The Microscopic and Macroscopic Structure of the Precursor Layer in Spreading Viscous Drops H. Pirouz Kavehpour(a), Ben Ovryn(b), Gareth H. McKinley(a) a)Hatsopoulos Microfluids Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA. b)Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH. Abstract: We study the spreading of viscous non-volatile 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.