The drag force is directly correlated to contact area between the bodies. This particular contribution comes from one surface adhering to the other, when not under the presence of a lubricant. To understand the adhesional contribution to friction, the concept of real area of contact is most important. The apparent contact area of a seal can easily be calculated by observing the geometry of contact, but to understand how much of this initial calculation is actually interacting with the rubber, surface roughness analysis must be done in tandem with the rubber’s intrinsic properties.
Taking the example of a shoeprint, where the entire shape of the shoe would be the nominal contact area(A0). With an ideally flat surface, the rubber sole would completely cover the shape. As the surface roughens, the real contact (A)area begins to show as just a fraction of the idea represented in the figure with wet footprint marks.