Spreading of non volatile liquids on smooth solid surfaces : role of long range forces

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Abstract
We present an experimental investigation of the spreading of non volatile polydimethylsiloxane drops on smooth horizontal silicone wafers. We distinguish the macroscopic part of the drop which can be seen by bare eye (or through a microscope) from the microscopic part, or precursor film, which progressively extends like a thin liquid tongue (thickness smaller than 1 000 A) all around the drop. The macroscopic drop spreads independently of the spreading parameter, in opposition to the precursor film which appears deeply influenced by the surface energies. We have characterized this precursor film (both its profile and its time evolution) by complementary techniques such as ellipsocontrast microscopic observations, spatially resolved ellipsometry, X-rays reflectivity. The results will be compared with recent theoretical predictions, and specific polymeric effects will be emphasized