An Instrument for Measuring the Static and Dynamic Response to Shear at the Air/Water Interface and of Insoluble Monolayers: Some New Insights into Two-Dimensional Phases

An apparatus has been designed, assembled, and employed in experiments to determine, as a function of molecular areal density, the viscoelastic behavior of insoluble monolayers spread on water. The apparatus has several unique features: (1) surface tension can be monitored constantly without touching the monolayer; (2) compression of the film is isotropic in an annular space entirely free of obstructions; (3) the stress transducer, located at the geometric center, defines the inner wall of the annulus and the trough wall defines the outer wall of the annulus. A wide range of viscosity and shear modulus can be measured by controlling the stress or strain applied to the film. The apparatus can be used to study the air/water interface of surfactant solutions as well as the surface of water covered by an insoluble monolayer.