The complex schematic used to describe the physics behind the interaction of a partly submerged cylinder with on the left-hand side a surfactant-laden and on the right-hand side a surfactant-free water surface

With a surfactant on the left-hand side water surface, we obtain asymmetric interfaces that are described by different curves, resulting in different forces applied to the cylinder. The horizontal force component of the force that acts on the cylinder is exactly equal to the difference between the surface tensions, when measured per unit length of the cylinder.