7
Fig. 2.7 With parallax
errors
Fig. 2.8 Without parallax
errors
Fig. 2.9 Point of reflection
Figure 2.10
2.5.1 Parallax errors
The rotating protractor has a set of parallel grid lines. The centre line is easily located, as it
has no grid lines next to it. The grid lines are used when taking contact angle readings from
the displayed droplet image. These readings can be quite accurate (error < 2°) but it is very
important to avoid parallax errors. This is why there are two centre lines on the protractor, one
at the top and one at the back of the rotating disk. If these lines are viewed from an angle they
appear as a “double centre line” as the two lines are not overlapping each other completely
(Figure 2.7). When the protractor is viewed from the correct (perpendicular) angle these two
lines will appear as a single line (Figure 2.8) and the parallax error is eliminated.
2.5.2 The Sliding Specimen Stage
For droplets wetting out on the surface it might not be possible to see both sides of the droplet
simultaneously. In this case it will be necessary to slide the specimen stage sideways or
simply reduce the droplet size when this can be done.
2.6 Taking a reading
First of all the protractor disk must be rotated into the correct position.
In general the objective is to match the parallel grid lines to the contour of the droplet image.
This can be done in different ways depending on the shape of the droplet, which falls into one
of three categories.
2.6.1 The Standard Method
This method is fast and allows contact angle measurements on both sides
without sliding the specimen stage.
a) Locate the point of reflection on the droplet at the edge of the contour
(Figure 2.9).
b) Turn the protractor until the grid lines are parallel to the
tangent at the point of reflection. Slide the specimen stage slightly
sideways until the point of reflection matches a grid line (Figure 2.10).
c) The centre line is now pointing at the contact angle for the droplet.
Take your reading on the scale avoiding parallax errors (Section 2.5.1).
PLEASE NOTE! The scale displays values from 180 degrees down to zero.
When the contact angle is measured on the left-hand side of the droplet
image, the reading is correct. When the reading is made on the right-hand
side of the droplet, however, the correct value is the complement angle (e.g.
122 degrees means 180-122= 58 degrees).
