Centro-symmetric technique for measuring shear modulus, viscosity, and surface tension of spread monolayers

We have constructed an apparatus which enables one to measure, simultaneously, the shear modulus, the viscosity, and the surface pressure of a monomolecular film on water under a hydrostatic compression. It is particularly suitable for measurements of the static shear modulus of insoluble monolayers. A capillary wave generator/detector system was also incorporated to determine the surface pressure. The trough is a Teflon cup 12.7 cm (5.4 in.) in inner diameter with sloping interior sides. As the cup is raised, while maintaining the water elevation fixed, the film is uniformly compressed. A rotor suspended at the center from a fine torsion fiber is used to measure the mechanical properties (surface viscosity and/or surface shear modulus). To measure the quasistatic shear properties, the cup is rotated through a small angle and the residual deflection of the rotor determined. Dynamic shear response is probed by studying transient torsional oscillations following a sudden angular displacement of the torsion fiber. There are no barrier seals and circular symmetry is maintained at all times. The apparatus has a resolution of 5×10^-4 surface poise of shear viscosity and 1×10^-4 dyn/cm of static shear modulus.