Textile Nonequilibrium Wetting

The physicochemical basis for this test was investigated by Fowkes (1953), who explained the well-known observation that the log of the wetting time (WOT) is a linear function of the log of the bulk phase concentration C of the surfactant when the latter is used at concentrations below its CMC. Fowkes stated that the penetration of the surfactant into the cotton proceeds at a rate that is a function of the contact angle 0 of the advancing liquid front such that 

When adsorption of the surfactant onto the substrate is strong, however, Fowkes found that the rate of wetting was determined not by the bulk phase concentration of the surfactant, but by the rate of diffusion of the surfactant to the wetting front. In this case the concentration of surfactant present at the advancing liquid front was so depleted by adsorption that the surface tension (or contact angle) there, and 

For a nonionic surfactant that is used at a concentration considerably above its CMC under diffusion-controlled conditions, the Fowkes (1953) equation for the factors determining WOT can be transformed (Cohen, 1981) to 

For various nonionic surfactants at concentrations considerably above their CMC, the WOT at a given temperature depends also on their diffusion, coefficients D and on their surface areas per gram of adsorbed surfactant S1 at the air-aqueous solution interface. Thus, from equation 6.21 

When nonionic surfactants are compared at similar temperatures and similar initial surfactant concentrations in the aqueous phase such that OC1, then 

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