A Spreading Wetting

Surfactants and Interfacial Phenomena, Third Edition. Milton J. Rosen ISBN 0-471-47818-0 2004 John Wiley Sons, Inc. 

The quantity ySA — (Jsl + Ila) is then a measure of the driving force behind the spreading process and is usually called the spreading coefficient Sus- If l/s, as defined by 

When a thin layer of liquid L is being spread over a second liquid L2 as substrate, SLl/Ll = JLlA — (Yl + YijA) and the value of S can be obtained directly by measuring the surface tensions of the two liquids and the interfacial tension 

In systems where there is no strong interaction between Lt and L2, ) is less than 1. Thus, in those systems yLiA must be less than yLlA for spreading to occur spontaneously (i.e., for the spreading coefficient to be positive, the spreading liquid must have a lower surface tension than the liquid over which it is spreading). The same is assumed to be true if the substrate over which the liquid is spreading is a solid  

This concept of a critical surface tension for spreading on low-energy surfaces is one that was developed by Zisman and coworkers (Fox, 1950 Shafrin, 1960 Zisman, 1964). They demonstrated that, at least for low-energy substrates, in order to wet the substrate the surface tension of the wetting liquid must not exceed a certain critical value that is characteristic of the particular substrate. 

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