DYNAMIC SURFACE PHENOMENA IN THE SPONTANEOUS SPREADING OF OILS ON SOLIDS

A detailed study has been made by interference microscopy and ellipsometry of the spontaneous spreading of a variety of nonpolar liquids on smooth, clean metal surfaces. Spreading is generally characterized by the advance from the bulk liquid of a primary film having a thickness of much less than 1000A; this primary film is usually followed by a considerably thicker secondary film. The movement of the secondary film is the result of a surface tension gradient across the transition zone between the primary and secondary films. This gradient is produced by the unequal evaporative depletion from these two regions of a volatile contaminant having a lower surface tension. If the volatile contaminant has a higher surface tension than the main component the direction of this surface tension gradient may be reversed so that the liquid recedes rather than spreads. Removal of the relatively more volatile constituents from the test liquid eliminates the spreading of the secondary film but not the spreading of the primary film. Under some conditions movement of liquid onto the solid surface may also occur by vapor transport and by capillary flow of liquid through microscratches. (Author)