The dynamics of thermal fluctuations of a medium reveals the properties of this medium, without the need to apply any stress. In order to exploit this principle, we have developed an optical technique for measuring spontaneous free surface height fluctuations, based on the measurement of the deflection of a laser reflected at the surface. We show that we can measure surface fluctuations of various materials, ranging from low viscosity liquids to viscoelastic solids. The viscoelastic properties of the medium probed can be determined from the experimental fluctuations spectrum. Comparing the data obtained with conventional rheometric measurements, the technique turns out to be a reliable way to characterize the rheological properties of frequencies ranging from 0.1 Hz to tens of kHz. In addition, we are interested in the influence of confinement on the surface fluctuations of a newtonian liquid. We study the effect of confinement using substrates of different types : solid plane, solid with a relief and liquid. We show that the surface fluctuations strongly depend on the substrate used. We study in particular the case where a liquid film is deposited onto a solid surface. By creating a Marangoni effect induced by a temperature gradient at the free surface of the liquid, it is possible to control the thickness of the probed film. We can therefore measure surface fluctuations of films whose thickness varies between 30 nanometers and a few micrometers. The measured spectra are sensitive to the hydrodynamic condition at the liquid-solid interface and makes it possible to estimate a possible slip length.
