Monolayers normal stress

Normal stress differences do not exist in the absence of interparticle forces. Moreover, the relative viscosity of the suspension is a function of only particle densities approaching the maximum possible that still allow the suspension to flow, cluster size (and, as a result, the viscosity of the two-dimensional monolayer) appears to scale as... 

To evaluate the boundary conditions, we consider the influence of the interaction forces and the properties of the adsorbed monolayers of soap molecules at both air-liquid interfaces. First we discuss the occurrence of the interaction forces in the boundary conditions. From (5.12) it is seen that the equilibrium condition for a flat, free film can be formulated in terms of the disjoining pressure. This way of describing the influence of interaction forces is followed in the continuity of the normal stress across the liquid interfaces. ... 

We have confined ourselves to a description of the dynamics of surface roughness and the influence of the interaction forces on these dynamics. In reality, however, there are many more dynamic processes in the film and especially in the adsorbed monolayers that should be considered to describe in full detail the film dynamics. Apart from dynamics of the film surfaces parallel to the normal of the interfaces, motions of the adsorbed surface molecules in the interface must be considered. According to Lucas-sen-Reynders and Lucassen, the actual stresses in an interface are described by four rheological coefficients, reflecting the viscoelastic properties of the interface. Two of these, the surface dilatational elasticity and the surface dilatational viscosity, measure the surface s resistance against changes in area. The dilatational module e, considered before, expresses the dilatational elasticity. In our description of the film system, we neglected the viscous behavior of the interface, which implies that no diffusion of surface active molecules between bulk and interface was considered. If, however, surface-to-bulk diffusion is taken into account, the expression... 

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