New ellipse-fitting method for contact angle measurement

Abstract

In this work, the authors examined the commonly used fitting methods for optical contact angle measurement. It was shown that currently, there is no available fitting method that can measure the contact angle of a non-axisymmetric drop without a clear view of the drop baseline. This work contributes to contact angle measurement methods by providing a new ellipse-fitting method. The new method fits the drop profile with a general equation of an ellipse whose major and minor axes are not necessarily parallel and normal with the substrate, respectively. The advantage of this method is that it can measure the contact angle of a non-axisymmetric drop without a clear view of the drop baseline. The measurement results were validated by comparing against the DropSnake and polynomial fitting methods. Generally good agreement was seen, which is important as it validates this method as the only available method for finding the contact angle of a droplet when the contact line is not visible – for example, in a condensation environment where many drops can block the view of each other’s contact line.