The parameters of a circular arc equation are matched to the optically determined drop shape. The contact angle is calculated as the angle between the circular arc and the baseline intersecting it.
In contrast to the height-width method, which also starts from a circular arc, the whole contour is evaluated in the circle method instead of only a few significant points. The measurement is therefore less susceptible to interference and can also be used, for example, for dynamic contact angles where needles are positioned in the drop.
The method is used particularly for small contact angles. As the real drop shape deviates from the circular arc for larger contact angles and volumes, other methods should be used for contact angles greater than 20° and volumes of more than 1 µL.