An Interfacial Stress Rheometer To Study Rheological Transitions in Monolayers at the Air−Water Interface

An interfacial stress rheometer has been constructed to study the rheology of Langmuir films subjected to time-dependent flows. A magnetized rod resides at the air−water interface and is set into oscillation by applying a sinusoidal magnetic field gradient. Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determination of the dynamic surface modulus, G s *(ω), and measurement of the relative elastic and viscous contributions of the monolayer. Measurements at 22 °C were conducted on eicosanol (C 20 ) and mixtures of a rigid-rod polymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol. The surface pressure dependence of the rheology for eicosanol reveals the presence of a maximum in the loss modulus, G s ‘ ‘(ω), within the L 2 ‘ phase at Π = 6 mN/m. In the LS I phase at pressures above 15 mN/m, the monolayer is Newtonian and has a surface viscosity of 0.03 mN·s/m. The mixtures of PcPS with eicosanol are known to have two-dimensional nematic behavior. The presence of PcPS in the film increased || G s *(ω)|| 100-fold, creating a non-Newtonian interface with a measurable elasticity. As the polymer rod concentration was increased further, G s *(ω) became less dependent on frequency, and above the isotropic−nematic transition, the storage modulus, G s ‘(ω), exceeded the loss modulus, G s ‘ ‘(ω). The results on eicosanol and the mixtures of the rigid rod with eicosanol demonstrate that the rheometer is capable of detecting microstructural transitions in a Langmuir monolayer.