Surface Tension as a Complex Parameter: A Critique of Experimental Evidence from Energy Dissipation Studies on Fibers Partially Immersed at an Air/Water Interface

The study focuses on important new experiments and to indicate additional experiments at the air/water interface and at the lines of contact of the liquid surface with the fiber and the container walls and edges, which can potentially show that the observed energy dissipation in the water surface may be assigned to factors other than or additional to a complex surface tension. The experiments are made on the surface of water filling a cylindrical cavity 10 mm in diameter and 5 mm deep in a stainless steel block that can be raised stepwise. A glass fiber a few microns in diameter is glued to an AFM-type cantilever mounted above the block and operated in tapping mode. The vertical force on the cantilever is measured as the water is raised to contact and partially immerse the glass fiber close to one side of the water pool. A standing wave is set up between the fiber and the adjacent wall and the power dissipation is measured over a range of frequencies. The results from this method are impressive but as yet insufficient to claim that a strictly surface-located viscoelasticity of the pure water/air interface has been identified to give an estimate of the imaginary component of a complex surface tension.