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 resid es at the air-water interface and is set into oscillation by applying a sin usoidal magnetic field gradient. Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determinatio n of the dynamic surface modulus, G(s)*(omega), and measurement of the rela tive elastic and viscous contributions of the monolayer. Measurements at 22 degrees C were conducted on eicosanol (C-20) and mixtures of a rigid-rod p olymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol. The su rface pressure dependence of the rheology for eicosanol reveals the presenc e of a maximum in the loss modulus, G(s)"(w), within the L-2' phase at Pi = 6 mN/m. In the LSI phase at pressures above 15 mN/m, the monolayer is Newt onian 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)*(omega)\\ 100-fold, creating a non-Ne wtonian interface with a measurable elasticity. As the polymer rod concentr ation was increased further, G(s)*(omega) became less dependent on frequenc y, and above the isotropic-nematic transition, the storage modulus, G(s)'(o mega), exceeded the loss modulus, G(s)"(omega). The results on eicosanol an d the mixtures of the rigid rod with eicosanol demonstrate that the rheomet er is capable of detecting microstructural transitions in a Langmuir monola yer.