Viscoelasticity of a protein monolayer from anisotropic surface pressure measurements
Nanoscience Center, University of Cambridge, J.J. Thomson Avenue, Cambridge, CB3 0FF, UK
2 Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK
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Published online: 22 February 2005
We present a method to completely characterize the viscoelasticity of Langmuir monolayers. In contrast to existing techniques, both the compression and shear moduli are determined at the same time, in a single experiment and with a standard apparatus. This approach relies on the measurement of anisotropy in the surface pressure: the tension is measured in orientations perpendicular and parallel to the compression direction. We apply this technique to the study of β-lactoglobulin spread monolayers, a system that has been shown to develop a shear modulus at high concentration. β-lactoglobulin monolayers are interesting both because of their importance in food science and because they exhibit universally slow dynamical behavior that is not fully understood. Our results confirm that the compressional modulus dominates the total viscoelastic response and also provide a complex shear modulus, emerging above a critical concentration. We are able to describe how each of the dynamical response moduli is related to the surface concentration and to the equilibrium osmotic pressure.
© EDP Sciences/Società Italiana di Fisica/Springer-Verlag, 2005