Microscopic and macroscopic structure of the precursor layer in spreading viscous drops.

We study the spreading of viscous nonvolatile liquids on smooth horizontal substrates using a phase-modulated interference microscope with sufficient dynamic range to enable the simultaneous measurement of both the inner ("microscopic") length scale and the outer ("macroscopic") flow scale in addition to the intermediate matching region. The resulting measurements of both the apparent contact angle and the lateral scale of the precursor "wetting" film agree quantitatively with theoretical predictions for a van der Waal's liquid over a wide range of capillary numbers.