rheology then is concerned withexcessquantities; in other words, the force
needed to deform the bulk materials is somehow subtracted.
As in bulk rheology, various modes of deformation can be applied in
interfacial rheology. Some important variants are depicted in Figure 10.32.
Bendingof an interface produces a Laplace pressure [Eq. (10.7)]; the higher
the surfactant concentration, the smaller the bending force needed. It may
further be noted that a close-packed surfactant layer can fairly strongly
resist bending, though only if the radius of curvature is of molecular
dimension (order of 1 nm). Bending will not be further discussed in this
section.
The other two modes of interfacial rheology only become manifest in
the presence of surfactant. An essential difference between deformation in
shear(Fig. 10.32b) and indilatation/compression(Fig. 10.32c) is that in the
former caseGis constant, whereas in the latter cases the local surface area is
enlarged or diminished, wherebyGvaries. This implies that interchange of
surfactant between bulk and interface will in most cases occur. It should be
added that a change in the area of a surface element does not necessarily
imply that the total surface area is changed: expansion at one place can be
compensated for by compression elsewhere. An increase of total surface
area would mean that the total surface free energy is increased, which needs
the application of forces. This is generally not included in surface
dilatational rheology.
In practice, interfaces are often subjected to a combination of the
deformations mentioned. As in bulk rheology, there are some other
variables. First, the response of a material to a force can be elastic or
viscous. Elastic response means immediate deformation, where the strain
(relative deformation, i.e., tanain shear andDA/Ain dilatation) is related
to the force; on release of the force, the strain immediately becomes zero. In
viscous deformation, the force causes flow or, more precisely, a strain rate
(d tana/dtor d lnA/dt); this occurs as long as the force lasts, and upon
release of the force the strain achieved remains. For most systems, the
behavior is viscoelastic. Second, deformation can be fast or slow, and time
scales between a microsecond and more than a day may be of importance.
Third, the relative deformation (strain) applied can be small—i.e., remain
close to the equilibrium situation—or be large.
Surface rheology is in two dimensions. The stresses involved are thus
given as unit force per unit length, i.e., in N?m^1 in SI units (as compared to
N?m^2 for three dimensions). Surface elastic moduli are expressed in
N?m^1 , and surface viscosities in N?s?m^1.
singke
(singke)
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