplet first de-
formed and then retracted, and bounced off the surface. Finally, the
droplet sat on the surface and had a high CA, which suggests the for-
mation of a solid-air-liquid interface. For the high-impact velocity
(0.88 m/s) case, the droplet did not bounce-off and wetting of the
surface (and possibly pinning of the droplet) occurred, which is re-
ferred to as the critical velocity. As the droplet strikes the surface, it
deforms and wets the surface (possibly the droplet was also pinned).
This was due to the lack of air pockets between the droplet and the
surface at that velocity. The droplet impalement by the pillars results
in a lower CA. These investigations argue the transition from the
Cassie-Baxter to Wenzel regimes on the basis of velocity at impact.
Jung et al. [30] further studied the critical impact velocity at
which the surface becomes wet (possibly by pinning of the droplet)
to prove the validity of the transition criterion (Eq. 4.9). For
calculations, they took the surface tension of the water-air interface
as 0.073 N m
-1
r
-3
, the mass density (
) is 1000 kg m
for water and
-2
1 kg m s
displays the measured critical impact
velocity as a function of geometric parameters (triangles) with a 1
mm radius droplet. The results show that the critical impact velocity
at which wetting occurs is in good quantitative agreement with the
theoretical predictions.
= 1 N. Figure 4.8b
4.2.6 Contact Angle Hysteresis and Self-Cleaning
There is another very important characteristic of a solid-liquid
interface called CAH. When a droplet sits on a tilted surface, the CAs
at the front and back of the droplet correspond to the advancing
(
) CA, respectively (Fig. 4.9a). CAH is defined
as the difference between the advancing angle, which is higher of
the two, and the receding angle. The main reason for the CAH is
surface roughness and heterogeneity. Careful control of the surface
roughness at the atomic scale could achieve a CAH as low as 1° or less
[31]. However, the hysteresis effect cannot be eliminated completely
because of local heterogeneity. CAH provides information about
energy dissipation during the flow of a droplet along a solid surface.
Surfaces with low CAH have a very low water roll-off angle, which
is the angle to which a surface must be tilted for roll-off of water
drops [19, 32, 33]. Low water roll-off angle is important in liquid-
flow applications, such as in micro/nanochannels and surfaces with
q
) and receding (
q
adv
rec
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