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Flows at molecular scales : probing and manipulating ultra-thin liquid films

Abstract : A hydrodynamic description of liquids at very small scales requires to consider effects specific to these scales, such as intermolecular forces. We have investigated these effects by studying the behavior of supported liquid films of thicknesses of a few tens of microns, that are driven by thermal gradients. The Marangoni flows resulting from the thermal gradients allow a controlled dewetting of the liquid films. Using two different optical techniques, we have measured the height profiles of these films as a function of time, for thicknesses spanning 3 decades and with a resolution of a few nanometers. The experimental thinning dynamics is discussed with the help of a numerical resolution of the thin-film equation. We show that the thermocapillarity effect produces a local regime which is solely characterised by the interplay of the surface tension gradient and the disjoining pressure. The resulting master evolution allows to fully characterise the gradient of the thermal field. Deviations from this master curve result from the effect of intermolecular forces, and the derivative of a disjoining pressure could be obtained from experimental data for thicknesses ranging from 10 to 100nm. In the case of wetting liquids (silicone oils on a glass substrate), the disjoining pressure follows a power-law with thickness that cannot be described by the commonly assumed van der Waals model. In addition, we provide results on unstable films of two alkanes on the same substrate, which break up by the combined effect of thermocapillarity and attractive intermolecular forces. In this case, an ultra-thin foot that recedes towards a macroscopic contact line is formed. Finally, the effect of the ultra-thin film in the relaxation dynamics of the films upon removing the driving thermal field is also studied. In the case of silicone oil, we show how the shape of the ultra-thin film affects the recovery of the initial homogeneous configuration. In the case of alkane films, the contact line remains fixed once the thermal field is removed.
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Submitted on : Thursday, December 2, 2021 - 10:35:08 AM
Last modification on : Tuesday, January 11, 2022 - 3:01:55 AM

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  • HAL Id : tel-03462926, version 1

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Martin Maza Cuello. Flows at molecular scales : probing and manipulating ultra-thin liquid films. Physics [physics]. Université Paris sciences et lettres, 2021. English. ⟨NNT : 2021UPSLS064⟩. ⟨tel-03462926⟩

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