Biomedical Engineering Reference
In-Depth Information
where the indices
S
1
and
S
2
denote the left and right solid surfaces. If the external
constraint is such that
q
continues to increase, the triple line is suddenly depinned
and the liquid is released and invades the lyophobic surface.
3.7.5 Pinning in Microwells
We have already presented the morphology of liquid in a microgroove. It is impor-
tant that the liquid does not spread out of the well. The maximum liquid volume
that a well can contain is that corresponding to an interface pinned to the rim
(Figure 3.44). The canthotaxis limit states that if
q
is the Young contact angle on
the upper surface, the interface can bulge up to this limit. On the other hand, if the
liquid volume decreases—by evaporation, for example—the liquid withdraws pro-
gressively to the inner corners before completely receding.
3.8 Microdrops
3.8.1 Shape of Microdrops
In this section, the shape of microdrops in different situations typical of microsys-
tems is investigated, assuming that these microdrops are in an equilibrium state
(i.e., at rest) or moving at a sufficiently low velocity that the inertial forces can be
neglected. Different situations will be examined: sessile droplets deposited on a
plate, droplets constrained between two horizontal planes, pendant droplets, drop-
lets on lyophilic strips, in corners and dihedrals, and in wells and cusps.
3.8.1.1 Sessile Droplets
It is easily observed that large droplets on horizontal surfaces have a flattened shape,
whereas small droplets have a spherical shape (Figure 3.45).
Figure 3.44
Pinning of liquid in a groove: the interface stays attached to the rim as long the volume
of liquid is such that it is comprised between the two limits 1 and 2 defined by the continuous black
lines.
