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In the present work, a polymeric transient viscoelastic network is used as a model system to investigate several fundamentals of interfacial viscoelasticity and nonlinear behavior, in simple shear, compression, and for simple mixed deformations. A supramolecular polymer bilayer, characterized by long but finite relaxation times, is created at the water-air interface using a layer-by-layer assembly method. The possibility of studying the individual layers starting from an unstrained reference state enabled the independent quantification of the equilibrium thermodynamic properties, and the viscoelastic response of the bilayer could be studied separately for shear and compressional deformations. Time- and frequency-dependent material functions of the layer were determined in simple shear and uniform compression. Moreover, a quasilinear neo-Hookean model for elastic interfaces was adapted to describe step strain experiments on a viscoelastic system by allowing the material properties to be time-dependent. The use of this model made it possible to calculate the response of the system to step deformations. Within the linear response regime, both stress-strain proportionality and the superposition principle were investigated. Furthermore, the onset of nonlinear behavior of the extra stresses was characterized in shear and for the first time in pure compression. We conclude by investigating the multilayer system in a rising bubble setup and show that the neo-Hookean model is able to predict the extra and deviatoric surface stresses well up to moderate deformations. [ABSTRACT FROM AUTHOR]

Thermal motion gives rise to fluctuations in free surfaces; measurement of the thermally excited waves on such surfaces provides information on the mechanical properties of the medium. We have developed an optical tool to probe the thermally excited waves on free surfaces: surface fluctuation specular reflection (SFSR) spectroscopy. It consists in measuring the fluctuations in the position of a laser beam that is specularly reflected onto the free surface of a medium. The position of the reflected beam is sensitive to the roughness of the probed surface; the thermal waves are detected by subtracting the light intensities collected on the two quadrants of a photodiode, on which the beam is centered. We show how the measured signal is related to the medium properties. We also present measurements performed on Newtonian liquids as well as on a viscoelastic solid; we show that in all cases, there is a very good agreement between experimental and computed spectra. SFSR thus applies to a broad range of materials. It moreover offers a very good temporal resolution and should provide a useful tool for dynamical measurements on complex fluids. [ABSTRACT FROM AUTHOR]