Molecular layering in the spreading of wetting liquid drops

THE details of the mechanisms involved in the spreading of liquids on solid substrates are still poorly understood, partly because dominant phenomena may occur in the range 0 to 100 A from the substrate. Experimental investigation at the molecular level should provide information on the basic physical process involved. Here we present the results of studies on the time-dependent thickness profile of tiny drops spreading completely on a smooth surface (a silicon wafer). We use simple liquids which lack any orientational or structural order in the bulk. Using ellipsometry, an optical technique that provides measurements of thickness with sub-molecular resolution, we show that there is strong, dynamic structuring of the fluid near the solid surface, in that the spreading droplet advances as a series of distinct molecular layers. These observations may provide a clue to the nature of the molecular interactions or fluid flow phenomena involved in the spreading process.