Instantaneous deformation interface

When thick oxide films are formed on a sessile drop, oxidation is evident because at melting, the surface of the liquid is not smooth. However, thin oxide skins of up to a few tens of nanometers are easily deformable, allowing a smooth surface to be formed. However, even when very thin, skins do not allow metal/substrate interface to be established and as a result the apparent contact angles are as high as 160°. A classical example is pure Al on A1203 (Figure 6.20) (Brennan and Pask 1968, Eustathopoulos et al. 1974, Weirauch 1988). At room temperature, Al is instantaneously covered by an oxide layer about 2 nm thick formed by the reaction ... 

They adapted an interfacial shear rheometer (plate/ rod) to measure the shear viscoelasticity of the system with and without dispersant. At an applied shear stress, creep curves for the system were monitored. There were no instantaneous elasticity and viscosity for the Kuwait and Tia Juana crudes with and wifliout dispersant. They attributed this to a network structure of flocculated asphaltenes in the films. They found that there was some dilatancy in their crude oil films, described as a stick/slip flow in their flow curves. However, fliis flow was attributed to thick films of asphaltene particles building up at the interface. Lfsing creep measurements, they examined a model system of as-phaltenes/n-heptane/toluene. They found a retarded elastic deformation, which was different from the response of the crade oils. This suggested to fliem that there was a different type of interfaeial slrueture formed with the model oil, and this may be attributed to die solveney of the medium and not to die lower asphaltenes eontent in the model system. 

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