文档介绍：7. Canthotaxis/Wetting Barriers/
Pinning Lines
Under conditions of weightlessness, the shape of a capillary surface is deter-
mined by the container shape and the contact angle of the liquid with the
adjacent solid faces. The location and handling of liquids require the appli-
cation of wetting aids and wetting barriers such as sharp edges or coatings.
At a wetting barrier, the liquid surface winds from the contact angle with
the better-wetting face to the contact angle with the worse-wetting face. This
effect has been termed “canthotaxis”. Hitherto, canthotaxis has been an art
rather than a science; exact results are scarce. Numerically, the length of the
interval of canthotaxis (pinning interval) is proportional to the square of the
angular interval in question. For surfaces with zero capillary pressure, the
length of the interval of canthotaxis varies in proportion to the cube of the
angular interval.
Introduction
Under the action of gravity, liquids generally rest on the bottom of their
containers. The liquid surface is plane and horizontal. Small liquid volumes
are likely to be found in wedges. In contrast, liquid volumes may be found
almost anywhere under conditions of weightlessness. The shape of a capillary
surface is determined by the shape of the container and the contact angle
γ of the liquid with the adjacent solid faces. Wetting liquids (γ<π/2) try
to maximize their area of contact with a solid, whereas nonwetting liquids
(γ>π/2) try to minimize it. Special measures regarding the geometry and
the wetting conditions have been taken essfully for manipulation and
transport of liquids under low-gravity conditions.
The drops on a solid surface shown in Fig. may rest in arbitrary posi-
tions if no measures are taken to locate them. For instance, if the solid surface
has been treated so that the liquid wets it well in some positions and wets it
badly in others, the liquid is most likely to be found in the former positions.