## Zero Degree Contact Angle Measurements

It is difficult to form a satisfactory profile image of a sessile drop as its contact angle approaches zero (say, is less than 5 degrees). The algorithms themselves have no inherent limitation in reaching zero if the image is satisfactory. The practical optical limitations include

Glare tends to form across the top of the drop, making the height of the drop obscure and therefore making the measurement inaccurate.

There is very little liquid at the "corners" of the drop, so the contrast is low and it is difficult to establish the baseline, the location of the liquid-solid interface, accurately.

The viewing angle of the camera is normally a few degrees down, which is usually beneficial. However, when the actual contact angle is similar to or smaller than the camera viewing angle, significant errors are introduced. These can make the drop appear to have a much larger contact angle.

For these reasons, which have to do with optics, we do not recommend planning on automatic measurements in the 5 degree range and lower. It is possible for the operator, with care, to pick out the true edges and click on these, at which point the FTA software will compute the contact angle and verify it by drawing graphics over the edges. This is a solution when you only occasionally need to make very low angle measurements.

For those requiring reliable automatic contact angle measurement in the low angle region, FTA offers top view systems that compute the contact angle by the diameter of the wetted area on the specimen and the original drop volume. These are accurate, automatic, and can be used to truly zero degrees.

Whether or not you need to resolve the difference between, say, 2 and 5 degrees contact angle depends on your application. Adhesion equations all depend on the cosine of the contact angle, so the difference in the contact angle factor for these two angles is negligible:

cosine 2 degrees = 0.9994

cosine 5 degrees = 0.9962

This small difference would not matter to a paint or adhesive. On the other hand, the cleanliness of some materials is measured by contact angles and the differences between 0 degrees (complete spreading) and 2 or 5 degrees is important because here it specifies the chemical nature of the surface. In these cases the precision of the top view measurement is worthwhile.