9
2.6.3 Contact Angles above 90 degrees
Figure 2.12 shows a droplet with a contact angle > 90 degrees. The recommended way to
measure a droplet with a contact angle > 90 degrees is described below.
PLEASE NOTE! This method is only applicable when the top of the droplet is easily
determined.
a) Rotate the protractor until centre line is at 90 degrees.
b) Locate the point of reflection on the droplet at the edge of the
contour (black circle in Figure 2.16).
c) Slide the specimen stage to the left until the point of reflection
is aligned with the centre line.
PLEASE NOTE! It is important
the point of reflection to be positioned exactly at the centre
line.
d) Locate the top of the droplet and rotate the protractor until the
centre line goes through the top of the droplet (Figure 2.17).
The top of the droplet is easier to locate with smaller droplets,
which also will increase the precision.
e) Determine the reading pointed to by the centre line.
f) Multiply the reading by two to get the contact angle value.
2.6.4 A “perfect sphere”
At 90-degrees contact angle the droplet shape will appear as a
perfect sphere together with its mirror image (Figure 2.18).
Figure 2.17
Fig. 2.16 Point of reflection
Fig. 2.18 A ”perfect sphere”
