Externally generated capillary wave propagation has been studied on water whose surface was covered with a monolayer in the liquid/gas coexistence phase up to 1 kHz. A simultaneous morphology observation with a fluorescence microscope enabled us to make a one-to-one correspondence between the fraction of the liquid film and the propagation characteristics. It was found that the liquid film does not affect the capillary wave significantly until it covers more than 90% of the water surface, which indicates that the dynamic elasticity of the liquid monolayer below 1 kHz is extremely low. It was also noted that the bubbles (the gas phase) were very stable against coalescence, implying that the line tension of the liquid phase is quite low. These findings suggest that the liquid phase in equilibrium with the gas phase is in a very shallow intermolecular potential. It was confirmed that the surface waves do not affect the morphology of the monolayer.
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