The sessile drop method is another popular way to measure contact angles, and is done by placing a two-dimensional drop on a solid surface and controlling the volume of liquid in the drop. The sessile drop method and the captive bubble method are usually interchangeable in performing experiments as they are both based on the properties of symmetry. Specifically, the axis of symmetry of the drop or bubble makes the contact line of the drop or bubble with the solid surface circular. This creates an observable contact angle corresponding to the contact radius of the drop or bubble.
However, interacting with a rough homogeneous surface in measurements of contact angles, the drop and bubble each present different behaviours in the measuring process, which are related to the volume of liquid and contact angles.
# On a rough homogeneous surface, the observed contact angle may not represent the actual contact angle with a local slope since it may not be observable on a rough surface. The observed contact angle on a rough surface is also called an apparent angle which is equivalent to the sum of the intrinsic contact angle and the local surface slope at the tangent of the contact slope for a drop or bubble. With the sessile drop method, the observed contact angle usually underestimated the intrinsic contact angle, while the observed contact angle in the captive bubble method overestimates the intrinsic contact angle of the rough surface.
# If a graph is plotted respectively for the measurements of contact angles using the sessile drop method and the captive bubble method concerning the volume of liquid within the drop or bubble and the measured contact angle, the geometrical relationships illustrate different characteristics for each method. In consideration of the relationship between contact angles and the position of the contact for a certain volume in the drop or bubble, the highest and lowest possible contact angles on volume are dependent on each other differently in the two methods.
# For the amplitude of oscillations shown in the graph, both of the drop and the captive bubble display a similar order of magnitude at a relatively low contact angle. On the other hand, on a rough surface with a relatively high contact angle, the amplitude shown for the drop is larger than that of a captive bubble. The amplitude of oscillation of the lowest highest possible contact angle demonstrates the difference for the drop method and the captive bubble method, in which the amplitude of the graph of the captive bubble method is comparatively larger than that of the graph of the sessile drop method.
# In terms of the wavelength of the graph, the wavelength for both methods spans over a large range of volume of liquid on the solid surface. Differences in behaviours of the drop and the bubble vary from the lowest possible contact angles to the highest possible contact angle.