Article,
Effect of Short-Range Interactions on Spreading
Journal of Colloid and Interface Science, 179 (1): 136--142 (Apr 15, 1996)
DOI: 10.1006/jcis.1996.0195
Abstract
Two decades ago, the author concluded that, even if the macroscopic spreading condition is satisfied, the droplet may not spread completely over the entire surface of the solid (assumed large enough) as single atoms, but may form a planar droplet of multimolecular thickness, which has a rapid variation of angle near the leading edge. In the present paper, a simple and approximate expression for the free energy of a thin planar film which accounts for the attractive dispersion interactions and the repulsive Born interactions is used to identify the conditions under which spreading as a planar droplet or spreading as single atoms occurs. In addition, the case of very small planar droplets, assumed to have the shape of a disc, is treated. The radius and height of the disc depend upon the volume of the droplet when the latter is of the order of 10-18cm3. Such small volumes are encountered in supported metal catalysts. If the macroscopic spreading condition is not satisfied, but sufficiently strong short-range attractive forces are acting near the leading edge, the angle at the leading edge may become zero and spreading of a monolayer, bilayer, or multilayer from the droplet will occur. At equilibrium, the droplet coexists with the spread layer, and the angle at the leading edge between the droplet and the covered solid surface is finite. Explanations are suggested for (i) the absence of spreading (autophobicity) over a surface covered with a monolayer of molecules of the liquid of the droplet, and (ii) the superspreading of droplets of aqueous solutions of trisiloxane oligo (ethylene oxide) surfactants over a hydrophobic surface.
