Existence and Role of the Precursor Film in the Spreading of Polymer Liquids

We present investigations of the spreading behavior of nonvolatile liquids (high-molecular-weight polydimethylsiloxane) on smooth horizontal surfaces, and establish the following: (1) the existence of a thin precursor film extending progressively ahead of the macroscopic front of the drops, and that we directly visualize by polarized reflection microscopy; (2) the universality of the spreading kinetics, followed by means of simultaneous size and contact-angle measurements, which appears insensitive to the spreading parameter $S$. Both points give strong experimental support to recent theoretical developments.