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This effect has been measured in collaboration between SIMM, SVI (Saint-Gobain-CNRS) and LOMA (Univ. Bordeaux)
As a result of thermal motion, the free surface of any liquid film fluctuates and the amplitude of its height fluctuations (less than 1nm !) are ruled by viscous and capillary effects. When the film is spread on a solid substrate and its thickness is smaller than about 100 nm, the fluctuation amplitudes are modified by the intermolecular forces. Quantitative predictions of this effect were made in the past decades but, up to date, it had not been fully measured. Using a dedicated experimental set-up we were able to form and measure the surface fluctuations of ultra-thin liquid films. The capillary spectra spanning three decades in frequency are in excellent agreement with theoretical predictions accounting for van der Waals forces. Our results emphasize the relevance of considering the effect of intermolecular forces on thermal fluctuations, which play a central role in phenomena such as drop formation, film break-up or dewetting.
C. Clavaud, M. Maza-Cuello, C. Frétigny, L. Talini, and T. Bickel
Phys. Rev. Lett. 126, 228004
