Rheology, Structure, and Dynamics of Langmuir films

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This is a grant to support the development and application an interfacial rheometer based on a new design that offers increased sensitivity and a faster dynamic response. Furthermore, its open architecture allows for the simultaneous acquisition of structural measurements using optical methods such as Brewster angle microscopy and absorptive dichroism. It is demonstrated that most interfaces comprised of complex materials (fatty acids, polymers, and lipids) respond in a highly nonlinear manner when subjected to flow. The evidence of this behavior includes recent measurements performed in the laboratory of the P.I. showing flow induced changes in interfacial microstructure on a variety of length scales. Macroscopically, these molecular and mesoscale deformations and orientations are manifested as shear thinning viscosities and first normal stresses in two dimensions. However, at the present time, very little is known about the connection of non-Newtonian interfacial rheology and molecular and microstructural mechanisms. Mechanical rheological data will be acquired, along with molecular level responses, to establish structure-property constitutive relationships. The proposed design of an interfacial rheometer is of the class of controlled stress instruments. The applied stress is generated using magnetic field gradients that subject a rod residing at the surface of a Langmuir films to flow. Both creep compliance and dynamic oscillatory testing will be possible with the instrument so that the linear and nonlinear viscoelastic properties of the films can be obtained. The use of a Langmuir trough as the platform for the measurements will make it possible to establish the desired thermodynamic condition of the film during the measurements. It is planned to study a variety of complex fluid interfaces including: (1) fatty acid monolayers in a number of phases including liquid condensed and solid phases, (2) polymer solution monolayers where the polymers are rigid, hairy rod materials that resemble two-dimensional liquid crystals, and (3) lipid monolayers in the liquid expanded, condensed and two phase states.